Modulation of rhythmic motor activity by pyrokinin peptides

Insect PRXamides: Evolutionary Divergence, Novelty, and Loss in a Conserved Neuropeptide System

The PRXamide neuropeptides have been described in both protostome and deuterostome species, including all major groups of the Panarthropoda. Best studied are the insect PRXamides consisting of three genes: pk/pban, capa, and eth, each encoding multiple short peptides that are cleaved post-translationally. Comparisons of genome and transcriptome sequences reveal that while retaining its fundamental ancestral organization, the products of the pk/pban gene have undergone significant change in the insect Order Diptera. Basal dipteran pk/pban genes are much like those of other holometabolous insects, while more crown species have lost two peptide coding sequences including the otherwise ubiquitous pheromone biosynthesis activating neuropeptide (PBAN). In the genomic model species Drosophila melanogaster, one of the remaining peptides (hugin) plays a potentially novel role in feeding and locomotor regulation tied to circadian rhythms. Comparison of peptide coding sequences of pk/pban across the Diptera pinpoints the acquisition or loss of the hugin and PBAN peptide sequences respectively, and provides clues to associated changes in life history, physiology, and/or behavior. Interestingly, the neural circuitry underlying pk/pban function is highly conserved across the insects regardless of the composition of the pk/pban gene. The rapid evolution and diversification of the Diptera provide many instances of adaptive novelties from genes to behavior that can be placed in the context of emerging selective pressures at key points in their phylogeny; further study of changing functional roles of pk/pban may then be facilitated by the high-resolution genetic tools available in Drosophila melanogaster.

Pyrokinin receptor silencing in females of the southern cattle tick Rhipicephalus (Boophilus) microplus is associated with a reproductive fitness cost

BACKGROUND

Rhipicephalus microplus is the vector of deadly cattle pathogens, especially Babesia spp., for which a recombinant vaccine is not available. Therefore, disease control depends on tick vector control. However, R. microplus populations worldwide have developed resistance to available acaricides, prompting the search for novel acaricide targets. G protein-coupled receptors (GPCRs) are involved in the regulation of many physiological processes and have been suggested as druggable targets for the control of arthropod vectors. Arthropod-specific signaling systems of small neuropeptides are being investigated for this purpose. The pyrokinin receptor (PKR) is a GPCR previously characterized in ticks. Myotropic activity of pyrokinins in feeding-related tissues of Rhipicephalus sanguineus and Ixodes scapularis was recently reported.

METHODS

The R. microplus pyrokinin receptor (Rhimi-PKR) was silenced through RNA interference (RNAi) in female ticks. To optimize RNAi, a dual-luciferase assay was applied to determine the silencing efficiency of two Rhimi-PKR double-stranded RNAs (dsRNA) prior to injecting dsRNA in ticks to be placed on cattle. Phenotypic variables of female ticks obtained at the endpoint of the RNAi experiment were compared to those of control female ticks (non-injected and beta-lactamase dsRNA-injected). Rhimi-PKR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

RESULTS

The Rhimi-PKR transcript was expressed in all developmental stages. Rhimi-PKR silencing was confirmed in whole ticks 4 days after injection, and in the tick carcass, ovary and synganglion 6 days after injection. Rhimi-PKR silencing was associated with an increased mortality and decreased weight of both surviving females and egg masses (P < 0.05). Delays in repletion, pre-oviposition and incubation periods were observed (P < 0.05).

CONCLUSIONS

Rhimi-PKR silencing negatively affected female reproductive fitness. The PKR appears to be directly or indirectly associated with the regulation of female feeding and/or reproductive output in R. microplus. Antagonists of the pyrokinin signaling system could be explored for tick control.

