Cloning and expression of a FMRFamide-gated Na(+) channel from Helisoma trivolvis and comparison with the native neuronal channel

A Na+ channel receptor of FMRFamide in the cephalopod Sepiella japonica: Identification, characterisation, and expression profiling during different stages of gonadal development

FMRFamide, a member of the neuropeptide family, is involved in numerous physiological processes. FMRFamide-activated sodium channels (FaNaCs) are a family of non-voltage-gated, amiloride-sensitive, Na+-selective channels triggered by the neuropeptide FMRFamide. In the present study, the full-length cDNA of the FaNaC receptor of Sepiella japonica (SjFaNaC) was cloned. The cDNA of SjFaNaC was 3004 bp long with an open reading frame (ORF) of 1812 bp, encoding 603 amino acid residues with no signal peptide at the N-terminus. Sequence analysis indicated that SjFaNaC shared a high identity with other cephalopods FaNaCs and formed a sister clade with bivalves. The protein structure was predicted using SWISS-MODEL with AcFaNaC as the template. Quantitative real-time PCR (qRT-PCR) revealed that SjFaNaC transcripts were highly expressed in both female and male reproductive organs, as well as in the optic lobe and brain of the central nervous system (CNS). Results of in situ hybridisation (ISH) showed that SjFaNaC mRNA was mainly distributed in the medulla and deep retina of the optic lobe and in both the supraesophageal and subesophageal masses of the brain. Subcellular localisation indicated that the SjFaNaC protein was localised intracellularly and on the cell surface of HEK293T cells. In summary, these findings may lay the foundation for future exploration of the functions of SjFaNaC in cephalopods.

Structure and mechanism of a neuropeptide-activated channel in the ENaC/DEG superfamily

Phe-Met-Arg-Phe-amide (FMRFamide)-activated sodium channels (FaNaCs) are a family of channels activated by the neuropeptide FMRFamide, and, to date, the underlying ligand gating mechanism remains unknown. Here we present the high-resolution cryo-electron microscopy structures of Aplysia californica FaNaC in both apo and FMRFamide-bound states. AcFaNaC forms a chalice-shaped trimer and possesses several notable features, including two FaNaC-specific insertion regions, a distinct finger domain and non-domain-swapped transmembrane helix 2 in the transmembrane domain (TMD). One FMRFamide binds to each subunit in a cleft located in the top-most region of the extracellular domain, with participation of residues from the neighboring subunit. Bound FMRFamide adopts an extended conformation. FMRFamide binds tightly to A. californica FaNaC in an N terminus-in manner, which causes collapse of the binding cleft and induces large local conformational rearrangements. Such conformational changes are propagated downward toward the TMD via the palm domain, possibly resulting in outward movement of the TMD and dilation of the ion conduction pore.

Aromatic amino acids in the finger domain of the FMRFamide-gated Na[Formula: see text] channel are involved in the FMRFamide recognition and the activation

FMRFamide-gated Na[Formula: see text] channel (FaNaC) is a member of the DEG/ENaC family and activated by a neuropeptide, FMRFamide. Structural information about the FMRFamide-dependent gating is, however, still elusive. Because two phenylalanines of FMRFamide are essential for the activation of FaNaC, we hypothesized that aromatic-aromatic interaction between FaNaC and FMRFamide is critical for FMRFamide recognition and/or the activation gating. Here, we focused on eight conserved aromatic residues in the finger domain of FaNaCs and tested our hypothesis by mutagenic analysis and in silico docking simulations. The mutation of conserved aromatic residues in the finger domain reduced the FMRFamide potency, suggesting that the conserved aromatic residues are involved in the FMRFamide-dependent activation. The kinetics of the FMRFamide-gated currents were also modified substantially in some mutants. Some results of docking simulations were consistent with a hypothesis that the aromatic-aromatic interaction between the aromatic residues in FaNaC and FMRFamide is involved in the FMRFamide recognition. Collectively, our results suggest that the conserved aromatic residues in the finger domain of FaNaC are important determinants of the ligand recognition and/or the activation gating in FaNaC.

Identification and expression analysis of receptors that mediate MIP regulating larval settlement in Urechis unicinctus

Larval attachment and metamorphosis are important processes during the development of some marine invertebrates. Myoinhibitory peptides (MIPs), a class of small molecular neuropeptides, have been revealed to be involved in regulating the larval settlement. In this paper, we identified two types of MIP membrane receptors, G-protein coupled receptor SPR and MIP-gated ion channel receptors MGIC1 and MGIC2 based on sequence homology with other species in the transcriptome database of Echiuroidea Urechis unicinctus (Xenopneusta, Urechidae). The results of in situ hybridization showed that positive signals of these receptors were obviously located in the apex of the segmentation larvae, a critical stage of U. unicinctus larval settlement. Further, these receptors were determined on the membrane of HEK293 cells by immunohistochemistry. Also, we verified that U. unicinctus MIP can activate its SPR receptor based on the results of the significantly decreased cAMP concentration in HEK293 cells. Our data will provide scientific reference for elucidating mechanism of neuropeptide regulating the larval attachment and metamorphosis in marine invertebrates.

