Structure-activity relationships of KNEFIRFamide (AF1), a nematode FMRFamide-related peptide, on Ascaris suum muscle

FMRFamide-Like Peptide 22 Influences the Head Movement, Host Finding, and Infection of Heterodera glycines

The FMRFamide-like peptides (FLPs) represent the largest family of nematode neuropeptides and are involved in multiple parasitic activities. The immunoreactivity to FMRFamide within the nervous system of Heterodera glycines, the most economically damaging parasite of soybean [Glycine max L. (Merr)], has been reported in previous research. However, the family of genes encoding FLPs of H. glycines were not identified and functionally characterized. In this study, an FLP encoding gene Hg-flp-22 was cloned from H. glycines, and its functional characterization was uncovered by using in vitro RNA interference and application of synthetic peptides. Bioinformatics analysis showed that flp-22 is widely expressed in multiple nematode species, where they encode the highly conserved KWMRFamide motifs. Quantitative real-time (qRT)-PCR results revealed that Hg-flp-22 was highly expressed in the infective second-stage juveniles (J2s) and adult males. Silencing of Hg-flp-22 resulted in the reduced movement of J2s to the host root and reduced penetration ability, as well as a reduction in their subsequent number of females. Behavior and infection assays demonstrated that application of synthetic peptides Hg-FLP-22b (TPQGKWMRFa) and Hg-FLP-22c (KMAIEGGKWVRFa) significantly increased the head movement frequency and host invasion abilities in H. glycines but not in Meloidogyne incognita. In addition, the number of H. glycines females on the host roots was found to be significantly higher in Hg-FLP-22b treated nematodes than the ddH₂O-treated control J2s. These results presented in this study elucidated that Hg-flp-22 plays a role in regulating locomotion and infection of H. glycines. This suggests the potential of FLP signaling as putative control targets for H. glycines in soybean production.

The FMRFamide-Like Peptide Family in Nematodes

In the three decades since the FMRFamide peptide was isolated from the mollusk Macrocallista nimbosa, structurally similar peptides sharing a C-terminal RFamide motif have been identified across the animal kingdom. FMRFamide-like peptides (FLPs) represent the largest known family of neuropeptides in invertebrates. In the phylum Nematoda, at least 32 flp-genes are classified, making the FLP system of nematodes unusually complex. The diversity of the nematode FLP complement is most extensively mapped in Caenorhabditis elegans, where over 70 FLPs have been predicted. FLPs have shown to be expressed in the majority of the 302 C. elegans neurons including interneurons, sensory neurons, and motor neurons. The vast expression of FLPs is reflected in the broad functional repertoire of nematode FLP signaling, including neuroendocrine and neuromodulatory effects on locomotory activity, reproduction, feeding, and behavior. In contrast to the many identified nematode FLPs, only few peptides have been assigned a receptor and there is the need to clarify the pathway components and working mechanisms of the FLP signaling network. Here, we review the diversity, distribution, and functions of FLPs in nematodes.

Family of FLP Peptides in Caenorhabditis elegans and Related Nematodes

Neuropeptides regulate all aspects of behavior in multicellular organisms. Because of their ability to act at long distances, neuropeptides can exert their effects beyond the conventional synaptic connections, thereby adding an intricate layer of complexity to the activity of neural networks. In the nematode Caenorhabditis elegans, a large number of neuropeptide genes that are expressed throughout the nervous system have been identified. The actions of these peptides supplement the synaptic connections of the 302 neurons, allowing for fine tuning of neural networks and increasing the ways in which behaviors can be regulated. In this review, we focus on a large family of genes encoding FMRFamide-related peptides (FaRPs). These genes, the flp genes, have been used as a starting point to identifying flp genes throughout Nematoda. Nematodes have the largest family of FaRPs described thus far. The challenges in the future are the elucidation of their functions and the identification of the receptors and signaling pathways through which they function.

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.

Changes in cyclic nucleotides, locomotory behavior, and body length produced by novel endogenous neuropeptides in the parasitic nematode Ascaris suum

Recent technical advances have rapidly advanced the discovery of novel peptides, as well as the transcripts that encode them, in the parasitic nematode Ascaris suum. Here we report that many of these novel peptides produce profound and varied effects on locomotory behavior and levels of cyclic nucleotides in A. suum. We investigated the effects of 31 endogenous neuropeptides encoded by transcripts afp-1, afp-2, afp-4, afp-6, afp-7, and afp-9-14 (afp: Ascaris FMRFamide-like Precursor protein) on cyclic nucleotide levels, body length and locomotory behavior. Worms were induced to generate anteriorly propagating waveforms, peptides were injected into the pseudocoelomic cavity, and changes in the specific activity (nmol/mg protein) of second messengers cAMP (3'5' cyclic adenosine monophosphate) and cGMP (3'5' cyclic guanosine monophosphate) were determined. Many of these neuropeptides changed the levels of cAMP (both increases and decreases were found), whereas few neuropeptides changed the level of cGMP. A subset of the peptides that lowered cAMP was investigated for effects on the locomotory waveform and on body length. Injection of AF19, or AF34 (afp-13), AF9 (afp-14), AF26 or AF41 (afp-11) caused immediate paralysis and cessation of propagating body waveforms. These neuropeptides also significantly increased body length. In contrast, injection of AF15 (afp-9) reduced the body length, and decreased the amplitude of waves in the body waveform. AF30 (afp-10) produced worms with tight ventral coils. Although injection of neuropeptides encoded by afp-1 (AF3, AF4, AF10 or AF13) produced an increased number of exaggerated body waves, there were no effects on either cAMP or cGMP. By injecting peptides into behaving A. suum, we have provided an initial screen of the effects of novel peptides on several behavioral and biochemical parameters.

The RFamide neuropeptide 26RFa and its role in the control of neuroendocrine functions

Identification of novel neuropeptides and their cognate G protein-coupled receptors is essential for a better understanding of neuroendocrine regulations. The RFamide peptides represent a family of regulatory peptides that all possess the Arg-Phe-NH2 motif at their C-terminus. In mammals, seven RFamide peptides encoded by five distinct genes have been characterized. The present review focuses on 26RFa (or QRFP) which is the latest member identified in this family. 26RFa is present in all vertebrate phyla and its C-terminal domain (KGGFXFRF-NH2), which is responsible for its biological activity, has been fully conserved during evolution. 26RFa is the cognate ligand of the orphan G protein-coupled receptor GPR103 that is also present from fish to human. In all vertebrate species studied so far, 26RFa-expressing neurons show a discrete localization in the hypothalamus, suggesting important neuroendocrine activities for this RFamide peptide. Indeed, 26RFa plays a crucial role in the control of feeding behavior in mammals, birds and fish. In addition, 26RFa up-regulates the gonadotropic axis in mammals and fish. Finally, evidence that the 26RFa/GPR103 system regulates steroidogenesis, bone formation, nociceptive transmission and arterial blood pressure has also been reported. Thus, 26RFa appears to act as a key neuropeptide in vertebrates controlling vital neuroendocrine functions. The pathophysiological implication of the 26RFa/GPR103 system in human is totally unknown and some fields of investigation are proposed.