Characterization, expression, and function of the pyrokinins (PKs) in the giant freshwater prawn, Macrobrachium rosenbergii

Pyrokinins (PKs) are neuropeptides that have been found to regulate a variety of physiological activities including reproduction in various insect and crustacean species. However, the reproductive roles of PKs in the giant freshwater prawn have not yet been investigated. In this study, we identified the MroPK gene from next-generation sequence resources, which encodes a MroPK precursor that shares a high degree of conservation with the C-terminal sequence of FxPRLamide in other arthropods. MroPK is expressed within most tissues, except the hepatopancreas, stomach, and gill. Within developing ovarian tissue, MroPK expression was found to be significantly higher during the early stages (stages 1-2) compared with the late stages (stages 3-4), and could be localized to the oogonia, previtellogenic, and early vitellogenic oocytes. A role for PK in M. rosenbergii reproduction was supported following experimental administration of MroPK to ovarian explant cultures, showing an increase in the productions of progesterone and estradiol and upregulation of steroidogenesis-related genes (3β-HSD and 17β-HSD) and vitellogenin (Vg) expressions. Together, these results support a role for MroPK in regulating ovarian maturation via steroidogenesis.

Neuromodulation Enables Temperature Robustness and Coupling Between Fast and Slow Oscillator Circuits

Acute temperature changes can disrupt neuronal activity and coordination with severe consequences for animal behavior and survival. Nonetheless, two rhythmic neuronal circuits in the crustacean stomatogastric ganglion (STG) and their coordination are maintained across a broad temperature range. However, it remains unclear how this temperature robustness is achieved. Here, we dissociate temperature effects on the rhythm generating circuits from those on upstream ganglia. We demonstrate that heat-activated factors extrinsic to the rhythm generators are essential to the slow gastric mill rhythm’s temperature robustness and contribute to the temperature response of the fast pyloric rhythm. The gastric mill rhythm crashed when its rhythm generator in the STG was heated. It was restored when upstream ganglia were heated and temperature-matched to the STG. This also increased the activity of the peptidergic modulatory projection neuron (MCN1), which innervates the gastric mill circuit. Correspondingly, MCN1’s neuropeptide transmitter stabilized the rhythm and maintained it over a broad temperature range. Extrinsic neuromodulation is thus essential for the oscillatory circuits in the STG and enables neural circuits to maintain function in temperature-compromised conditions. In contrast, integer coupling between pyloric and gastric mill rhythms was independent of whether extrinsic inputs and STG pattern generators were temperature-matched or not, demonstrating that the temperature robustness of the coupling is enabled by properties intrinsic to the rhythm generators. However, at near-crash temperature, integer coupling was maintained only in some animals while it was absent in others. This was true despite regular rhythmic activity in all animals, supporting that degenerate circuit properties result in idiosyncratic responses to environmental challenges.

Blue light responses in Cancer borealis stomatogastric ganglion neurons

In many animals, the daily cycling of light is a key environmental cue, encoded in part by specialized light-sensitive neurons without visual functions. We serendipitously discovered innate light-responsiveness while imaging the extensively studied stomatogastric ganglion (STG) of the crab, Cancer borealis. The STG houses a motor circuit that controls the rhythmic contractions of the foregut, and the system has facilitated deep understanding of circuit function and neuromodulation. We illuminated the crab STG in vitro with different wavelengths and amplitudes of light and found a dose-dependent increase in neuronal activity upon exposure to blue light (λ460-500 nm). The response was elevated in the absence of neuromodulatory inputs to the STG. The pacemaker kernel that drives the network rhythm was responsive to light when synaptically isolated, and light shifted the threshold for slow wave and spike activity in the hyperpolarized direction, accounting for the increased activity patterns. Cryptochromes are evolutionarily conserved blue-light photoreceptors that are involved in circadian behaviors.¹ Their activation by light can lead to enhanced neuronal activity.² We identified cryptochrome sequences in the C. borealis transcriptome as potential mediators of this response and confirmed their expression in pyloric dilator (PD) neurons, which are part of the pacemaker kernel, by single-cell RNA-seq analysis.