Acid-sensing ion channels in sensory signaling

Acid-sensing ion channels (ASICs) are cation-permeable channels that in the periphery are primarily expressed in sensory neurons that innervate tissues and organs. Soon after the cloning of the ASIC subunits, almost 20 yr ago, investigators began to use genetically modified mice to assess the role of these channels in physiological processes. These studies provide critical insights about the participation of ASICs in sensory processes, including mechanotransduction, chemoreception, and nociception. Here, we provide an extensive assessment of these findings and discuss the current gaps in knowledge with regard to the functions of ASICs in the peripheral nervous system.

A Na+ leak channel cloned from Trichoplax adhaerens extends extracellular pH and Ca2+ sensing for the DEG/ENaC family close to the base of Metazoa

Acid-sensitive ion channels belonging to the degenerin/epithelial sodium channel (DEG/ENaC) family activate in response to extracellular protons and are considered unique to deuterostomes. However, sensitivity to pH/protons is more widespread, where, for example, human ENaC Na⁺ leak channels are potentiated and mouse BASIC and Caenorhabditis elegans ACD-1 Na⁺ leak channels are blocked by extracellular protons. For many DEG/ENaC channels, extracellular Ca²⁺ ions modulate gating, and in some cases, the binding of protons and Ca²⁺ is interdependent. Here, we functionally characterize a DEG/ENaC channel from the early-diverging animal Trichoplax adhaerens, TadNaC6, that conducts Na⁺-selective leak currents in vitro sensitive to blockade by both extracellular protons and Ca²⁺. We determine that proton block is enhanced in low external Ca²⁺ concentration, whereas calcium block is enhanced in low external proton concentration, indicative of competitive binding of these two ligands to extracellular sites of the channel protein. TadNaC6 lacks most determinant residues for proton and Ca²⁺ sensitivity in other DEG/ENaC channels, and a mutation of one conserved residue (S353A) associated with Ca²⁺ block in rodent BASIC channels instead affected proton sensitivity, all indicative of independent evolution of H⁺ and Ca²⁺ sensitivity. Strikingly, TadNaC6 was potently activated by the general DEG/ENaC channel blocker amiloride, a rare feature only reported for the acid-activated channel ASIC3. The sequence and structural divergence of TadNaC6, coupled with its noncanonical functional features, provide unique opportunities for probing the proton, Ca²⁺, and amiloride regulation of DEG/ENaC channels and insight into the possible core-gating features of ancestral ion channels.

New techniques, applications and perspectives in neuropeptide research

Neuropeptides are one of the most diverse classes of signaling molecules and have attracted great interest over the years owing to their roles in regulation of a wide range of physiological processes. However, there are unique challenges associated with neuropeptide studies stemming from the highly variable molecular sizes of the peptides, low in vivo concentrations, high degree of structural diversity and large number of isoforms. As a result, much effort has been focused on developing new techniques for studying neuropeptides, as well as novel applications directed towards learning more about these endogenous peptides. The areas of importance for neuropeptide studies include structure, localization within tissues, interaction with their receptors, including ion channels, and physiological function. Here, we discuss these aspects and the associated techniques, focusing on technologies that have demonstrated potential in advancing the field in recent years. Most identification and structural information has been gained by mass spectrometry, either alone or with confirmations from other techniques, such as nuclear magnetic resonance spectroscopy and other spectroscopic tools. While mass spectrometry and bioinformatic tools have proven to be the most powerful for large-scale analyses, they still rely heavily on complementary methods for confirmation. Localization within tissues, for example, can be probed by mass spectrometry imaging, immunohistochemistry and radioimmunoassays. Functional information has been gained primarily from behavioral studies coupled with tissue-specific assays, electrophysiology, mass spectrometry and optogenetic tools. Concerning the receptors for neuropeptides, the discovery of ion channels that are directly gated by neuropeptides opens up the possibility of developing a new generation of tools for neuroscience, which could be used to monitor neuropeptide release or to specifically change the membrane potential of neurons. It is expected that future neuropeptide research will involve the integration of complementary bioanalytical technologies and functional assays.

