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

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.

Structural basis for excitatory neuropeptide signaling

Rapid signaling between neurons is mediated by ligand-gated ion channels, cell-surface proteins with an extracellular ligand-binding domain and a membrane-spanning ion channel domain. The degenerin/epithelial sodium channel (DEG/ENaC) superfamily is diverse in terms of its gating stimuli, with some DEG/ENaCs gated by neuropeptides, and others gated by pH, mechanical force or enzymatic activity. The mechanism by which ligands bind to and activate DEG/ENaCs is poorly understood. Here we dissected the structural basis for neuropeptide-gated activity of a neuropeptide-gated DEG/ENaC, FMRFamide-gated sodium channel 1 (FaNaC1) from the annelid worm Malacoceros fuliginosus, using cryo-electron microscopy. Structures of FaNaC1 in the ligand-free resting state and in several ligand-bound states reveal the ligand-binding site and capture the ligand-induced conformational changes of channel gating, which we verified with complementary mutagenesis experiments. Our results illuminate channel gating in DEG/ENaCs and offer a structural template for experimental dissection of channel pharmacology and ion conduction.

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.

Review: The evolution of peptidergic signaling in Cnidaria and Placozoa, including a comparison with Bilateria

Bilateria have bilateral symmetry and are subdivided into Deuterostomia (animals like vertebrates) and Protostomia (animals like insects and mollusks). Neuropeptides occur in both Proto- and Deuterostomia and they are frequently structurally related across these two lineages. For example, peptides belonging to the oxytocin/vasopressin family exist in both clades. The same is true for the G protein-coupled receptors (GPCRs) of these peptides. These observations suggest that these neuropeptides and their GPCRs were already present in the common ancestor of Proto- and Deuterostomia, which lived about 700 million years ago (MYA). Furthermore, neuropeptides and their GPCRs occur in two early-branching phyla that diverged before the emergence of Bilateria: Cnidaria (animals like corals and sea anemones), and Placozoa (small disk-like animals, feeding on algae). The sequences of these neuropeptides and their GPCRs, however, are not closely related to those from Bilateria. In addition, cnidarian neuropeptides and their receptors are not closely related to those from Placozoa. We propose that the divergence times between Cnidaria, Placozoa, and Bilateria might be too long for recognizing sequence identities. Leucine-rich repeats-containing GPCRs (LGRs) are a special class of GPCRs that are characterized by a long N-terminus containing 10-20 leucine-rich domains, which are used for ligand binding. Among the ligands for LGRs are dimeric glycoprotein hormones, and insulin-like peptides, such as relaxin. LGRs have been found not only in Proto- and Deuterostomia, but also in early emerging phyla, such as Cnidaria and Placozoa. Humans have eight LGRs. In our current review, we have revisited the annotations of LGRs from the sea anemone Nematostella vectensis and the placozoan Trichoplax adhaerens. We identified 13 sea anemone LGRs and no less than 46 LGRs from T. adhaerens. All eight human LGRs appear to have orthologues in sea anemones and placozoans. LGRs and their ligands, therefore, have a long evolutionary history, going back to the common ancestor of Cnidaria and Placozoa.

Comparative analysis defines a broader FMRFamide-gated sodium channel family and determinants of neuropeptide sensitivity

FMRFamide (Phe-Met-Arg-Phe-amide, FMRFa) and similar neuropeptides are important physiological modulators in most invertebrates, but the molecular basis of FMRFa activity at its receptors is unknown. We therefore sought to identify the molecular determinants of FMRFa potency against one of its native targets, the excitatory FMRFa-gated sodium channel (FaNaC) from gastropod mollusks. Using molecular phylogenetics and electrophysiological measurement of neuropeptide activity, we identified a broad FaNaC family that includes mollusk and annelid channels gated by FMRFa, FVRIamides, and/or Wamides (or myoinhibitory peptides). A comparative analysis of this broader FaNaC family and other channels from the overarching degenerin (DEG)/epithelial sodium channel (ENaC) superfamily, incorporating mutagenesis and experimental dissection of channel function, identified a pocket of amino acid residues that determines activation of FaNaCs by neuropeptides. Although this pocket has diverged in distantly related DEG/ENaC channels that are activated by other ligands but enhanced by FMRFa, such as mammalian acid-sensing ion channels, we show that it nonetheless contains residues that determine enhancement of those channels by similar peptides. This study thus identifies amino acid residues that determine FMRFa neuropeptide activity at FaNaC receptor channels and illuminates the evolution of ligand recognition in one branch of the DEG/ENaC superfamily of ion channels.