Discovery of neuropeptides in the nematode Ascaris suum by database mining and tandem mass spectrometry

Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) was used to discover peptides in extracts of the large parasitic nematode Ascaris suum. This required the assembly of a new database of known and predicted peptides. In addition to those already sequenced, peptides were either previously predicted to be processed from precursor proteins identified in an A. suum library of expressed sequence tags (ESTs) or newly predicted from a library of A. suum genome survey sequences (GSSs). The predicted MS/MS fragmentation patterns of this collection of real and putative peptides were compared with the actual fragmentation patterns found in the MS/MS spectra of peptides fractionated by MS; this enabled individual peptides to be sequenced. Many previously identified peptides were found, and 21 novel peptides were discovered. Thus, this approach is very useful, despite the fact that the available GSS database is still preliminary, having only 1× coverage.

Neuropeptide physiology in helminths

Parasitic worms come from two distinct, distant phyla, Nematoda (roundworms) and Platyhelminthes (flatworms). The nervous systems of worms from both phyla are replete with neuropeptides and there is ample physiological evidence that these neuropeptides control vital aspects of worm biology. In each phyla, the physiological evidence for critical roles for helminth neuropeptides is derived from both parasitic and free-living members. In the nematodes, the intestinal parasite Ascaris suum and the free-living Caenorhabditis elegans have yielded most of the data; in the platyhelminths, the most physiological data has come from the blood fluke Schistosoma mansoni. FMRFamide-like peptides (FLPs) have many varied effects (excitation, relaxation, or a combination) on somatic musculature, reproductive musculature, the pharynx and motor neurons in nematodes. Insulin-like peptides (INSs) play an essential role in nematode dauer formation and other developmental processes. There is also some evidence for a role in somatic muscle control for the somewhat heterogeneous grouping ofpeptides known as neuropeptide-like proteins (NLPs). In platyhelminths, as in nematodes, FLPs have a central role in somatic muscle function. Reports of FLP physiological action in platyhelminths are limited to a potent excitation of the somatic musculature. Platyhelminths are also abundantly endowed with neuropeptide Fs (NPFs), which appear absent from nematodes. There is not yet any data linking platyhelminth NPF to any particular physiological outcome, but this neuropeptide does potently and specifically inhibit cAMP accumulation in schistosomes. In nematodes and platyhelminths, there is an abundance of physiological evidence demonstrating that neuropeptides play critical roles in the biology of both free-living and parasitic helminths. While it is certainly true that there remains a great deal to learn about the biology of neuropeptides in both phyla, physiological evidence presently available points to neuropeptidergic signaling as a very promising field from which to harvest future drug targets.

Neuropeptide gene families in Caenorhabditis elegans

Neuropeptides are short sequences ofamino acids that function in all multicellular organisms to communicate information between cells. The first sequence ofa neuropeptide was reported in 1970' and the number of identified neuropeptides remained relatively small until the 1990s when the DNA sequence of multiple genomes revealed treasure troves ofinformation. Byblasting away at the genome, gene families, the sizes ofwhich were previously unknown, could now be determined. This information has led to an exponential increase in the number of putative neuropeptides and their respective gene families. The molecular biology age greatly benefited the neuropeptide field in the nematode Caenorhabditis elegans. Its genome was among the first to be sequenced and this allowed us the opportunity to screen the genome for neuropeptide genes. Initially, the screeningwas slow, as the Genefinder and BLAST programs had difficulty identifying small genes and peptides. However, as the bioinformatics programs improved, the extent of the neuropeptide gene families in C. elegans gradually emerged.

Neuropeptides

The role of neuropeptides in modulating behavior is slowly being elucidated. With the sequencing of the C. elegans genome, the extent of the neuropeptide genes in C. elegans can be determined. To date, 113 neuropeptide genes encoding over 250 distinct neuropeptides have been identified. Of these, 40 genes encode insulin-like peptides, 31 genes encode FMRFamide-related peptides, and 42 genes encode non-insulin, non-FMRFamide-related neuropeptides. As in other systems, C. elegans neuropeptides are derived from precursor molecules that must be post-translationally processed to yield the active peptides. These precursor molecules contain a single peptide, multiple copies of a single peptide, multiple distinct peptides, or any combination thereof. The neuropeptide genes are expressed extensively throughout the nervous system, including in sensory, motor, and interneurons. In addition, some of the genes are also expressed in non-neuronal tissues, such as the somatic gonad, intestine, and vulval hypodermis. To address the effects of neuropeptides on C. elegans behavior, animals in which the different neuropeptide genes are inactivated or overexpressed are being isolated. In a complementary approach the receptors to which the neuropeptides bind are also being identified and examined. Among the knockout animals analyzed thus far, defects in locomotion, dauer formation, egg laying, ethanol response, and social behavior have been reported. These data suggest that neuropeptides have a modulatory role in many, if not all, behaviors in C. elegans.

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 related peptide ligands activate the Caenorhabditis elegans orphan GPCR Y59H11AL.1

G-protein coupled receptors (GPCRs) are ancient molecules that can sense environmental and physiological signals. Currently, the majority of the predicted Caenorhabditis elegans GPCRs are orphan. Here, we describe the characterization of such an orphan C. elegans GPCR, which is categorized in the tachykinin-like group of receptors. Since the C. elegans genome predicts only one tachykinin-like peptide (SFDRMGGTEFGLM), which could not activate the receptor, we hypothesized that one or some of the numerous FMRFamide related peptides (FaRPs) could be the cognate ligands for this receptor. This hypothesis was based on the suggestion that RFamides may be ancestral neuropeptides, from which a lot of the amidated neuropeptides, including tachykinins, derived. Indeed, we found that the orphan receptor encoded by the Y59H11AL.1 gene is activated by several C. elegans neuropeptides, including SPMERSAMVRFamide. These peptides activate the receptor in a concentration-dependent way.