Perturbation-specific responses by two neural circuits generating similar activity patterns

A fundamental question in neuroscience is whether neuronal circuits with variable circuit parameters that produce similar outputs respond comparably to equivalent perturbations.^1-4 Work on the pyloric rhythm of the crustacean stomatogastric ganglion (STG) showed that highly variable sets of intrinsic and synaptic conductances can generate similar circuit activity patterns.^5-9 Importantly, in response to physiologically relevant perturbations, these disparate circuit solutions can respond robustly and reliably,^10-12 but when exposed to extreme perturbations the underlying circuit parameter differences produce diverse patterns of disrupted activity.^7,12,13 In this example, the pyloric circuit is unchanged; only the conductance values vary. In contrast, the gastric mill rhythm in the STG can be generated by distinct circuits when activated by different modulatory neurons and/or neuropeptides.^14-21 Generally, these distinct circuits produce different gastric mill rhythms. However, the rhythms driven by stimulating modulatory commissural neuron 1 (MCN1) and bath-applying CabPK (Cancer borealis pyrokinin) peptide generate comparable output patterns, despite having distinct circuits that use separate cellular and synaptic mechanisms.^22-25 Here, we use these two gastric mill circuits to determine whether such circuits respond comparably when challenged with persisting (hormonal: CCAP) or acute (sensory: GPR neuron) metabotropic influences. Surprisingly, the hormone-mediated action separates these two rhythms despite activating the same ionic current in the same circuit neuron during both rhythms, whereas the sensory neuron evokes comparable responses despite acting via different synapses during each rhythm. These results highlight the need for caution when inferring the circuit response to a perturbation when that circuit is not well defined physiologically.

Mass spectrometry profiling and quantitation of changes in circulating hormones secreted over time in Cancer borealis hemolymph due to feeding behavior

The crustacean stomatogastric ganglion (STG) is a valuable model for understanding circuit dynamics in neuroscience as it contains a small number of neurons, all easily distinguishable and most of which contribute to two complementary feeding-related neural circuits. These circuits are modulated by numerous neuropeptides, with many gaining access to the STG as hemolymph-transported hormones. Previous work characterized neuropeptides in the hemolymph of the crab Cancer borealis but was limited by low peptide abundance in the presence of a complex biological matrix and the propensity for rapid peptide degradation. To improve their detection, a data-independent acquisition (DIA) mass spectrometry (MS) method was implemented. This approach improved the number of neuropeptides detected by approximately twofold and showed greater reproducibility between experimental and biological replicates. This method was then used to profile neuropeptides at different stages of the feeding process, including hemolymph from crabs that were unfed, or 0 min, 15 min, 1 h, and 2 h post-feeding. The results show differences both in the presence and relative abundance of neuropeptides at the various time points. Additionally, 96 putative neuropeptide sequences were identified with de novo sequencing, indicating there may be more key modulators within this system than is currently known. These results suggest that a distinct cohort of neuropeptides provides modulation to the STG at different times in the feeding process, providing groundwork for targeted follow-up electrophysiological studies to better understand the functional role of circulating hormones in the neural basis of feeding behavior.

Mass Spectrometry Quantification, Localization, and Discovery of Feeding-Related Neuropeptides in Cancer borealis

The crab Cancer borealis nervous system is an important model for understanding neural circuit dynamics and modulation, but the identity of neuromodulatory substances and their influence on circuit dynamics in this system remains incomplete, particularly with respect to behavioral state-dependent modulation. Therefore, we used a multifaceted mass spectrometry (MS) method to identify neuropeptides that differentiate the unfed and fed states. Duplex stable isotope labeling revealed that the abundance of 80 of 278 identified neuropeptides was distinct in ganglia and/or neurohemal tissue from fed vs unfed animals. MS imaging revealed that an additional 7 and 11 neuropeptides exhibited altered spatial distributions in the brain and the neuroendocrine pericardial organs (POs), respectively, during these two feeding states. Furthermore, de novo sequencing yielded 69 newly identified putative neuropeptides that may influence feeding state-related neuromodulation. Two of these latter neuropeptides were determined to be upregulated in PO tissue from fed crabs, and one of these two peptides influenced heartbeat in ex vivo preparations. Overall, the results presented here identify a cohort of neuropeptides that are poised to influence feeding-related behaviors, providing valuable opportunities for future functional studies.

