3 REVIEW The role of RFamide peptides in feeding David A Bechtold and Simon M Luckman Faculty of Life Sciences, University of Manchester, 1.124 Stopford Building, Oxford Road, Manchester M13 9PT, UK (Requests for offprints should be addressed to D A Bechtold; Email: [email protected]) Abstract In the three decades since FMRFamide was isolated from the evolution. Even so, questions remain as to whether feeding- clam Macrocallista nimbosa, the list of RFamide peptides has related actions represent a primary function of the RFamides, been steadily growing. These peptides occur widely across the especially within mammals. However, as we will discuss here, animal kingdom, including five groups of RFamide peptides the study of RFamide function is rapidly expanding and with identified in mammals. Although there is tremendous it so is our understanding of how these peptides can influence diversity in structure and biological activity in the RFamides, food intake directly as well as related aspects of feeding the involvement of these peptides in the regulation of energy behaviour and energy expenditure. balance and feeding behaviour appears consistently through Journal of Endocrinology (2007) 192, 3–15 Introduction co-localised with classical neurotransmitters, including acetyl- choline, serotonin and gamma-amino bulyric acid (GABA). The first recognised member of the RFamide neuropeptide Although a role for RFamides in feeding behaviour was family was the cardioexcitatory peptide, FMRFamide, first suggested over 20 years ago, when FMRFamide was isolated from ganglia of the clam Macrocallista nimbosa (Price shown to be anorexigenic in mice (Kavaliers et al. 1985), the & Greenberg 1977). Since then a large number of these question of whether regulating food intake represents a peptides, defined by their carboxy-terminal arginine (R) and primary function of RFamide signalling remains. One amidated phenylalanine (F) residues (hence RFamide), have convincing piece of evidence is that RFamide involvement been identified in the nervous systems of animals within all in feeding behaviour has been demonstrated across a wide major phyla. Vertebrates and more especially invertebrates can range of animal classes, including coelenterates, molluscs, each express an array of RFamide peptides, owing to the fact amphibians, birds and mammals (see below), suggesting that this function has been conserved through evolution (Dockray that multiple genes encoding RFamides are often present in a 2004). Here, we review the evidence that explores the ability single species, and multiple mature RFamide peptides can be of RFamide peptides to influence feeding behaviour in both generated by a single polypeptide precursor. The prevalence mammalian and non-mammalian species. of RFamides in invertebrates is illustrated by the nematode Caenorhabditis elegans, in which 22 known genes encoding RFamide peptides (referred to as flp genes) and 59 distinct Mammalian RFamide peptides RFamide peptides have been identified (Li et al. 1999). Impressively, flp gene products are expressed in almost half of Immunoreactive staining of vertebrate tissues with antibodies the 302 neurons found in the adult nematode (Li et al. 1999, raised against the molluscan FMRFamide peptide first Kim & Li 2004). suggested that the RFamide peptides were to be found in RFamide peptides show a remarkable diversity in N-terminal higher animal groups (Boer et al. 1980, Dockray et al. 1981, sequence, and as a likely consequence, a broad pattern of Weber et al. 1981). Soon after, LPLRFamide was purified biological activities. Pharmacological studies have implicated from chicken brain (Dockray et al. 1983) and since then the RFamide peptides in roles that include cardiovascular function, list of vertebrate RFamides has grown steadily, including five modulation of muscle contraction, control of locomotor genes encoding RFamide peptides identified in mammals activity, water balance, neuroendocrine and neuromodulatory (Table 1; Fig. 1). In addition to the increasing number of activities (Dockray 2004, Sun et al. 2005, Fukusumi et al. 2006). peptides, several G-protein-coupled receptors (GPRs) are As with most neuropeptides, the RFamides are often now recognised as receptors for the mammalian RFamides. Journal of Endocrinology (2007) 192, 3–15 DOI: 10.1677/JOE-06-0069 0022–0795/07/0192–003 q 2007 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology-journals.org Downloaded from Bioscientifica.com at 09/29/2021 01:02:27AM via free access 4 D A BECHTOLD and S M LUCKMAN . RFamide peptides Table 1 Primary sequences of RFamide peptides Species Sequence RFamide peptide FMRFamide Clam FMRF-NH2 NPFF Human FLFQPQRF-NH2 NPAF Human AGEGLSSPFWSLAAPQRF-NH2 PrRP20 Human TPDINPAWYASRGIRPVGRF-NH2 