Structure and functions of the novel hypothalamic RFamide neuropeptides R-RFa and 26RFa in vertebrates

A number of RFamide peptides have been characterized in invertebrate species and these peptides have been found to exert a broad spectrum of biological activities. In contrast, in vertebrates, our knowledge on RFamide peptides is far more limited and only a few members of the RFamide peptide family have been identified in various vertebrate classes during the last years. The present review focuses on two novel RFamide peptides, Rana RFamide (R-RFa) and 26RFa, that have been recently isolated from the amphibian brain. R-RFa shares the C-terminal LPLRFamide motif with other RFamide peptides previously identified in mammals, birds and fish. The distribution of R-RFa in the frog brain exhibits strong similarities with those of other LPLRFamide peptides, notably in the periventricular region of the hypothalamus. There is also evidence that the physiological functions of R-RFa and other LPLRFamide peptides have been conserved from fish to mammals; in particular, all these peptides appear to be involved in the control of pituitary hormone secretion. 26RFa does not exhibit any significant structural identity with other RFamide peptides and this peptide is the only member of the family that possesses an FRFamide motif at its C-terminus. The strong conservation of the primary structure of 26RFa from amphibians to mammals suggests that this RFamide peptide is involved in important biological functions in vertebrates. As for several other RFamide peptides, 26RFa-containing neurons are present in the hypothalamus, notably in two nuclei involved in the control of feeding behavior. Indeed, 26RFa is a potent stimulator of appetite in mammals. Concurrently, recent data suggest that 26RFa exerts various neuroendocrine regulatory activities at the pituitary and adrenal level.

Neuromuscular function in plant parasitic nematodes: a target for novel control strategies?

Over the last decade the need for new strategies and compounds to control parasitic helminths has become increasingly urgent. The neuromuscular systems of these worms have been espoused as potential sources of target molecules for new drugs which may address this need. One facet of helminth neuromuscular biology which has garnered considerable research interest is that of neuropeptidergic neurotransmission, particularly regarding parasites of humans and animals, as well as free-living nematode model species. This research interest has been piqued by the fact that neuropeptides have been demonstrated to be fundamentally important to nematode biology and thus may be of utility in this search for new drug targets. This review focuses on the neuropeptide biology of plant parasitic nematodes, a subject which has been comparatively neglected despite the fact that the search for alternative control measures also extends to these economically important parasites. We focus on the FMRFamide-like peptide (FLP) neuropeptides and the complexity and distribution of this peptide family in plant parasitic nematodes. Possible roles for FLPs in plant parasitic nematode behaviour, as elucidated by a combination of molecular imaging techniques and RNA interference (RNAi), are discussed. We propose that disruption of FLP neurosignalling in plant parasitic nematodes represents a novel form of pest control and speculate as to how this may be achieved.

The ever-expanding neuropeptide gene families in the nematode Caenorhabditis elegans

Neuropeptides act as chemical signals in the nervous system to modulate behaviour. With the ongoing EST projects and DNA sequence determination of different genomes, the identification of neuropeptide genes has been made easier. Despite the relatively 'simple' repertoire of behaviours in the nematode Caenorhabditis elegans, this worm contains a surprisingly large and diverse set of neuropeptide genes. At least 109 genes encoding over 250 potential neuropeptides have been identified in C. elegans; all genes are likely to be expressed and many, if not all, of the predicted peptides are produced. The predicted peptides include: 38 insulin-like peptides, several of which are involved in development and reproductive growth, and over 70 FMRFamide-related peptides, some of which are involved in locomotion, reproduction, and social behaviour. Many of the C. elegans peptides are identical or highly similar to those isolated or predicted in parasitic nematodes, such as Ascaris suum, Haemonchus contortus, Ancylostoma caninum, Heterodera glycines and Meloidogyne arenaria, suggesting that the function of these peptides is similar across species. The challenge for the future is to determine the function of all the genes and individual peptides and to identify the receptors through which the peptides signal.

Role of a FMRFamide-like family of neuropeptides in the pharyngeal nervous system of Caenorhabditis elegans

The nervous system of C. elegans has a remarkable abundance of flp genes encoding FMRFamide-like (FLP) neuropeptides. To provide insight into the physiological relevance of this neuropeptide diversity, we have tested more than 30 FLPs (encoded by 23 flps) for bioactivity on C. elegans pharynx. Eleven flp genes encode peptides that inhibit pharyngeal activity, while eight flp genes encode peptides that are excitatory. Three potent peptides (inhibitory, FLP-13A, APEASPFIRFamide; excitatory, FLP-17A, KSAFVRFamide; excitatory, FLP-17B, KSQYIRFamide) are encoded by flp genes, which, according to reporter gene constructs, are expressed in pharyngeal motoneurons. Thus, they may act through receptors localized on the pharyngeal muscle. The two other potent peptides, FLP-8 (excitatory AF1, KNEFIRFamide,) and FLP-11A (inhibitory, AMRNALVRFamide), appear to be expressed in extrapharyngeal neurons and are therefore likely to act either indirectly or as neurohormones. Intriguingly, a single neuron can express peptides that have potent but opposing biological activity in the pharynx. Only five flp genes encode neuropeptides that have no observable effect on the pharynx, but none of these have shown reporter gene expression in the pharyngeal nervous system. To examine the roles of multiple peptides produced from single precursors, a comparison was made between the bioactivity of different neuropeptides for five flp genes (flp-3, flp-13, flp-14, flp-17, and flp-18). For all but one gene (flp-14), the effects of peptides encoded by the same gene were similar. Overall, this study demonstrates the impressive neurochemical complexity of the simple circuit that regulates feeding in the nematode, C. elegans.

Molecular characterization of two G protein-coupled receptor splice variants as FLP2 receptors in Caenorhabditis elegans

Two alternatively spliced Caenorhabditis elegans G protein-coupled receptors, T19F4.1a and T19F4.1b, were cloned and functionally characterized. The T19F4.1b receptor protein is 30 amino acids longer than T19F4.1a, and the difference in amino acid constitution is exclusively conferred to the intracellular C-terminal region, suggesting a potential difference in G protein-coupling specificity. Following cloning of the receptor cDNAs into the pcDNA3 vector and stable or transient transfection into Chinese hamster ovary cells, the aequorin bioluminescence/Ca2+ assay was used to investigate receptor activation. This is the first report of the construction of a cell line stably expressing a C. elegans neuropeptide receptor. Our experiments identified both receptors as being cognate receptors for two FMRFamide-related peptides encoded by the flp-2 precursor: SPREPIRFamide (FLP2-A) and LRGEPIRFamide (FLP2-B). Pharmacological profiling using truncated forms of FLP2-A and -B revealed that the active core of both peptides is EPIRFamide. Screening of peptides encoded by other flps did not result in a significant activation of the receptor. In contrast to other C. elegans receptors tested in heterologous expression systems, the functional activation of both T19F4.1a and T19F4.1b was not temperature-dependent. Screening in cells lacking the promiscuous Galpha16 suggests that T19F4.1a and b are both linked to the Gq pathway.