Coupling between fast and slow oscillator circuits in Cancer borealis is temperature-compensated

Coupled oscillatory circuits are ubiquitous in nervous systems. Given that most biological processes are temperature-sensitive, it is remarkable that the neuronal circuits of poikilothermic animals can maintain coupling across a wide range of temperatures. Within the stomatogastric ganglion (STG) of the crab, Cancer borealis, the fast pyloric rhythm (~1 Hz) and the slow gastric mill rhythm (~0.1 Hz) are precisely coordinated at ~11°C such that there is an integer number of pyloric cycles per gastric mill cycle (integer coupling). Upon increasing temperature from 7°C to 23°C, both oscillators showed similar temperature-dependent increases in cycle frequency, and integer coupling between the circuits was conserved. Thus, although both rhythms show temperature-dependent changes in rhythm frequency, the processes that couple these circuits maintain their coordination over a wide range of temperatures. Such robustness to temperature changes could be part of a toolbox of processes that enables neural circuits to maintain function despite global perturbations.

Molecular cloning of crustacean hyperglycemic hormone (CHH) family members (CHH, molt-inhibiting hormone and mandibular organ-inhibiting hormone) and their expression levels in the Jonah crab, Cancer borealis

The crustacean hyperglycemic hormone (CHH) neuropeptide family has multiple functions in the regulation of hemolymph glucose levels, molting, ion, and water balance and reproduction. In crab species, three neuroendocrine tissues: the eyestalk ganglia (medulla terminalis X-organ and -sinus gland = ES), the pericardial organ (PO), and guts synthesize a tissue-specific isoforms of CHH neuropeptides. Recently the presence of the mandibular organ-inhibiting hormone (MOIH) was reported in the stomatogastric nervous system (STNS) that regulates the rhythmic muscle movements in esophagus, cardiac sac, gastric and pyloric ports of the foregut. In this study, we aimed to determine the presence of a tissue-specific CHH isoform in the Jonah crab, Cancer borealis using PCR with degenerate primers and 5', 3' rapid amplification of cDNA ends (RACE) in the ES. PO, and STNS. The analysis of CHH sequences shows that C. borealis has one type of CHH isoform, unlike other crab species. We also isolated the cDNA sequence of molt-inhibiting hormone (MIH) in the ES and MOIH in the ES and STNS. The presence of CHH, MOIH and MIH in the sinus gland of adult females and males is confirmed by using a dot-blot assay with the putative peaks collected from RP-HPLC and anti-Cancer sera for CHH, MIH, and MOIH. The present of crustacean female sex hormone (CFSH) in the sinus gland of adult females was examined with a dot-blot assay with anti-Callinectes CFSH serum. Levels of CHH, MOIH, and MIH in the sinus gland and their expressions in the eyestalk ganglia are estimated in the adult males, where CHH is the predominant form among these neuropeptides.

PRESnovo: Prescreening Prior to de novo Sequencing to Improve Accuracy and Sensitivity of Neuropeptide Identification

Identification of peptides in species lacking fully sequenced genomes is challenging due to the lack of prior knowledge. De novo sequencing is the method of choice, but its performance is less than satisfactory due to algorithmic bias and interference in complex MS/MS spectra. The task becomes even more challenging for endogenous peptides that do not involve an enzymatic digestion step, such as neuropeptides. However, many neuropeptides possess common sequence motifs that are conserved across members of the same family. Taking advantage of this feature to improve de novo sequencing of neuropeptides, we have developed a method named PRESnovo (prescreening precursors prior to de novo sequencing) to predict the motif from a MS/MS spectrum. A neuropeptide sequence is broken into a motif with conserved amino acid residues and the remaining partial sequence. By searching against a predefined motif database constructed from known homologous sequences, PRESnovo assigns the most probable motif to each precursor via a sophisticated scoring function. Performance analysis was conducted with 15 neuropeptide standards, and 11 neuropeptides were correctly identified with PRESnovo compared to 1 identification by PEAKS only. We applied PRESnovo to assign motifs to peptide sequences in conjunction with PEAKS for assigning the rest of the peptide sequence in order to discover neuropeptides in tissue samples of green crab, C. maenas, and Jonah crab, C. borealis. Collectively, a large number of neuropeptides were identified, including 13 putative neuropeptides identified in green crab brain, 77 in Jonah crab brain, and 47 in Jonah crab sinus glands for the first time. This PRESnovo strategy greatly simplifies de novo sequencing and enhances the accuracy and sensitivity of neuropeptide identification when common motifs are present.