The nonproton ligand of acid-sensing ion channel 3 activates mollusk-specific FaNaC channels via a mechanism independent of the native FMRFamide peptide

The degenerin/epithelial sodium channel (DEG/ENaC) superfamily of ion channels contains subfamilies with diverse functions that are fundamental to many physiological and pathological processes, ranging from synaptic transmission to epileptogenesis. The absence in mammals of some DEG/ENaCs subfamily orthologues such as FMRFamide peptide-activated sodium channels (FaNaCs), which have been identified only in mollusks, indicates that the various subfamilies diverged early in evolution. We recently reported that the nonproton agonist 2-guanidine-4-methylquinazoline (GMQ) activates acid-sensing ion channels (ASICs), a DEG/ENaC subfamily mainly in mammals, in the absence of acidosis. Here, we show that GMQ also could directly activate the mollusk-specific FaNaCs. Differences in ion selectivity and unitary conductance and effects of substitutions at key residues revealed that GMQ and FMRFamide activate FaNaCs via distinct mechanisms. The presence of two activation mechanisms in the FaNaC subfamily diverging early in the evolution of DEG/ENaCs suggested that dual gating is an ancient feature in this superfamily. Notably, the GMQ-gating mode is still preserved in the mammalian ASIC subfamily, whereas FMRFamide-mediated channel gating was lost during evolution. This implied that GMQ activation may be essential for the functions of mammalian DEG/ENaCs. Our findings provide new insights into the evolution of DEG/ENaCs and may facilitate the discovery and characterization of their endogenous agonists.

Modulation of the FMRFamide-gated Na+ channel by external Ca2

FMRFamide-gated Na⁺ channel (FaNaC) is a member of the DEG/ENaC family. Amino acid sequence of the second transmembrane region (TM2) of FaNaC is quite similar to that of the acid-sensing ion channels (ASIC) of the same family. In the upper part of TM2, there are two aspartate residues (D552 and D556 in Aplysia FaNaC, AkFaNaC) which construct two negative rings in the external vestibule. In the present study, we examined the function of D552/D556 mutants of AkFaNaC in Xenopus oocytes with special interest in Ca²⁺ sensitivity of FaNaC. The FMRFamide-evoked current through AkFaNaC was depressed by submillimolar Ca²⁺ such that the current in Ca²⁺-free condition was 2-3-fold larger than that in the control solution which contained 1.8 mM CaCl ₂. Both D552 and D556 were found to be indispensable for the sensitivity of FaNaC to submillimolar Ca²⁺. Unexpectedly, however, both acidic residues were not essential for the inhibition by millimolar Ca²⁺ concentrations. The Ca²⁺-sensitive gating of FaNaC was recapitulated by an allosteric model in which Ca²⁺-bound channels are more difficult to open. The desensitization of FaNaC was also inhibited by Ca²⁺, which was abolished in some D552/D556 mutants. Structural models of FaNaC made by homology modeling showed that the distance between oxygen atoms of D552 and D556 on the adjacent subunits is close enough to coordinate Ca²⁺ in the nonconducting desensitized channel but not in the open channel. The results suggest that Ca²⁺ coordination between oxygen atoms of D552 and D556 disturbs the opening transition as well as the desensitization of FaNaC.

Proton and non-proton activation of ASIC channels

The Acid-Sensing Ion Channels (ASIC) exhibit a fast desensitizing current when activated by pH values below 7.0. By contrast, non-proton ligands are able to trigger sustained ASIC currents at physiological pHs. To analyze the functional basis of the ASIC desensitizing and sustained currents, we have used ASIC1a and ASIC2a mutants with a cysteine in the pore vestibule for covalent binding of different sulfhydryl reagents. We found that ASIC1a and ASIC2a exhibit two distinct currents, a proton-induced desensitizing current and a sustained current triggered by sulfhydryl reagents. These currents differ in their pH dependency, their sensitivity to the sulfhydryl reagents, their ionic selectivity and their relative magnitude. We propose a model for ASIC1 and ASIC2 activity where the channels can function in two distinct modes, a desensitizing mode and a sustained mode depending on the activating ligands. The pore vestibule of the channel represents a functional site for binding non-proton ligands to activate ASIC1 and ASIC2 at neutral pH and to prevent channel desensitization.

Exploration of the Peptide Recognition of an Amiloride-sensitive FMRFamide Peptide-gated Sodium Channel

FMRFamide (Phe-Met-Arg-Phe-NH2)-activated sodium channel (FaNaC) is an amiloride-sensitive sodium channel activated by endogenous tetrapeptide in invertebrates, and belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. The ENaC/DEG superfamily differs markedly in its means of activation, such as spontaneously opening or gating by mechanical stimuli or tissue acidosis. Recently, it has been observed that a number of ENaC/DEG channels can be activated by small molecules or peptides, indicating that the ligand-gating may be an important feature of this superfamily. The peptide ligand control of the channel gating might be an ancient ligand-gating feature in this superfamily. Therefore, studying the peptide recognition of FaNaC channels would advance our understanding of the ligand-gating properties of this superfamily of ion channels. Here we demonstrate that Tyr-131, Asn-134, Asp-154, and Ile-160, located in the putative upper finger domain ofHelix aspersaFaNaC (HaFaNaC) channels, are key residues for peptide recognition of this ion channel. Two HaFaNaC specific-insertion motifs among the ENaC/DEG superfamily, residing at the putative α4-α5 linker of the upper thumb domain and the α6-α7 linker of the upper knuckle domain, are also essential for the peptide recognition of HaFaNaC channels. Chemical modifications and double mutant cycle analysis further indicated that those two specific inserts and key residues in the upper finger domain together participate in peptide recognition of HaFaNaC channels. This ligand recognition site is distinct from that of acid-sensing ion channels (ASICs) by a longer distance between the recognition site and the channel gate, carrying useful information about the ligand gating and the evolution of the trimeric ENaC/DEG superfamily of ion channels.