Brief application of AF2 produces long lasting potentiation of nAChR responses in Ascaris suum

Resistance of parasitic nematodes to the cholinergic anthelmintic levamisole is associated with a reduction in the proportion of time that acetylcholine receptor ion-channels are in the open state decreasing the response of nematode parasites to the drug. Here we examine electrophysiological and contractile responses to acetylcholine and the cholinergic agonist, levamisole, in Ascaris suum muscle looking for a pharmacological approach that may be developed to increase the response to cholinergic agonists. We found that short application of the FMRFamide, AF2, produced modulation (long lasting potentiation) of the peak membrane potential response to acetylcholine but not to levamisole. Since levamisole preferentially activates L-type acetylcholine receptors, we also tested the effect of nicotine (selective activator of N-type acetylcholine receptors) and bephenium (selective activator of B-type acetylcholine receptors) and found again no effect of AF2 on peak membrane potential responses. We then tested atropine on the AF2 potentiation of acetylcholine and found it to inhibit the peak potentiation suggesting that AF2 receptors interact with muscarinic receptors to produce the potentiation of acetylcholine. We saw similar atropine sensitive potentiation of acetylcholine responses in our muscle contraction experiments. The potentiation of the acetylcholine responses shows that nematode acetylcholine receptors are capable of a level of plasticity. A model involving calcium release from the sarcoplasmic reticulum, CaM Kinase, calcineurin, muscarinic receptors and AF2 receptors is proposed to explain our observations. These observations are important because they point to a pharmacological approach that may be developed to counter resistance to cholinergic anthelmintics.

Functional characterization of the putative orphan neuropeptide G-protein coupled receptor C26F1.6 in Caenorhabditis elegans

In this study, we describe the cloning and the characterization of the third FMRFamide-related peptide (FaRP) receptor in Caenorhabditis elegans, the VRFa receptor 1. Numerous structurally different FaRPs were synthesized and used to screen the orphan C26F1.6 receptor for activation. Two peptides ending in M(orL)VRFamide elicited a calcium response in receptor expressing mammalian cells. The response is dose-dependent and appeared to be very specific, since very closely related FaRPs were less active, even the other peptides ending in M(orL)VRFamide. Pharmacological profiling of the most active peptide suggests that SMVRFa is the most active binding core. N-terminal extension decreases peptide activity.

AF2 interaction with Ascaris suum body wall muscle membranes involves G-protein activation

KHEYLRF-NH(2) (AF2) is the most abundant FMRFamide-related peptide (FaRP) in Ascaris suum and also in many other parasitic and free-living nematodes. The AF2 abundance in the highly diverse nematodes and its potent and profound effects on the neuromuscular systems make AF2 and its receptor(s) very attractive targets for the discovery of novel broad-spectrum anthelmintics. Although FaRP receptors are believed to belong to the large family of G-protein coupled receptors (GPCRs), to date no AF2 receptor(s) have been cloned so there is no final proof to show that they are indeed G-protein coupled. In this study, using A. suum body wall muscle membranes, we showed that: (1) AF2 effectively (EC(50) 57 nM) induced a dose-dependent stimulation of [35S]GTP gamma S binding to the membranes, which is a hallmark of G-protein activation; (2) the high affinity binding of [125I-Tyr(4)]AF2 was inhibited in a dose-dependent manner by GTP with a K(i) of 10.5 nM (so-called guanine nucleotide effect, characteristic for GPCRs). Collectively, our results provide direct evidence for G-protein involvement in AF2-triggered receptor activation and thus confirm that the receptor for AF2 in A. suum is a GPCR.

Effects of KHEYLRFamide and KNEFIRFamide on cyclic adenosine monophosphate levels in Ascaris suum somatic muscle

KHEYLRF-NH(2) (AF2) is a FMRFamide-related peptide (FaRP) present in parasitic and free-living nematodes. At concentrations as low as 10 pM, AF2 induces a biphasic tension response, consisting of a transient relaxation followed by profound excitation, in neuromuscular strips prepared from Ascaris suum. In the present study, the effects of AF2 on cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP) and inositol-1,4,5-triphosphate (IP(3)) levels were measured following muscle tension recordings from 2 cm neuromuscular strips prepared from adult A. suum. AF2 induced a concentration- and time-dependent increase in cAMP, beginning at 1 nM; cAMP levels increased by 84-fold following 1 h exposure to 1 microM AF2. cGMP and IP(3) levels were unaffected by AF2 at concentrations </=1 microM. AF2-induced stimulation of cAMP was unaffected by removal of the dorsal or ventral nerve cord, even though this form of denervation abolished the excitatory phase of the tension response. The effects of 0.1 and 1 microM AF2 on cAMP were also unaffected by 10 microM SDPNFLRF-NH(2) (PF1, an inhibitory FaRP) and 10 microM PF1022A (an inhibitory cyclodepsipeptide), even though each of these peptides abolished the excitatory phase of the tension response induced by AF2. Within an alanine-scan series of AF2 analogues, only KHAYLRF-NH(2) stimulated cAMP production with equipotency to AF2; the effects of this peptide on muscle tension also mimicked AF2. Another excitatory FaRP present in nematodes, KNEFIRF-NH(2) (AF1), also stimulated cAMP production, but was 100-fold less potent than AF2. The stimulatory effects of AF1 on tension and cAMP levels were blocked by an alanine-substituted analogue of this peptide (Ala(6)-AF1, KNEFIAF-NH(2)), while the stimulatory effects of AF2 on tension and cAMP were not affected by this analogue. AF2 and AF1 increase A. suum somatic muscle cAMP by targeting different receptors. Increases in cAMP stimulated by AF2 can be decoupled from the excitatory response caused by this peptide, and it is not possible to establish a causal linkage between the contractile response elicited by this peptide and its effects on cAMP accumulation.

Structure-activity relationships of an inhibitory nematode FMRFamide-related peptide, SDPNFLRFamide (PF1), on Ascaris suum muscle

FMRFamide-related peptides are widespread among the Nematoda. Among them is a family of extended PNFLRFamide peptides encoded on the flp-1 peptide precursor gene in Caenorhabditis elegans. The most studied peptide from this series is SDPNFLRFamide (PF1). Each residue in this peptide was sequentially substituted with either alanine or the corresponding d-isomer of the native amino acid in order to define structure-function relationships in this peptide using an Ascaris suum muscle tension assay. In general, substitutions in the N-terminal tetrapeptide had only minor consequences for efficacy, while substitutions in the C-terminal tetrapeptide caused more dramatic changes. Such substitutions typically markedly diminished efficacy, but d-isomer substitution at either position 5 (Phe) or 6 (Leu) converted the inhibitory activity of the prototype into excitation. In addition, it has been evident that KPNFLRFamide and SDPNFLRFamide, though encoded on flp-1 and sharing a PNFLRFamide hexapeptide, act through different receptors. KPNFLRFamide directly gates a chloride channel in A. suum muscle cells, while SDPNFLRFamide acts through nitric oxide synthase to open K+ channels in the same tissue. The use of K+ channel blockers and nitric oxide synthase inhibitors in electrophysiological experiments employing A. suum muscle membranes allowed the unambiguous conclusion that the N-terminal lysine is absolutely required for activation of the chloride channel and excludes interaction with the SDPNFLRFamide receptor.