To what extent may peptide receptor gene diversity/complement contribute to functional flexibility in a simple pattern-generating neural network?

Peptides are known to contribute to central pattern generator (CPG) flexibility throughout the animal kingdom. However, the role played by receptor diversity/complement in determining this functional flexibility is not clear. The stomatogastric ganglion (STG) of the crab, Cancer borealis, contains CPGs that are models for investigating peptidergic control of rhythmic behavior. Although many Cancer peptides have been identified, their peptide receptors are largely unknown. Thus, the extent to which receptor diversity/complement contributes to modulatory flexibility in this system remains unresolved. Here, a Cancer mixed nervous system transcriptome was used to determine the peptide receptor complement for the crab nervous system as a whole. Receptors for 27 peptide families, including multiple receptors for some groups, were identified. To increase confidence in the predicted sequences, receptors for allatostatin-A, allatostatin-B, and allatostatin-C were cloned, sequenced, and expressed in an insect cell line; as expected, all three receptors trafficked to the cell membrane. RT-PCR was used to determine whether each receptor was expressed in the Cancer STG. Transcripts for 36 of the 46 identified receptors were amplified; these included at least one for each peptide family except RYamide. Finally, two peptides untested on the crab STG were assessed for their influence on its motor outputs. Myosuppressin, for which STG receptors were identified, exhibited clear modulatory effects on the motor patterns of the ganglion, while a native RYamide, for which no STG receptors were found, elicited no consistent modulatory effects. These data support receptor diversity/complement as a major contributor to the functional flexibility of CPGs.

Similarities and differences in circuit responses to applied Gly1-SIFamide and peptidergic (Gly1-SIFamide) neuron stimulation

Microcircuit modulation by peptides is well established, but the cellular/synaptic mechanisms whereby identified neurons with identified peptide transmitters modulate microcircuits remain unknown for most systems. Here, we describe the distribution of GYRKPPFNGSIFamide (Gly1-SIFamide) immunoreactivity (Gly1-SIFamide-IR) in the stomatogastric nervous system (STNS) of the crab Cancer borealis and the Gly1-SIFamide actions on the two feeding-related circuits in the stomatogastric ganglion (STG). Gly1-SIFamide-IR localized to somata in the paired commissural ganglia (CoGs), two axons in the nerves connecting each CoG with the STG, and the CoG and STG neuropil. We identified one Gly1-SIFamide-IR projection neuron innervating the STG as the previously identified modulatory commissural neuron 5 (MCN5). Brief (~10 s) MCN5 stimulation excites some pyloric circuit neurons. We now find that bath applying Gly1-SIFamide to the isolated STG also enhanced pyloric rhythm activity and activated an imperfectly coordinated gastric mill rhythm that included unusually prolonged bursts in two circuit neurons [inferior cardiac (IC), lateral posterior gastric (LPG)]. Furthermore, longer duration (>30 s) MCN5 stimulation activated a Gly1-SIFamide-like gastric mill rhythm, including prolonged IC and LPG bursting. The prolonged LPG bursting decreased the coincidence of its activity with neurons to which it is electrically coupled. We also identified local circuit feedback onto the MCN5 axon terminals, which may contribute to some distinctions between the responses to MCN5 stimulation and Gly1-SIFamide application. Thus, MCN5 adds to the few identified projection neurons that modulate a well-defined circuit at least partly via an identified neuropeptide transmitter and provides an opportunity to study peptide regulation of electrical coupled neurons in a functional context. NEW & NOTEWORTHY Limited insight exists regarding how identified peptidergic neurons modulate microcircuits. We show that the modulatory projection neuron modulatory commissural neuron 5 (MCN5) is peptidergic, containing Gly1-SIFamide. MCN5 and Gly1-SIFamide elicit similar output from two well-defined motor circuits. Their distinct actions may result partly from circuit feedback onto the MCN5 axon terminals. Their similar actions include eliciting divergent activity patterns in normally coactive, electrically coupled neurons, providing an opportunity to examine peptide modulation of electrically coupled neurons in a functional context.