Diversity of the RFamide Peptide Family in Mollusks

Since the initial characterization of the cardioexcitatory peptide FMRFamide in the bivalve mollusk Macrocallista nimbosa, a great number of FMRFamide-like peptides (FLPs) have been identified in mollusks. FLPs were initially isolated and molecularly characterized in model mollusks using biochemical methods. The development of recombinant technologies and, more recently, of genomics has boosted knowledge on their diversity in various mollusk classes. Today, mollusk FLPs represent approximately 75 distinct RFamide peptides that appear to result from the expression of only five genes: the FMRFamide-related peptide gene, the LFRFamide gene, the luqin gene, the neuropeptide F gene, and the cholecystokinin/sulfakinin gene. FLPs display a complex spatiotemporal pattern of expression in the central and peripheral nervous system. Working as neurotransmitters, neuromodulators, or neurohormones, FLPs are involved in the control of a great variety of biological and physiological processes including cardiovascular regulation, osmoregulation, reproduction, digestion, and feeding behavior. From an evolutionary viewpoint, the major challenge will then logically concern the elucidation of the FLP repertoire of orphan mollusk classes and the way they are functionally related. In this respect, deciphering FLP signaling pathways by characterizing the specific receptors these peptides bind remains another exciting objective.

Parasite neuropeptide biology: Seeding rational drug target selection?

The rationale for identifying drug targets within helminth neuromuscular signalling systems is based on the premise that adequate nerve and muscle function is essential for many of the key behavioural determinants of helminth parasitism, including sensory perception/host location, invasion, locomotion/orientation, attachment, feeding and reproduction. This premise is validated by the tendency of current anthelmintics to act on classical neurotransmitter-gated ion channels present on helminth nerve and/or muscle, yielding therapeutic endpoints associated with paralysis and/or death. Supplementary to classical neurotransmitters, helminth nervous systems are peptide-rich and encompass associated biosynthetic and signal transduction components - putative drug targets that remain to be exploited by anthelmintic chemotherapy. At this time, no neuropeptide system-targeting lead compounds have been reported, and given that our basic knowledge of neuropeptide biology in parasitic helminths remains inadequate, the short-term prospects for such drugs remain poor. Here, we review current knowledge of neuropeptide signalling in Nematoda and Platyhelminthes, and highlight a suite of 19 protein families that yield deleterious phenotypes in helminth reverse genetics screens. We suggest that orthologues of some of these peptidergic signalling components represent appealing therapeutic targets in parasitic helminths.

The neuronal control of cardiac functions in Molluscs

In this manuscript, I review the current and relevant classical studies on properties of the Mollusca heart and their central nervous system including ganglia, neurons, and nerves involved in cardiomodulation. Similar to mammalian brain hemispheres, these invertebrates possess symmetrical pairs of ganglia albeit visceral (only one) ganglion and the parietal ganglia (the right ganglion is bigger than the left one). Furthermore, there are two major regulatory drives into the compartments (pericard, auricle, and ventricle) and cardiomyocytes of the heart. These are the excitatory and inhibitory signals that originate from a few designated neurons and their putative neurotransmitters. Many of these neurons are well-identified, their specific locations within the corresponding ganglion are mapped, and some are termed as either heart excitatory (HE) or inhibitory (HI) cells. The remaining neurons are classified as cardio-regulatory, and their direct and indirect actions on the heart's function have been documented. The cardiovascular anatomy of frequently used experimental animals, Achatina, Aplysia, Helix, and Lymnaea is relatively simple. However, as in humans, it possesses all major components including even trabeculae and atrio-ventricular valves. Since the myocardial cells are enzymatically dispersible, multiple voltage dependent cationic currents in isolated cardiomyocytes are described. The latter include at least the A-type K(+), delayed rectifier K(+), TTX-sensitive Na(+), and L-type Ca(2+) channels.