Isolation, characterization, and distribution of a novel neuropeptide, Rana RFamide (R-RFa), in the brain of the European green frog Rana esculenta

A novel neuropeptide of the RFamide peptide family was isolated in pure form from a frog (Rana esculenta) brain extract by using reversed-phase high performance liquid chromatography in combination with a radioimmunoassay for mammalian neuropeptide FF (NPFF). The primary structure of the peptide was established as Ser-Leu-Lys- Pro-Ala-Ala-Asn-Leu-Pro-Leu- Arg-Phe-NH(2). The sequence of this neuropeptide, designated Rana RFamide (R-RFa), exhibits substantial similarities with those of avian LPLRFamide, gonadotropin-inhibitory hormone, and human RFRP-1. The distribution of R-RFa was investigated in the frog central nervous system by using an antiserum directed against bovine NPFF. In the brain, immunoreactive cell bodies were primarily located in the hypothalamus, i.e., the anterior preoptic area, the suprachiasmatic nucleus, and the dorsal and ventral hypothalamic nuclei. The most abundant population of R-RFa-containing neurons was found in the periependymal region of the suprachiasmatic nucleus. R-RFa- containing fibers were widely distributed throughout the brain from the olfactory bulb to the brainstem, and were particularly abundant in the external layer of the median eminence. In the spinal cord, scattered immunoreactive neurons were found in the gray matter. R-RFa-positive processes were found in all regions of the spinal cord, but they were more abundant in the dorsal horn. This study provides the first characterization of a member of the RFamide peptide family in amphibians. The occurrence of this novel neuropeptide in the hypothalamus and median eminence and in the dorsal region of the spinal cord suggests that, in frog, R-RFa may exert neuroendocrine activities and/or may be involved in the transmission of nociceptive stimuli.

Structure-activity relationships of 18 endogenous neuropeptides on the motor nervous system of the nematode Ascaris suum

Neuropeptides play an important role in all nervous systems and structure-activity studies of related peptides is one approach to understanding this role. This study of the motor nervous system of the parasitic nematode Ascaris suum describes the physiological effects of a family of 18 endogenous Ascaris FMRFamide-like peptides (AF peptides) on the membrane potential and input resistance of the dorsal excitatory type 2 (DE2) and dorsal inhibitory (DI) motor neurons. These motor neurons are part of the final common output pathway from the motor nervous system to the somatic muscle cells responsible for locomotion. AF peptide effects on the frequency of excitatory postsynaptic potentials (EPSPs) in DE2 motor neurons were also measured to infer peptide effects on central presynaptic spiking neurons. AF peptide injections into intact worms were made to assess their qualitative effects on behavior, providing a context for interpreting motor neuron data. One category of AF peptides, N-terminally extended -FIRFa peptides (AF5, AF7 and AF1), has pronounced behavioral effects and qualitatively similar, but quantitatively different effects on DE2 and DI motor neurons. A second category of AF peptides (AF2, AF9, and AF8) also produces dramatic behavioral effects and strong electrophysiological effects on DE2 and/or DI motor neurons. A third category of AF peptides, consisting of six members of the -PGVLRFa group (which are encoded by the same gene and have closely related sequences) and peptide AF11, have pronounced behavioral effects, but relatively weak or negligible effects on DE2 and DI motor neurons. A fourth category of AF peptides, also consisting of structurally unrelated members, has pronounced behavioral effects and, as individual peptides, similar effects on both DE2 and DI motor neurons; AF15 is excitatory, while AF17 and AF19 are inhibitory, on both motor neuron types. Finally, two AF peptides (AF6, AF16) are relatively weak or inactive in producing behavioral or motor neuronal effects. Based on comparisons of the effects of AF peptides on DE2 and DI motor neurons, a tentative list of 5 major response-types is proposed as a working hypothesis to guide the search for AF peptide receptors. The findings attest to the potential complexity of neurosignaling in this comparatively simple nervous system.

Changes in locomotory behavior and cAMP produced in Ascaris suum by neuropeptides from Ascaris suum or Caenorhabditis elegans

Injection of Ascaris FMRFamide-like (AF) peptides and peptides encoded by genes in Caenorhabditis elegans were analyzed for effects on locomotion, body waveforms, and cAMP concentrations in adult female Ascaris suum. Injection of AF1 (KNEFIRFamide) or AF2 (KHEYLRFamide) inhibited the propagation of locomotory waves and reduced the number of waveforms, decreased the body length, and caused a large, long-lasting increase in cAMP. Muscle tissue was identified as a major source of the cAMP response induced by AF1. The AF1 analog AF1R6A did not affect cAMP levels by itself, but inhibited the cAMP response produced by AF1. AF8 (KSAYMRFamide) produced ventral coiling in the behavioral assay, and AF10 (GFGDEMSMPGVLRFamide) decreased the body length and increased the number of body waveforms. In dorsal muscle strips, AF10 produced a long-lasting contraction. Neither AF8 nor AF10 changed cAMP concentrations. AF17 (FDRDFMHFamide) increased body length and decreased cAMP. The neuropeptides encoded by C. elegans genes flp-4, flp-7, flp-9, and flp-13 produced paralysis and loss of waveforms, increased body length and, like AF17, decreased cAMP. Three new predicted peptides from C. elegans genome sequences were synthesized and tested. One produced ventral coiling but no change in cAMP; the other two gave no detectable responses. The fact that C. elegans neuropeptides produce behavioral and physiological effects in A. suum suggests that structurally related peptides may exist in A. suum. The profound changes in cAMP produced by some neuropeptides has important implications for understanding cAMP signaling and shows that neuropeptide-mediated signal transduction pathways are potential targets for anthelmintic drug development.

The range and biological activity of FMRFamide-related peptides and classical neurotransmitters in nematodes

Nematodes include both major parasites of humans, livestock and plants in addition to free-living species such as Caenorhabditis elegans. The nematode nervous system (especially in C. elegans) is exceptionally well defined in terms of the number, location and projections of the small number of neurons in the nervous system and their integration into circuits involved in regulatory behaviours vital to their survival. This review will summarize what is known about the biological activity of neurotransmitters in nematodes: the biosynthetic pathways and genes involved, their receptors, inactivation mechanisms and secondary messenger signalling systems. It will cover the 'classical' transmitters, such as acetylcholine (ACh), GABA, glutamate, serotonin, dopamine, octopamine, noradrenaline and nitric oxide. The localization of peptides throughout the nematode nervous system is summarized, in addition to the isolation of nematode neuropeptides by both traditional biochemical techniques and more modern genetic means. The major contribution of the completion of the C. elegans genome-sequencing program is highlighted throughout. Efforts to unravel neurotransmitter action in various physiological actions such as locomotion, feeding and reproduction are detailed as well as the various inactivation mechanisms for the current complement of nematode transmitters.