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.

Peripheral sensory neurons govern development of the nervous system in bivalve larvae

Recent findings regarding early lophotrochozoan development have altered the conventional model of neurogenesis and revealed that peripheral sensory elements play a key role in the initial organization of the larval nervous system. Here, we describe the main neurogenetic events in bivalve mollusks in comparison with other Lophotrochozoa, emphasizing a novel role for early neurons in establishing larval nervous systems and speculating about the morphogenetic function of the apical organ. We demonstrate that during bivalve development, peripheral sensory neurons utilizing various transmitters differentiate before the apical organ emerges. The first neurons and their neurites serve as a scaffold for the development of the nervous system. During veliger stage, cerebral, pleural, and visceral ganglia form along the lateral (visceral) nerve cords in anterior-to-posterior axis. The pedal ganglia and corresponding ventral (pedal) nerve cords develop much later, after larval settlement and metamorphosis. Pharmacological abolishment of the serotonin gradient within the larval body disrupts the navigation of "pioneer" axons resulting in malformation of the whole nervous system architecture. Comparative morphological data on neurogenetic events in bivalve mollusks shed new light on the origin of the nervous system, mechanisms of early axon navigation, and sequence of the tetraneurous nervous system formation. Furthermore, this information improves our understanding of the basic nervous system architecture in larval Bivalvia and Mollusca.

An ancient FMRFamide-related peptide-receptor pair induces defence behaviour in a brachiopod larva

Animal behaviour often comprises spatially separated sub-reactions and even ciliated larvae are able to coordinate sub-reactions of complex behaviours (metamorphosis, feeding). How these sub-reactions are coordinated is currently not well understood. Neuropeptides are potential candidates for triggering larval behaviour. However, although their immunoreactivity has been widely analysed, their function in trochozoan larvae has only been studied for a few cases. Here, we investigate the role of neuropeptides in the defence behaviour of brachiopod larvae. When mechanically disturbed, the planktonic larvae of Terebratalia transversa protrude their stiff chaetae and sink down slowly. We identified endogenous FLRFamide-type neuropeptides (AFLRFamide and DFLRFamide) in T. transversa larvae and show that the protrusion of the chaetae as well as the sinking reaction can both be induced by each of these peptides. This also correlates with the presence of FLRFamidergic neurons in the apical lobe and adjacent to the trunk musculature. We deorphanized the AFLRFamide/DFLRFamide receptor and detected its expression in the same tissues. Furthermore, the ability of native and modified FLRFamide-type peptides to activate this receptor was found to correspond with their ability to trigger behavioural responses. Our results show how FLRFamide-type neuropeptides can induce two coherent sub-reactions in a larva with a simple nervous system.

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.

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.

ASICs and neuropeptides

The acid sensing ion channels (ASICs) are proton-gated cation channels expressed throughout the nervous system. ASICs are activated during acidic pH fluctuations, and recent work suggests that they are involved in excitatory synaptic transmission. ASICs can also induce neuronal degeneration and death during pathological extracellular acidosis caused by ischemia, autoimmune inflammation, and traumatic injury. Many endogenous neuromodulators target ASICs to affect their biophysical characteristics and contributions to neuronal activity. One of the most unconventional types of modulation occurs with the interaction of ASICs and neuropeptides. Collectively, FMRFamide-related peptides and dynorphins potentiate ASIC activity by decreasing the proton-sensitivity of steady state desensitization independent of G protein-coupled receptor activation. By decreasing the proton-sensitivity of steady state desensitization, the FMRFamide-related peptides and dynorphins permit ASICs to remain active at more acidic basal pH. Unlike the dynorphins, some FMRFamide-related peptides also potentiate ASIC activity by slowing inactivation and increasing the sustained current. Through mechanistic studies, the modulation of ASICs by FMRFamide-related peptides and dynorphins appears to be through distinct interactions with the extracellular domain of ASICs. Dynorphins are expressed throughout the nervous system and can increase neuronal death during prolonged extracellular acidosis, suggesting that the interaction between dynorphins and ASICs may have important consequences for the prevention of neurological injury. The overlap in expression of FMRFamide-related peptides with ASICs in the dorsal horn of the spinal cord suggests that their interaction may have important consequences for the treatment of pain during injury and inflammation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