AMGSEFLamide, a member of a broadly conserved peptide family, modulates multiple neural networks in Homarus americanus

Recent genomic/transcriptomic studies have identified a novel peptide family whose members share the carboxyl terminal sequence -GSEFLamide. However, the presence/identity of the predicted isoforms of this peptide group have yet to be confirmed biochemically, and no physiological function has yet been ascribed to any member of this peptide family. To determine the extent to which GSEFLamides are conserved within the Arthropoda, we searched publicly accessible databases for genomic/transcriptomic evidence of their presence. GSEFLamides appear to be highly conserved within the Arthropoda, with the possible exception of the Insecta, in which sequence evidence was limited to the more basal orders. One crustacean in which GSEFLamides have been predicted using transcriptomics is the lobster, Homarus americanus Expression of the previously published transcriptome-derived sequences was confirmed by reverse transcription (RT)-PCR of brain and eyestalk ganglia cDNAs; mass spectral analyses confirmed the presence of all six of the predicted GSEFLamide isoforms - IGSEFLamide, MGSEFLamide, AMGSEFLamide, VMGSEFLamide, ALGSEFLamide and AVGSEFLamide - in H. americanus brain extracts. AMGSEFLamide, of which there are multiple copies in the cloned transcripts, was the most abundant isoform detected in the brain. Because the GSEFLamides are present in the lobster nervous system, we hypothesized that they might function as neuromodulators, as is common for neuropeptides. We thus asked whether AMGSEFLamide modulates the rhythmic outputs of the cardiac ganglion and the stomatogastric ganglion. Physiological recordings showed that AMGSEFLamide potently modulates the motor patterns produced by both ganglia, suggesting that the GSEFLamides may serve as important and conserved modulators of rhythmic motor activity in arthropods.

Neuropeptide discovery in Proasellus cavaticus: Prediction of the first large-scale peptidome for a member of the Isopoda using a publicly accessible transcriptome

In silico transcriptome mining is one of the most effective methods for neuropeptide discovery in crustaceans, particularly for species that are small, rare or from geographically inaccessible habitats that make obtaining the large pools of tissue needed for other peptide discovery platforms impractical. Via this approach, large peptidomes have recently been described for members of many of the higher crustacean taxa, one notable exception being the Isopoda; no peptidome has been predicted for any member of this malacostracan order. Using a publicly accessible transcriptome for the isopod Proasellus cavaticus, a subcentimeter subterranean ground water dweller, the first in silico-predicted peptidome for a member of the Isopoda is presented here. BLAST searches employing known arthropod neuropeptide pre/preprohormone queries identified 49 transcripts as encoding putative homologs within the P. cavaticus transcriptome. The proteins deduced from these transcripts allowed for the prediction of 171 distinct mature neuropeptides. The P. cavaticus peptidome includes members of the adipokinetic hormone-corazonin-like peptide, allatostatin A, allatostatin B, allatostatin C, allatotropin, bursicon α, bursicon β, CCHamide, crustacean cardioactive peptide, crustacean hyperglycemic hormone/molt-inhibiting hormone, diuretic hormone 31, eclosion hormone, elevenin, FMRFamide-like peptide, glycoprotein hormone α2, leucokinin, myosuppressin, neuroparsin, neuropeptide F, pigment dispersing hormone, pyrokinin, red pigment concentrating hormone, RYamide, short neuropeptide F, sulfakinin, tachykinin-related peptide and trissin families, as well as many linker/precursor-related sequences that may or may not represent additional bioactive molecules. Interestingly, many of the predicted P. cavaticus neuropeptides possess structures identical (or nearly so) to those previously described from members of several other malacostracan orders, i.e., the Decapoda, Amphipoda and Euphausiacea, a finding that suggests broad phylogenetic conservation of bioactive peptide structures, and possibly functions, may exist within the Malacostraca.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Peptidergic signaling in the crab Cancer borealis: Tapping the power of transcriptomics for neuropeptidome expansion