Pharmacology of FMRFamide-related peptides in helminths

Nervous systems of helminths are highly peptidergic. Species in the phylum Nematoda (roundworms) possess at least 50 FMRFamide-related peptides (FaRPs), with more yet to be identified. To date, few non-FaRP neuropeptides have been identified in these organisms, though evidence suggests that other families are present. FaRPergic systems have important functions in nematode neuromuscular control. In contrast, species in the phylum Platyhelminthes (flatworms) apparently utilize fewer FaRPs than do nematodes; those species examined possess one or two FaRPs. Other neuropeptides, such as neuropeptide F (NPF), play key roles in flatworm physiology. Although progress has been made in the characterization of FaRP pharmacology in helminths, much remains to be learned. Most studies on nematodes have been done with Ascaris suum because of its large size. However, thanks to the Caenorhabditis elegans genome project, we know most about the FaRP complement of this free-living animal. That essentially all C. elegans FaRPs are active on at least one A. suum neuromuscular system argues for conservation of ligand-receptor recognition features among the Nematoda. Structure-activity studies on nematode FaRPs have revealed that structure-activity relationship (SAR) "rules" differ considerably among the FaRPs. Second messenger studies, along with experiments on ionic dependence and anatomical requirements for activity, reveal that FaRPs act through many different mechanisms. Platyhelminth FaRPs are myoexcitatory, and no evidence exists of multiple FaRP receptors in flatworms. Interestingly, there are examples of cross-phylum activity, with some nematode FaRPs being active on flatworm muscle. The extent to which other invertebrate FaRPs show cross-phylum activity remains to be determined. How FaRPergic nerves contribute to the control of behavior in helminths, and are integrated with non-neuropeptidergic systems, also remains to be elucidated.

Actions of nematode FMRFamide-related peptides on the pharyngeal muscle of the parasitic nematode, Ascaris suum

The endogenous nematode peptides known as FMRFamide-related peptides (FaRPs) and various "classical" transmitters have a range of effects on nematodes that result in changes in behavior, particularly locomotion, including paralysis and inhibition of feeding. This study describes the application of an in vitro pharmacological approach to further delineate the action of a number of FaRP neurotransmitters on feeding behavior. Contraction of Ascaris suum pharyngeal muscle was monitored using a modified pressure transducer system that detects changes in intrapharyngeal pressure and therefore contraction of the radial muscle of the pharynx. The pharynx did not contract spontaneously. However, serotonin (5-HT, 100 microM) stimulated rhythmic contractions and relaxations (pumping) at a frequency of 0.5 Hz. The native nematode peptide, KNEFIRFamide (AF1), inhibited the pumping elicited by 5-HT. The duration of inhibition was concentration-dependent (1-1000 nM) with a threshold of 1 nM (n = 7). KSAYMRFamide (AF8/PF3) also inhibited pharyngeal pumping. There was no observable effect of any of the following nematode peptides on pharyngeal pumping behavior (1-1000 nM; n = 8): AF2, AF3, AF4, AF6, AF16, PF1/CF1, PF2/CF2, or PF4. Thus, interruption of pharyngeal processes, such as feeding, regulation of hydrostatic pressure, and secretion, may provide a new site of anthelmintic action.

FMRFamide-related neuropeptide gene family in Caenorhabditis elegans

Neuropeptides are used as signaling molecules in the nervous system of most organisms, including mammals. The family of FMRFamide (Phe-Met-Arg-Phe-NH2)-like neuropeptides (FaRPs) all share an RFamide sequence at their C-termini and have been shown to have diverse functions in the central and peripheral nervous systems. In the nematode Caenorhabditis elegans, FMRFamide-like peptides (FaRPs) are expressed in at least 10% of the neurons, including motor, sensory, and interneurons that are involved in movement, feeding, defecation, and reproduction. Twenty-two genes, designated flp-1 through flp-22, encode FaRPs in C. elegans, although there are likely to be additional flp genes to be identified. Each flp gene encodes a different set of FaRPs, yielding a predicted total of 59 distinct FaRPs; a few of the genes may also encode non-FaRPs. Inactivation of some of the flp genes indicates that at least one flp gene has unique functions, while at least two flp genes appear to have overlapping functions with other flp genes. These results suggest that a complex family of FaRPs have varied roles through all stages of development and in adulthood in C. elegans.

Structural characterisation and pharmacology of KHEYLRFamide (AF2) and KSAYMRFamide (PF3/AF8) from Haemonchus contortus

The FMRFamide-related peptides (FaRPs), KHEYLRFamide (AF2) and KSAYMRFamide (PF3) were structurally characterised from the parasitic nematode of sheep, Haemonchus contortus (MH isolate). Both peptides were sequenced in a single gas-phase sequencing run and their structure confirmed by mass spectrometry which identified peptides of 920 Da (C-terminally amidated AF2) and 902/918 Da (C-terminally amidated non-oxidised/oxidised PF3, respectively). AF2 had inhibitory effects on H. contortus muscle and inhibited acetylcholine (ACh, 10 microM)-induced contractions, with a threshold for activity of 1 microM. PF3 induced concentration-dependent contractions of H. contortus (activity threshold, 10 nM) and enhanced ACh contractions. Compared with the MH isolate, an isolate of H. contortus which has reduced sensitivity to cholinergic drugs (Lawes isolate) was less sensitive to the effects of PF3. The concentration-response curves for the cholinergic compounds ACh and levamisole (LEV), and PF3, but not a control, KPNFIRFamide (PF4), showed a statistically similar shift. This study implicates PF3 in the modulation of cholinergic function in H. contortus.