The comprehensive analysis of DEG/ENaC subunits in Hydra reveals a large variety of peptide-gated channels, potentially involved in neuromuscular transmission

Background

It is generally the case that fast transmission at neural synapses is mediated by small molecule neurotransmitters. The simple nervous system of the cnidarian Hydra, however, contains a large repertoire of neuropeptides and it has been suggested that neuropeptides are the principal transmitters of Hydra. An ion channel directly gated by Hydra-RFamide neuropeptides has indeed been identified in Hydra – the Hydra Na⁺ channel (HyNaC) 2/3/5, which is expressed at the oral side of the tentacle base. Hydra-RFamides are more widely expressed, however, being found in neurons of the head and peduncle region. Here, we explore whether further peptide-gated HyNaCs exist, where in the animal they are expressed, and whether they are all gated by Hydra-RFamides.

Results

We report molecular cloning of seven new HyNaC subunits – HyNaC6 to HyNaC12, all of which are members of the DEG/ENaC gene family. In Xenopus oocytes, these subunits assemble together with the four already known subunits into thirteen different ion channels that are directly gated by Hydra-RFamide neuropeptides with high affinity (up to 40 nM). In situ hybridization suggests that HyNaCs are expressed in epitheliomuscular cells at the oral and the aboral side of the tentacle base and at the peduncle. Moreover, diminazene, an inhibitor of HyNaCs, delayed tentacle movement in live Hydra.

Conclusions

Our results show that Hydra has a large variety of peptide-gated ion channels that are activated by a restricted number of related neuropeptides. The existence and expression pattern of these channels, and behavioral effects induced by channel blockers, suggests that Hydra co-opted neuropeptides for fast neuromuscular transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0084-2) contains supplementary material, which is available to authorized users.

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.

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.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

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.

NMR analysis of Caenorhabditis elegans FLP-18 neuropeptides: implications for NPR-1 activation

Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides (FLPs) are the largest neuropeptide family in animals, particularly invertebrates. FLPs are characterized by a C-N-terminal gradient of decreasing amino acid conservation. Neuropeptide receptor 1 (NPR-1) is a G-protein coupled receptor (GPCR), which has been shown to be a strong regulator of foraging behavior and aggregation responses in Caenorhabditis elegans. Recently, ligands for NPR-1 were identified as neuropeptides coded by the precursor genes flp-18 and flp-21 in C. elegans. The flp-18 gene encodes eight FLPs including DFDGAMPGVLRF-NH2 and EMPGVLRF-NH2. These peptides exhibit considerably different activities on NPR-1, with the longer one showing a lower potency. We have used nuclear magnetic resonance and biological activity to investigate structural features that may explain these activity differences. Our data demonstrate that long-range electrostatic interactions exist between N-terminal aspartates and the C-terminal penultimate arginine as well as N-terminal hydrogen-bonding interactions that form transient loops within DFDGAMPGVLRF-NH2. We hypothesize that these loops, along with peptide charge, diminish the activity of this peptide on NPR-1 relative to that of EMPGVLRF-NH2. These results provide some insight into the large amino acid diversity in FLPs.

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.

Phe-met-arg-phe (FMRF)-amide is a substrate source of NO synthase in the gastropod nervous system