Eukaryotic mechanosensitive channels

Mechanosensitive ion channels are gated directly by physical stimuli and transduce these stimuli into electrical signals. Several criteria must apply for a channel to be considered mechanically gated. Mechanosensitive channels from bacterial systems have met these criteria, but few eukaryotic channels have been confirmed by the same standards. Recent work has suggested or confirmed that diverse types of channels, including TRP channels, K(2P) channels, MscS-like proteins, and DEG/ENaC channels, are mechanically gated. Several studies point to the importance of the plasma membrane for channel gating, but intracellular and/or extracellular structures may also be required.

Position 552 in a FMRFamide-gated Na(+) channel affects the gating properties and the potency of FMRFamide

FMRFamide-gated Na(+) channel (FaNaC) is a peptide-gated sodium channel in the epithelial Na(+) channel/degenerin family. Although there are some data on the location of the putative peptide binding site, there is no structural information on the activation gating of FaNaC. Here, we addressed the function of a conserved aspartate residue in the second transmembrane domain of FaNaC. We used Aplysia kurodai FaNaC (AkFaNaC) and examined the function of the aspartate (D552) by site-directed mutagenesis and electrophysiological recording in Xenopus oocytes. We found that the macroscopic activation, desensitization, and potency of FMRFamide and its modification by external Ca(2+) and Mg(2+) are greatly affected by physicochemical properties of the amino acid at position 552. We conclude that D552 is situated in a key position that affects the gating properties of FaNaC.

Three homologous subunits form a high affinity peptide-gated ion channel in Hydra

Recently, three ion channel subunits of the degenerin (DEG)/epithelial Na(+) channel (ENaC) gene family have been cloned from the freshwater polyp Hydra magnipapillata, the Hydra Na(+) channels (HyNaCs) 2-4. Two of them, HyNaC2 and HyNaC3, co-assemble to form an ion channel that is gated by the neuropeptides Hydra-RFamides I and II. The HyNaC2/3 channel is so far the only cloned ionotropic receptor from cnidarians and, together with the related ionotropic receptor FMRFamide-activated Na(+) channel (FaNaC) from snails, the only known peptide-gated ionotropic receptor. The HyNaC2/3 channel has pore properties, like a low Na(+) selectivity and a low amiloride affinity, that are different from other channels of the DEG/ENaC gene family, suggesting that a component of the native Hydra channel might still be lacking. Here, we report the cloning of a new ion channel subunit from Hydra, HyNaC5. The new subunit is closely related to HyNaC2 and -3 and co-localizes with HyNaC2 and -3 to the base of the tentacles. Coexpression in Xenopus oocytes of HyNaC5 with HyNaC2 and -3 largely increases current amplitude after peptide stimulation and affinity of the channel to Hydra-RFamides I and II. Moreover, the HyNaC2/3/5 channel has altered pore properties and amiloride affinity, more similarly to other DEG/ENaC channels. Collectively, our results suggest that the three homologous subunits HyNaC2, -3, and -5 form a peptide-gated ion channel in Hydra that could contribute to fast synaptic transmission.

Heterologous expression of the invertebrate FMRFamide-gated sodium channel as a mechanism to selectively activate mammalian neurons

Considerable effort has been directed toward the development of methods to selectively activate specific subtypes of neurons. Focus has been placed on the heterologous expression of proteins that are capable of exciting neurons in which they are expressed. Here we describe the heterologous expression of the invertebrate FMRFamide (H-phenylalanine-methionine-arginine-phenylalanine-NH2) -gated sodium channel from Helix aspersa (HaFaNaC) in hippocampal slice cultures. HaFaNaC was co-expressed with a fluorescent protein (green fluorescent protein (GFP), red fluorescent protein from Discosoma sp (dsRed) or mutated form of red fluorescent protein from Discosoma sp (tdTomato)) in CA3 pyramidal neurons of rat hippocampal slice cultures using single cell electroporation. Pressure application of the agonist FMRFamide to HaFaNaC-expressing neuronal somata produced large prolonged depolarizations and bursts of action potentials (APs). FMRFamide responses were inhibited by amiloride (100 microM). In contrast, pressure application of FMRFamide to the axons of neurons expressing HaFaNaC produced no response. Fusion of GFP to the N-terminus of HaFaNaC showed that GFP-HaFaNaC was absent from axons. Bath application of FMRFamide produced persistent AP firing in HaFaNaC-expressing neurons. This FMRFamide-induced increase in the frequency of APs was dose-dependent. The concentrations of FMRFamide required to activate HaFaNaC-expressing neurons were below that required to activate the homologous acid sensing ion channel normally found in mammalian neurons. Furthermore, the mammalian neuropeptides neuropeptide FF and RFamide-related peptide-1, which have amidated RF C-termini, did not affect HaFaNaC-expressing neurons. Antagonists of NPFF receptors (BIBP3226) also had no effect on HaFaNaC. Therefore, we suggest that heterologous-expression of HaFaNaC in mammalian neurons could be a useful method to selectively and persistently excite specific subtypes of neurons in intact nervous tissue.