The crab Cancer borealis has long been used as a model for understanding neural control of rhythmic behavior. One significant discovery made through its use is that even numerically simple neural circuits are capable of producing an essentially infinite array of distinct motor outputs via the actions of locally released and circulating neuromodulators, the largest class being peptides. While much work has focused on elucidating the peptidome of C. borealis, no investigation has used in silico transcriptome mining for peptide discovery in this species, a strategy proven highly effective for identifying neuropeptides in other crustaceans. Here, we mined a C. borealis neural transcriptome for putative peptide-encoding transcripts, and predicted 200 distinct mature neuropeptides from the proteins deduced from these sequences. The identified peptides include isoforms of allatostatin A, allatostatin B, allatostatin C, CCHamide, crustacean cardioactive peptide, crustacean hyperglycemic hormone, diuretic hormone 31 (DH31), diuretic hormone 44 (DH44), FMRFamide-like peptide, GSEFLamide, HIGSLYRamide, insulin-like peptide (ILP), intocin, leucokinin, neuroparsin, pigment dispersing hormone, pyrokinin, red pigment concentrating hormone, short neuropeptide F and SIFamide. While some of the predicted peptides were known previously from C. borealis, most (159) are new discoveries for the species, e.g., the isoforms of CCHamide, DH31, DH44, GSEFLamide, ILP, intocin and neuroparsin, which are the first members of these peptide families identified from C. borealis. Collectively, the peptides predicted here approximately double the peptidome known for C. borealis, and in so doing provide an expanded platform from which to launch new investigations of peptidergic neuromodulation in this species.

Neuropeptide modulation of pattern-generating systems in crustaceans: comparative studies and approaches

Central pattern generators are subject to modulation by peptides, allowing for flexibility in patterned output. Current techniques used to characterize peptides include mass spectrometry and transcriptomics. In recent years, hundreds of neuropeptides have been sequenced from crustaceans; mass spectrometry has been used to identify peptides and to determine their levels and locations, setting the stage for comparative studies investigating the physiological roles of peptides. Such studies suggest that there is some evolutionary conservation of function, but also divergence of function even within a species. With current baseline data, it should be possible to begin using comparative approaches to ask fundamental questions about why peptides are encoded the way that they are and how this affects nervous system function.

Neuropeptidergic Signaling in the American Lobster Homarus americanus: New Insights from High-Throughput Nucleotide Sequencing