The possible involvement of the L-arginine-containing Phe-met-arg-phe (FMRF)-amide (FMRFa) in neuronal nitric oxide (NO) biosynthesis was studied in a gastropod species. We found NADPH-diaphorase-positive neurons and FMRFa-containing fibers in close proximity in the enteric nervous system. Administration of L-arginine and FMRFa induced quantitatively similar nitrite production in both intact intestinal tissues and tissue homogenates. These changes could be prevented by the presence of NOARG (an NO synthase inhibitor). Neither chemically modified FMRFa (D-arginine instead of L-arginine) nor amino acid constituents of FMRFa (methionine, phenylalanine) affected basal nitrite production. FMRFa-induced alterations were reduced in the presence of Na+ channel blockers (tetrodotoxin, amiloride, lidocaine), the Na+/K+ATPase inhibitor ouabain, or protease inhibitors (leupeptine, pepstatine-a). FMRFa and its amino acid constituents were analyzed by paper chromatography. When FMRFa was added to tissue homogenates, the peptide was eliminated within 1-2 min, whereas methionine, phenylalanine, arginine, and citrulline levels were elevated simultaneously. We tested the effects of FMRFa, L-arginine, and NOARG on intestinal contractile activity. FMRFa relaxed the intestine for 1-2 min and then induced contractions for 20-40 min. In the presence of NOARG, no relaxant effect of FMRFa was recorded. As administration of L-arginine strongly inhibits the mechanical activity of the intestinal muscle, NO production presumably plays a substantial role in the action of FMRFa, at least in the initial phase. Our biochemical data indicate a direct involvement of FMRFa in NO biosynthesis. FMRFa might be hydrolyzed by extracellular peptidases and then the locally released arginine might be transported into the cells and broken-down to produce NO. Depolarization-induced NO production attributable to the activation of amiloride-sensitive Na+ channels might also be involved.

The receptor site of the spider toxin PcTx1 on the proton-gated cation channel ASIC1a

Acid-sensing ion channels (ASICs) are excitatory neuronal cation channels, involved in physiopathological processes related to extracellular pH fluctuation such as nociception, ischaemia, perception of sour taste and synaptic transmission. The spider peptide toxin psalmotoxin 1 (PcTx1) has previously been shown to inhibit specifically the proton-gated cation channel ASIC1a. To identify the binding site of PcTx1, we produced an iodinated form of the toxin ((125)I-PcTx1Y(N)) and developed a set of binding and electrophysiological experiments on several chimeras of ASIC1a and the PcTx1-insensitive channels ASIC1b and ASIC2a. We show that (125)I-PcTx1Y(N) binds specifically to ASIC1a at a single site, with an IC(50) of 128 pM, distinct from the amiloride blocking site. Results obtained from chimeras indicate that PcTx1 does not bind to ASIC1a transmembrane domains (M1 and M2), involved in formation of the ion pore, but binds principally on both cysteine-rich domains I and II (CRDI and CRDII) of the extracellular loop. The post-M1 and pre-M2 regions, although not involved in the binding site, are crucial for the ability of PcTx1 to inhibit ASIC1a current. The linker domain between CRDI and CRDII is important for their correct spatial positioning to form the PcTx1 binding site. These results will be useful for the future identification or design of new molecules acting on 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.

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.

The effects of crustacean cardioactive peptide on locust oviducts are calcium-dependent

The role of calcium as a second messenger in the crustacean cardioactive peptide (CCAP)-induced contractions of the locust oviducts was investigated. Incubation of the oviducts in a calcium-free saline containing, a preferential calcium cation chelator, or an extracellular calcium channel blocker, abolished CCAP-induced contractions, indicating that the effects of CCAP on the oviducts are calcium-dependent. In contrast, sodium free saline did not affect CCAP-induced contractions. Co-application of CCAP to the oviducts with preferential L-type voltage-dependent calcium channel blockers reduced CCAP-induced contractions by 32-54%. Two preferential T-type voltage-dependent calcium channel blockers both inhibited CCAP-induced oviduct contractions although affecting different components of the contractions. Amiloride decreased the tonic component of CCAP-induced contractions by 40-55% and flunarizine dihydrochloride decreased the frequency of CCAP-induced phasic contractions by as much as 65%, without affecting tonus. Flunarizine dihydrochloride did not alter the proctolin-induced contractions of the oviducts. Results suggest that the actions of CCAP are partially mediated by voltage-dependent calcium channels similar to vertebrate L-type and T-type channels. High-potassium saline does not abolish CCAP-induced contractions indicating the presence of receptor-operated calcium channels that mediate the actions of CCAP on the oviducts. The involvement of calcium from intracellular stores in CCAP-induced contractions of the oviducts is likely since, an intracellular calcium antagonist decreased CCAP-induced contractions by 30-35%.