FMRFamide and related peptides in the phylum mollusca

FMRFamide is one of the well-known peptides studied within the phylum Mollusca. It was first isolated from the clam Macrocallista nimbosa during the end of the 1960s. Since then, a number of reports related to FMRFamide have been published from different experimental approaches, revealing that it and its related peptides (FaRPs) are implicated in a variety of physiological processes. As this year is the 30th anniversary since its discovery, this review focuses on diverse findings related to both FMRFamide and FaRPs in the phylum Mollusca.

FMRFamide-gated sodium channel and ASIC channels: a new class of ionotropic receptors for FMRFamide and related peptides

FMRFamide and related peptides typically exert their action through G-protein coupled receptors. However, two ionotropic receptors for these peptides have recently been identified. They are both members of the epithelial amiloride-sensitive Na+ channel and degenerin (ENaC/DEG) family of ion channels. The invertebrate FMRFamide-gated Na+ channel (FaNaC) is a neuronal Na+-selective channel which is directly gated by micromolar concentrations of FMRFamide and related tetrapeptides. Its response is fast and partially desensitizing, and FaNaC has been proposed to participate in peptidergic neurotransmission. On the other hand, mammalian acid-sensing ion channels (ASICs) are not gated but are directly modulated by FMRFamide and related mammalian peptides like NPFF and NPSF. ASICs are activated by external protons and are therefore extracellular pH sensors. They are expressed both in the central and peripheral nervous system and appear to be involved in many physiological and pathophysiological processes such as hippocampal long-term potentiation and defects in learning and memory, acquired fear-related behavior, retinal function, brain ischemia, pain sensation in ischemia and inflammation, taste perception, hearing functions, and mechanoperception. The potentiation of ASIC activity by endogenous RFamide neuropeptides probably participates in the response to noxious acidosis in sensory and central neurons. Available data also raises the possibility of the existence of still unknown FMRFamide related endogenous peptides acting as direct agonists for ASICs.

Molecular cloning and functional characterization of the Aplysia FMRFamide-gated Na+ channel

FMRFamide-gated Na+ channel (FaNaC) is the only known peptide-gated ion channel, which belongs to the epithelial Na+ channel/degenerin (ENaC/DEG) family. We have cloned a putative FaNaC from the Aplysia kurodai CNS library using PCR, and examined its characteristics in Xenopus oocytes. A. kurodai FaNaC (AkFaNaC) comprised with 653 amino acids, and the sequence predicts two putative membrane domains and a large extracellular domain as in other members of the ENaC/DEG family. In oocytes expressing AkFaNaC, FMRFamide evoked amiloride-sensitive Na+ current. Different from the known FaNaCs (Helix and Helisoma FaNaCs), AkFaNaC was blocked by external Ca2+ but not by Mg2+. Also, desensitization of the current was enhanced by Mg2+ but not by Ca2+. The FMRFamide-gated current was depressed in both low and high pH. These results indicate that AkFaNaC is an FaNaC of Aplysia, and that the channel has Aplysia specific functional domains.

Domain near TM1 influences agonist and antagonist responses of peptide-gated Na+ channels

A molecular biological approach was used to analyse the importance of different amino acids for ligand activation and for determining the action of amiloride on peptide- (Phe-Met-Arg-Phe-NH(2), FMRFamide)-gated Na(+) channels, members of the degenerin/epithelial Na channel (DEG/ENaC) family. Amiloride is a low-affinity blocker of most DEG/ENa channels, but has an unusual enhancing effect on responses of some of them. Chimeras were expressed in Xenopus oocytes and studied electrophysiologically. Differences in properties of channels from Helix aspersa and Helisoma trivolvis highlighted a sequence of 50 residues of the extracellular domain, near the first transmembrane segment (TM1), that affected sensitivity to FMRFamide, and whether amiloride blocked or enhanced the response to FMRFamide. Comparisons of chimeras prepared from H. aspersa and the extracellular domains of two other species, Aplysia californica and Lymnaea stagnalis and the preparation of further constructs, showed that amino acids 128-134 in the H. aspersa sequence are important in determining the predominant effect of amiloride and influencing the EC(50) of FMRFamide. The results also showed that amino acids in this region are influenced by amino acids in other regions of the extracellular domain so as to affect not only the magnitude of responses, but also their time course and desensitisation.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

Neuropeptide signalling: a repository of targets for novel endectocides?

The only available parasiticides with a spectrum of action that includes a broad range of helminth and arthropod parasites are the macrocyclic lactones. Designated endectocides, these drugs have action against both endoparasitic nematodes and ectoparasitic arthropods. Unfortunately, the discovery of such drugs is exceedingly rare and there is no evidence that novel endectocidal agents will be identified and developed in the short to medium term. However, the discovery of neuropeptides with motor-modulatory activities in both arthropods and helminths, coupled with recent progress in the characterization of invertebrate neuropeptide receptors, has the potential to propel neuropeptide signalling to the forefront of efforts to develop a novel endectocide.