PrRP31 Human SRTHRHSMEIRTPDINPAWYASRGIRPVGRF-NH2 C-RFa Carp SPEIDPFWYVGRGVRPIGRF-NH2 LPLRFamide Chicken LPLRF-NH2 RFRP-1 Bovine SLTFEEVKDWAPKIKMNKPVVNKMPPSAANLPLRF-NH2 RFRP-3 Bovine AMAHLPLRLGKNREDSLSRWVPNLPQRF-NH2 GnIH Quail SIKPSAYLPLRF-NH2 Metastin Human GTSLSPPPESSGSRQQPGLSAPHSRQIPAPQGAVLVQREKDLPNYNWNSFGLRF-NH2 ! QRFP Human EDEGSEATGFLPAAGEKTSGPLGNLAEELNGYSRKKGGFSFRF-NH2 26RFa Frog VGTALGSLAEELNGYNRKKGGFSFRF-NH2 Pyroglutamic acid is shown as !E. NPFF family raised against FMRFamide (Yang et al. 1985). The two Neuropeptide FF (NPFF, shortened from F8Famide) and peptides are derived from the same gene and precursor neuropeptide AF (NPAF, from A18F) were isolated originally peptide, and their expression has been demonstrated in the from bovine brain by affinity purification using antibodies brains of several mammalian species (Perry et al. 1997, Vilim Kisspeptin PrRP Human group group Bovine Kiss1 Rat Human PrRP Kiss1 Rat PrRP Mouse PrRP Kiss1 Carassius Bovine PrRP QFRP QFRP Human 26RFa/QFRP (26RFa) Fugu group PQRFa Rat 26RFa/QFRP Zebrafish PQRFa Mouse QFRP Rat NPFF Rat Mouse RFRP NPFF PQRFa Mouse (NPFF) Bovine RFRP NPFF Human group Human RFRP NPFF Bovine RFRP Lamprey PQRFa Sparrow Chicken Quail GnIH Goldfish Zebrafish GnIH GnIH LPXRFa Frog 0.1 LPXRFa GRP LPXRFa group Figure 1 Unrooted phylogenetic tree of the identified and the putative RFamide peptides in mammals and other vertebrates. The neighbour-joining method was used to construct this phylogenetic tree. Data were re-sampled by 1000 bootstrap replicates to determine the confidence indices within the phylogenetic tree. Scale bar refers to a phylogenetic distance of 0.1 amino acid substitutions per site. Frog 26RFa does not appear on the map and non-mammalian homologues of kisspeptin have not yet been identified. However, receptors related to GPR54 and GPR103 have been predicted from the zebrafish and chicken genomes respectively. Figure reprinted with minor modification from Osugi et al. (2006), FEBS Journal 273; copyright, with permission from Blackwell-Synergy. Journal of Endocrinology (2007) 192, 3–15 www.endocrinology-journals.org Downloaded from Bioscientifica.com at 09/29/2021 01:02:27AM via free access RFamide peptides . D A BECHTOLD and S M LUCKMAN 5 et al. 1999, and references cited below). The best- 2006). Sunter et al. (2001) have reported that the attenuation documented function of NPFF is its ability to modulate of feeding in rats by NPFF is accompanied by an acute opioid-induced analgesia (Yang et al. 1985, Panula et al. 1999, stimulation of water intake; we have not observed this effect Dong et al. 2001), although cardiovascular (Allard et al. 1995) in mice (unpublished results). As described above, both NPFF and feeding (Murase et al. 1996, Sunter et al. 2001, Bechtold and its receptors are localised in the hypothalamus, consistent & Luckman 2006) effects have also clearly been demonstrated. with a direct action of NPFF on hypothalamic neurons Two GPRs have been identified as putative receptors for regulating food intake. NPFF immunoreactive fibres densely NPFF and consequently designated NPFF1 and NPFF2 innervate another area implicated in feeding behaviour, the (Bonini et al. 2000). Subsequent studies have shown that PBN. Administration of NPFF into the PBN in relatively low NPFF1 exhibits a higher affinity for RFRP than for NPFF, doses (!10 nmol) can inhibit the stimulation of food intake and NPFF displays a higher potency for NPFF2 (Liu et al. in response to the m-opioid receptor agonist, [D-Ala, N-Me- 2001, Yoshida et al. 2003). Therefore, NPFF2 is now Phe, Gly-oL]-enkephalin (DAMGO; Nicklous & Simansky generally considered the endogenous receptor for NPFF, 2003). Interestingly, higher doses of NPFF caused an increase with NPFF1 as the RFRP receptor. However, NPFF2 in food intake which could be blocked by naloxone, an opioid appears to be fairly promiscuous, showing relatively high receptor antagonist. Combined with the well-documented affinities for NPFF, RFRPs as well as prolactin-releasing ability of NPFF to modulate opioid-induced analgesia (Panula peptide (PrRP; Engstrom et al. 2003). Human NPFF1 and et al. 1999), these findings suggest that the modulation of NPFF2 share 46% sequence identity and exhibit 31–37% feeding behaviour by NPFF and opioids involve similar identities with the receptors for orexin, neuropeptide Y, pathways (Murase et al. 1996). NPFF has been shown to CCK1 and PrRP. depress excitatory glutamatergic synaptic transmission in The major population of NPFF-expressing neurons in the PBN neurons, evidently acting via pre-synaptic d-opioid rat brain lies within the nucleus of the tractus solitarius (NTS; receptors (Chen et al. 2000). However, NPFF shows no Kivipelto et al. 1989, Boersma et al. 1993,