Structure-activity relationships of the Helix neuropeptide, SEPYLRFamide, and its N-terminally modified analogues on identified Helix lucorum neurones

Metabotropic and ionotropic effects evoked by the endogenous Helix heptapeptide, SEPYLRFamide, and four analogues, i.e., where the amino acid sequences at the N-terminal (EPYLRFamide, SEGYLRFamide, SRPYLRFamide and SKPYLRFamide) were modified, were compared on identified Helix lucorum LPa2, LPa3, RPa3, RPa2 neurones using two electrode voltage clamp and current clamp techniques. All peptides (bath application) reduce reversibly the inward current to local ionophoretic application of acetylcholine onto the neurone soma with an order of potency: EPYLRFamide=SEGYLRFamide=SRPYLRFamide>SEPYLRFamide+ ++>SKPYLRFamide. The reductions of the acetylcholine-induced inward current evoked by SEPYLRFamide and its analogues at concentrations of 0.01-10 microM are not accompanied by a change of amplitude of the leak inward current caused by constant negative shift of a holding potential. At concentration of 50 microM all peptides increase reversibly the resting membrane conductance to an equal degree. Local application under pressure of SEPYLRFamide and its analogues onto the soma of neurones evoke hyperpolarizations with similar values. These results indicate that the N-terminal three amino acids of the peptide molecule are not responsible for the degree of ionotropic effect on the neurones studied. In contrast the amino acid sequence at the N-terminal modifies the degree of the modulatory effects of the YLRFamide-related analogues. Changes at the SEPYLRFamide N-terminal (Ser1-Glu2-Pro3) intensify the inhibitory action of the analogues as compared with effect evoked by the endogenous peptide, that is, removal of Ser1 (Glu1-Pro2), replacement of Pro3 with Gly3 (Ser1-Glu2-Gly3), replacement of Glu2 with Arg2 (Ser1-Arg2-Pro3). Replacement of Glu2 with Lys2 (Ser1-Lys2-Pro3) reduces the modulatory potency. It is concluded that ionotropic and metabotropic effects of these YLRFamide-related peptides may occur at different membrane binding sites.

Parasitic peptides! The structure and function of neuropeptides in parasitic worms

Parasitic worms come from two very different phyla-Platyhelminthes (flatworms) and Nematoda (roundworms). Although both phyla possess nervous systems with highly developed peptidergic components, there are key differences in the structure and action of native neuropeptides in the two groups. For example, the most abundant neuropeptide known in platyhelminths is the pancreatic polypeptide-like neuropeptide F, whereas the most prevalent neuropeptides in nematodes are FMRFamide-related peptides (FaRPs), which are also present in platyhelminths. With respect to neuropeptide diversity, platyhelminth species possess only one or two distinct FaRPs, whereas nematodes have upwards of 50 unique FaRPs. FaRP bioactivity in platyhelminths appears to be restricted to myoexcitation, whereas both excitatory and inhibitory effects have been reported in nematodes. Recently interest has focused on the peptidergic signaling systems of both phyla because elucidation of these systems will do much to clarify the basic biology of the worms and because the peptidergic systems hold the promise of yielding novel targets for a new generation of antiparasitic drugs.

FMRFamide-related gene family in the nematode, Caenorhabditis elegans

Many organisms, including mammals, use short peptides as neurotransmitters. The family of FMRFamide (Phe-Met-Arg-Phe-NH2)-like neuropeptides, which all share an -RFamide sequence at their C-termini, has been shown to have diverse functions, including neuromodulation and stimulation or inhibition of muscle contraction. In the nematode, Caenorhabditis elegans, FMRFamide-like peptides (FaRPs) are expressed in approximately 10% of the neurons, including motor, sensory, and interneurons that are involved in movement, feeding, defecation, and reproduction. At least 14 genes, designated flp-1 through flp-14, encode FaRPs in C. elegans. Here, we present data that all 14 flp genes are transcribed in C. elegans, and several of these genes are alternatively spliced. Each flp gene encodes a different set of FaRPs, yielding a predicted total of 44 distinct FaRPs. Using staged RNA for reverse-transcription/polymerase chain reactions (RT/PCR), we determined that most flp genes are expressed throughout development. These results suggest that a complex family of FaRPs have varied roles through all stages of development and in adulthood in C. elegans.

Purification and properties of a membrane aminopeptidase from Ascaris suum muscle that degrades neuropeptides AF1 and AF2

We have identified on the membranes of the locomotory muscle of Ascaris suum an amastatin-sensitive aminopeptidase that hydrolyses the bioactive neuropeptides AF1 (KNEFIRF-NH2) and AF2 (KHEYLRF-NH2), by cleavage of the Lys1-Asn2 and Lys1-His2 peptide bonds, respectively. AF2 (1.2 nmol of HEYLRF-NH2 formed min[-1] (mg protein[-1])) was hydrolysed at a faster rate compared to AF1 (0.2 nmol of NEFIRF-NH2 formed min[-1] (mg protein[-1])). AF1 hydrolysis by the aminopeptidase was inhibited by the amastatin (IC50, 9.0 microM), leuhistin (IC50, 1.25 microM) but was insensitive to puromycin, indicating a similarity to mammalian aminopeptidase N. The enzyme was also inhibited by arphamenine B (IC50, 9.0 microM), (2S, 3R)-3-amino-2-hydroxy-4-(4-nitrophenyl)butanoyl-L-leucine (IC50, 8.0 microM), bestatin (IC50, 15.0 microM) and 1 mM 1-10 bis-phenanthroline. The detergent Triton X-100 solubilised enzyme had a pI of 5.0 and after 1000-fold purification by ion-exchange chromatography, appeared to have a Mr of around 240,000 by SDS-PAGE. The purified aminopeptidase had a Km of 534 microM for the hydrolysis of AF1 and cleaved Phe1 from FMRF-NH2, but was unable to hydrolyse DFMRF-NH2 or FDMRF-NH2. The aminopeptidase that we have described in this report might have a role in the extracellular metabolism and inactivation of neuropeptides acting on the locomotory muscle of A. suum.

Structure-activity relationships of FMRFamide-related peptides contracting Schistosoma mansoni muscle

This study reports the potent myoactivity of flatworm FMRFamide-related peptides (FaRPs) on isolated muscle fibers of the human blood fluke, Schistosoma mansoni. The turbellarian peptides YIRFamide (EC50 4 eta M), GYIRFamide (EC50 1 eta M), and RYIRFamide (EC50 7 eta M), all induced muscle contraction more potently than the cestode FaRP GNFFRFamide (EC50 500 eta M). Using a series of synthetic analogs of the flatworm peptides YIRFamide, GYIRFamide and RYIRFamide, the structure-activity relationships of the muscle FaRP receptor were examined. With a few exceptions, each residue in YIRFamide is important in the maintenance of its myoactivity. Alanine scans resulted in peptides that were inactive (Ala1, Ala2, Ala3 and Ala4 YIRFamide; Ala4 and Ala5 RYIRFamide) or had much reduced potencies (Ala1, Ala2 and Ala3 RYIRFamide). Substitution of the N-terminal (Tyr1) residue of YIRFamide with the non-aromatic residues Thr or Arg produced analogs with greatly reduced potency. Replacement of the N-terminal Tyr with aromatic amino acids resulted in myoactive peptides (FIRFamide, EC50 100 eta M; WIRFamide, EC50 0.5 eta M). The activity of YIRFamide analogs which possessed a Leu2, Phe2 or Met2 residue (EC50's 10, 1 and 3 eta M, respectively) instead of Ile2 was not significantly altered, whereas, YVRFamide had a greatly reduced (EC50 200 eta M) activity. Replacement of the Phe4 with a Tyr4 (YIRYamide) also greatly lowered potency. Truncated analogs were either inactive (FRFamide, YRFamide, HRFamide, RFamide, Famide) or had very low potency (IRFamide and MRFamide), with the exception of nLRFamide (EC50 20 eta M). YIRF free acid was inactive. In summary, these data show the general structural requirements of this schistosome muscle FaRP receptor to be similar, but not identical, to those of previously characterized molluscan FaRP receptors.