ASIC3 and ASIC1 mediate FMRFamide-related peptide enhancement of H+-gated currents in cultured dorsal root ganglion neurons

The acid-sensing ion channels (ASICs) form cation channels that are transiently activated by extracellular protons. They are expressed in dorsal root ganglia (DRG) neurons and in the periphery where they play a function in nociception and mechanosensation. Previous studies showed that FMRFamide and related peptides potentiate H(+)-gated currents. To better understand this potentiation, we examined the effect of FMRFamide-related peptides on DRG neurons from wild-type mice and animals missing individual ASIC subunits. We found that FMRFamide and FRRFamide potentiated H(+)-gated currents of wild-type DRG in a dose-dependent manner. They increased current amplitude and slowed desensitization following a proton stimulus. Deletion of ASIC3 attenuated the response to FMRFamide-related peptides, whereas the loss of ASIC1 increased the response. The loss of ASIC2 had no effect on FMRFamide-dependent enhancement of H(+)-gated currents. These data suggest that FMRFamide-related peptides modulate DRG H(+)-gated currents through an effect on both ASIC1 and ASIC3 and that ASIC3 plays the major role. The recent discovery of RFamide-related peptides (RFRP) in mammals suggested that they might also modulate H(+)-gated current. We found that RFRP-1 slowed desensitization of H(+)-gated DRG currents, whereas RFRP-2 increased the peak amplitude. COS-7 cells heterologously expressing ASIC1 or ASIC3 showed similar effects. These results suggest that FMRFamide-related peptides, including the newly identified RFRPs, modulate H(+)-gated DRG currents through ASIC1 and ASIC3. The presence of several ASIC subunits, the diversity of FMRFamide-related peptides, and the distinct effects on H(+)-gated currents suggest the possibility of substantial complexity in modulation of current in DRG sensory neurons.

The structure of the FMRFamide receptor and activity of the cardioexcitatory neuropeptide are conserved in mosquito

Numerous peptides are structurally related to the cardioexcitatory tetrapeptide FMRFamide. One subgroup of FMRFamide-related peptides (FaRPs) contains an FMRFamide C terminus. Searches of the Drosophila melanogaster genome database identified the first invertebrate FMRFamide G-protein coupled receptor (GPCR), DrmFMRFa-R (Cazzamali and Grimmelikhuijzen, Meeusen et al., 2002). In order to explore molecular mechanisms involved in FMRFamide signal transduction we identified a receptor from the malaria mosquito Anopheles gambiae genome (Holt et al., 2002), AngFMRFa-R, and compared its structure to DrmFMRFa-R. The cytoplasmic loops, extracellular loops, and transmembrane regions are highly conserved between these two FMRFamide receptors. Another subgroup of FaRPs is the sulfakinins which are represented by the consensus structure -XDYGHMRFamide, where X is D or E (Nichols, 2003). We compared AngFMRFa-R and DrmFMRFa-R to the A. gambiae sulfakinin receptors, ASK-R1 and ASK-R2 ( Duttlinger et al., 2003), and the D. melanogaster sulfakinin receptors, DSK-R1 and DSK-R2 Brody and Cravchik, 2000; Hewes and Taghert, 2001 ). The cytoplasmic loops, extracellular loops, and the transmembrane regions are not highly conserved between the FMRFamide and sulfakinin receptors. In order to explore the role of FMRFamide in mosquito biology we measured the effect of the tetrapeptide on in vivo heart rate. The tetrapeptide increased the frequency of spontaneous contractions of the larval mosquito heart and, thus, increased heart rate. These data support the conclusion that the structure of the FMRFamide receptor and activity of the cardioexcitatory FMRFamide neuropeptide are conserved in mosquito.

Identification in Drosophila melanogaster of the invertebrate G protein-coupled FMRFamide receptor

We here describe the cloning and characterization of the functionally active Drosophila melanogaster (Drm) FMRFamide receptor, which we designated as DrmFMRFa-R. The full-length ORF of a D. melanogaster orphan receptor, CG 2114 (Berkeley Drosophila Genome Project), was cloned from genomic DNA. This receptor is distantly related to mammalian thyroid-stimulating hormone-releasing hormone receptors and to a set of Caenorhabditis elegans orphan receptors. An extract of 5,000 central nervous systems from the related but bigger flesh fly, Neobellieria bullata (Neb), was used to screen cells expressing the orphan receptor. Successive purification steps, followed by MS, revealed the sequence of two previously uncharacterized endogenous peptides, APPQPSDNFIRFamide (Neb-FIRFamide) and pQPSQDFMRFamide (Neb-FMRFamide). These are reminiscent of other insect FMRFamide peptides, having neurohormonal as well as neurotransmitter functions. Nanomolar concentrations of the Drm FMRFamides (DPKQDFMRFamide, TPAEDFMRFamide, SDNFMRFamide, SPKQDFMRFamide, and PDNFMRFamide) activated the cognate receptor in a dose-dependent manner. To our knowledge, the cloned DrmFMRFa-R is the first functionally active FMRFamide G protein-coupled receptor described in invertebrates to date.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.