The Pharmacology of Nematode FMRFamide-related Peptides

FMRFamide-related peptides (FaRPs) are the largest known family of invertebrate neuropeptides. Immunocytochemical screens of nematode tissues using antisera raised to these peptides have localized extensive FaRP-immunostaining to their nervous systems. Although 21 FaRPs have been isolated and sequenced from extracts of free-living and parasitic nematodes, available evidence indicates that other FaRPs await discovery. While our knowledge of the pharmacology of these native nematode neuropeptides is extremely limited, reports on their physiological activity in nematodes are ever increasing. All the nematode FaRPs examined so far have been found to have potent and varied actions on nematode neuromuscular activity. It is only through the extensive pharmacological and physiological assessment of the tissue, cell and receptor interactions of these peptidic messengers that an understanding of their activity on nematode neuromusculature will be possible. In this review, Aaron Maule and colleagues examine the current understanding of the pharmacology of nematode FaRPs.

Nematode FMRFamide-related peptide (FaRP)-systems: occurrence, distribution and physiology

The application of rational (mechanism-based) approaches to anthelmintic discovery requires information about target proteins which are pharmacologically distinguishable from their vertebrate homologs. In helminths, several such targets (e.g., beta-tubulin, ATP-generating enzymes, cholinergic receptors, CI- channels) have been characterized only after the discovery, through empirical screening, of compounds that interfere with their function. From the perspective of anthelmintic discovery, the utility of these targets is diminishing due to the emergence of drug-resistant strains of parasites. This has motivated the search for compounds with novel modes-of-action. Recent basic research in helminth physiology and biochemistry has identified several potential targets for rational anthelmintic discovery, including receptors for FMRFamide-related peptides (FaRPs). To date, over 20 different nematode FaRPs have been identified and these peptides, which are broadly distributed in helminths, have been localized to all of the major neuronal subtypes in nematodes. The FaRPs that have been examined have been found profoundly to affect somatic muscle function in gastrointestinal nematodes. In this respect, complex inhibitory and excitatory actions have been identified for a number of these peptides. Although the transduction pathways for any of these peptides remain to be elucidated, the available evidence indicates that nematode FaRPs have numerous mechanisms of action. The employment of nematode neuropeptide receptors in mechanism-based screens has immense potential in the identification of novel anthelmintics.

FMRFamide-related peptides (FaRPs) in nematodes: occurrence and neuromuscular physiology

The occurrence of classical neurotransmitter molecules and numerous peptidic messenger molecules in nematode nervous systems indicate that although structurally simple, nematode nervous systems are chemically complex. Thus far, studies on one nematode neuropeptide family, namely the FMRFamide-related peptides (FaRPs), have revealed an unexpected variety of neuropeptide structures in both free-living and parasitic species. To date 23 nematode FaRPs have been structurally characterized including 12 from Ascaris suum, 8 from Caenorhabditis elegans, 5 from Panagrellus redivivus and 1 from Haemonchus contortus. Ten FaRP-encoding genes have been identified in Caenorhabditis elegans. However, the full complement of nematode neuronal messengers has yet to be described and unidentified nematode FaRPs await detection. Preliminary characterization of the actions of nematode neuropeptides on the somatic musculature and neurones of A, suum has revealed that these peptidic messengers have potent and complex effects. Identified complexities include the biphasic effects of KNEFIRFamide/KHEYLRFamide (AF1/2; relaxation of tone followed by oscillatory contractile activity) and KPNFIRFamide (PF4; rapid relaxation of tone followed by an increase in tone), the diverse actions of KSAYMRFamide (AF8 or PF3; relaxes dorsal muscles and contracts ventral muscles) and the apparent coupling of the relaxatory effects of SDPNFLRFamide/SADPNFLRFamide (PF1/PF2) to nitric oxide release. Indeed, all of the nematode FaRPs which have been tested on somatic muscle strips of A. suum have actions which are clearly physiologically distinguishable. Although we are a very long way from understanding how the actions of these peptides are co-ordinated, not only with those of each other but also with those of the classical transmitter molecules, to control nematode behaviour, their abundance coupled with their diversity of structure and function indicates a hitherto unidentified sophistication to nematode neuromuscular intergration.

Inhibitory effects of nematode FMRFamide-related peptides (FaRPs) on muscle strips from Ascaris suum

A large number of FMRFamide-related peptides (FaRPs) are found in nematodes, and some of these are known to influence tension and contractility of neuromuscular strips isolated from Ascaris suum body wall. Relaxation of these strips has been noted with five nematode FaRPs. The inhibitory actions of SDPNFLRFamide (PF1) and SADPNFLRFamide (PF2) appear to be mediated by nitric oxide, as previously demonstrated with inhibitors of nitric oxide synthase (NOS). This present study showed that the effects of PF1 were also depended on external Ca++ and were reduced by the Ca(++)-channel blocker verapamil, observations consistent with the finding that nematode NOS is Ca(++)-dependent. KSAYMRFamide (PF3), KNIRFamide (PF4) and KNAFIRFamide (an alanine substituted analog of KNEFIRFamide, AF1, termed A3AF1) also relaxed A. suum muscle strips, but these responses were not affected by NOS inhibitors. PF3 inhibited the activity of strips prepared from the dorsal side of the worm, but contracted ventral strips. Both effects were dependent on the presence of ventral/dorsal nerve cords (unlike PF1/PF2) and were attenuated in medium which contained high K+ or low Ca++. PF4-induced muscle relaxation and hyperpolarization were independent of nerve cords, but were reversed in Cl-free medium, unlike PF1 or PF3. The PF4 effect physiologically desensitized muscle strips to subsequent treatment with PF4 and/or GABA. However, PF4 and GABA were not synergistic in this preparation. The effects of GABA, but not PF4, were reduced in muscle strips treated with the GABA antagonist, NCS 281-93. Following PF4 (or GABA) relaxation, subsequent treatment with higher doses of PF4 caused muscle strip contraction. A3AF1 was found to relax muscle strips and hyperpolarize muscle cells independently of the ventral and dorsal nerve cords, K+, Ca++, and Cl-, and mimicked the inhibitory phase associated with the exposure of these strips to AF1. On the basis of anatomical and ionic dependence, these data have delineated at least four distinct inhibitory activities attributable to nematode FaRPs. Clearly, a remarkably complex set of inhibitory mechanisms operate in the nematode neuromuscular system.