Neuronal expression of an FMRFamide-gated Na+ channel and its modulation by acid pH

The molluscan Phe-Met-Arg-Phe-amide (FMRFamide)-gated sodium channels (FaNaCs) show both structural and functional similarities to the mammalian acid-sensing ion channels (ASICs). Both channel types are related to the epithelial sodium channels and, although the neuropeptide FMRFamide directly gates the FaNaCs, it also modulates the proton-gating properties of ASICs. It is not yet known whether protons can alter the gating properties of the FaNaCs. We chose to examine this possibility at a site of FaNaC expression in the nervous system of the mollusk Lymnaea stagnalis. We cloned a putative L. stagnalis FaNaC (LsFaNaC) that exhibited a high degree of sequence identity to the Helix aspersa FaNaC (HaFaNaC, 60%), and a weaker homology to the ASICs (ASIC3, 22%). In situ hybridization was used to map the LsFaNaC expression pattern in the brain and to identify the right pedal giant1 (RPeD1) neuron as a site where the properties of the endogenous channel could be studied. In RPeD1 neurons isolated in culture, we demonstrated the presence of an FMRFamide-gated sodium current with features expected for a FaNaC: amiloride sensitivity, sodium selectivity, specificity for FMRFamide and Phe-Leu-Arg-Phe-amide (FLRFamide), and no dependency on G-protein coupling. The sodium current also exhibited rapid desensitization in response to repeated FMRFamide applications. Lowering of the pH of the bathing solution reduced the amplitude of the FMRFamide-gated inward current, while also activating an additional sustained weak inward current that was apparently not mediated by the FaNaC. Acidification also prevented the desensitization of the FMRFamide-induced inward current. The acid sensitivity of LsFaNaC is consistent with the hypothesis that FaNaCs share a common ancestry with the ASICs.

Neuronal expression of an FMRFamide-gated Na+ channel and its modulation by acid pH

The molluscan Phe-Met-Arg-Phe-amide (FMRFamide)-gated sodium channels (FaNaCs) show both structural and functional similarities to the mammalian acid-sensing ion channels (ASICs). Both channel types are related to the epithelial sodium channels and, although the neuropeptide FMRFamide directly gates the FaNaCs, it also modulates the proton-gating properties of ASICs. It is not yet known whether protons can alter the gating properties of the FaNaCs. We chose to examine this possibility at a site of FaNaC expression in the nervous system of the mollusk Lymnaea stagnalis. We cloned a putative L. stagnalis FaNaC (LsFaNaC) that exhibited a high degree of sequence identity to the Helix aspersa FaNaC (HaFaNaC, 60%), and a weaker homology to the ASICs (ASIC3, 22%). In situ hybridization was used to map the LsFaNaC expression pattern in the brain and to identify the right pedal giant1 (RPeD1) neuron as a site where the properties of the endogenous channel could be studied. In RPeD1 neurons isolated in culture, we demonstrated the presence of an FMRFamide-gated sodium current with features expected for a FaNaC: amiloride sensitivity, sodium selectivity, specificity for FMRFamide and Phe-Leu-Arg-Phe-amide (FLRFamide), and no dependency on G-protein coupling. The sodium current also exhibited rapid desensitization in response to repeated FMRFamide applications. Lowering of the pH of the bathing solution reduced the amplitude of the FMRFamide-gated inward current, while also activating an additional sustained weak inward current that was apparently not mediated by the FaNaC. Acidification also prevented the desensitization of the FMRFamide-induced inward current. The acid sensitivity of LsFaNaC is consistent with the hypothesis that FaNaCs share a common ancestry with the ASICs.

Location of a ligand recognition site of FMRFamide-gated Na(+) channels

The second FMRFamide-gated Na(+) channel (HtFaNaC), from Helisoma trivolvis, has been cloned. HtFaNaC has some different pharmacological properties to HaFaNaC, from Helix aspersa, which has enabled a rational approach to be made to start to identify the FMRFamide recognition site. Several chimeras were made by switching sections between the channels. The differences in sensitivity to FMRFamide, and amiloride, were assessed after expression in Xenopus oocytes. The data suggest that a recognition site for FMRFamide, and the potentiating action of amiloride, resides in a sequence of about 120 amino acids in the extracellular loop proximal to the first transmembrane segment.