Peptides are the largest and most diverse class of molecules used for neurochemical communication, playing key roles in the control of essentially all aspects of physiology and behavior. The American lobster, Homarus americanus, is a crustacean of commercial and biomedical importance; lobster growth and reproduction are under neuropeptidergic control, and portions of the lobster nervous system serve as models for understanding the general principles underlying rhythmic motor behavior (including peptidergic neuromodulation). While a number of neuropeptides have been identified from H. americanus, and the effects of some have been investigated at the cellular/systems levels, little is currently known about the molecular components of neuropeptidergic signaling in the lobster. Here, a H. americanus neural transcriptome was generated and mined for sequences encoding putative peptide precursors and receptors; 35 precursor- and 41 receptor-encoding transcripts were identified. We predicted 194 distinct neuropeptides from the deduced precursor proteins, including members of the adipokinetic hormone-corazonin-like peptide, allatostatin A, allatostatin C, bursicon, CCHamide, corazonin, crustacean cardioactive peptide, crustacean hyperglycemic hormone (CHH), CHH precursor-related peptide, diuretic hormone 31, diuretic hormone 44, eclosion hormone, FLRFamide, GSEFLamide, insulin-like peptide, intocin, leucokinin, myosuppressin, neuroparsin, neuropeptide F, orcokinin, pigment dispersing hormone, proctolin, pyrokinin, SIFamide, sulfakinin and tachykinin-related peptide families. While some of the predicted peptides are known H. americanus isoforms, most are novel identifications, more than doubling the extant lobster neuropeptidome. The deduced receptor proteins are the first descriptions of H. americanus neuropeptide receptors, and include ones for most of the peptide groups mentioned earlier, as well as those for ecdysis-triggering hormone, red pigment concentrating hormone and short neuropeptide F. Multiple receptors were identified for most peptide families. These data represent the most complete description of the molecular underpinnings of peptidergic signaling in H. americanus, and will serve as a foundation for future gene-based studies of neuropeptidergic control in the lobster.

Superfamily of genes encoding G protein-coupled receptors in the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae)

G protein-coupled receptors (GPCRs) are the largest and most versatile superfamily of cell membrane proteins, which mediate various physiological processes including reproduction, development and behaviour. The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), is one of the most notorious insect pests, preferentially feeding on cruciferous plants. P. xylostella is not only one of the world's most widespread lepidopteran insects, but has also developed resistance to nearly all classes of insecticides. Although the mechanisms of insecticide resistance have been studied extensively in many insect species, few investigations have been carried out on GPCRs in P. xylostella. In the present study, we identified 95 putative GPCRs in the P. xylostella genome. The identified GPCRs were compared with their homologues in Bombyx mori and Drosophila melanogaster. Our results suggest that GPCRs in different insect species may have evolved by a birth-and-death process. One of the differences among compared insects is the duplication of short neuropeptide F receptor and adipokinetic hormone receptors in P. xylostella and B. mori. Another divergence is the decrease in quantity and diversity of the stress-tolerance gene, Mth, in P. xylostella. The evolution by the birth-and-death process is probably involved in adaptation to the feeding behaviour, reproduction and stress responses of P. xylostella. Some of the genes identified in the present study could be potential targets for the development of novel pesticides.

Distinct or shared actions of peptide family isoforms: I. Peptide-specific actions of pyrokinins in the lobster cardiac neuromuscular system

Although the crustacean heart is modulated by a large number of peptides and amines, few of these molecules have been localized to the cardiac ganglion itself; most appear to reach the cardiac ganglion only by hormonal routes. Immunohistochemistry in the American lobster Homarus americanus indicates that pyrokinins are present not only in neuroendocrine organs (pericardial organ and sinus gland), but also in the cardiac ganglion itself, where pyrokinin-positive terminals were found in the pacemaker cell region, as well as surrounding the motor neurons. Surprisingly, the single pyrokinin peptide identified from H. americanus, FSPRLamide, which consists solely of the conserved FXPRLamide residues that characterize pyrokinins, did not alter the activity of the cardiac neuromuscular system. However, a pyrokinin from the shrimp Litopenaeus vannamei [ADFAFNPRLamide, also known as Penaeus vannamei pyrokinin 2 (PevPK2)] increased both the frequency and amplitude of heart contractions when perfused through the isolated whole heart. None of the other crustacean pyrokinins tested (another from L. vannamei and two from the crab Cancer borealis) had any effect on the lobster heart. Similarly, altering the PevPK2 sequence either by truncation or by the substitution of single amino acids resulted in much lower or no activity in all cases; only the conservative substitution of serine for alanine at position 1 resulted in any activity on the heart. Thus, in contrast to other systems (cockroach and crab) in which all tested pyrokinins elicit similar bioactivities, activation of the pyrokinin receptor in the lobster heart appears to be highly isoform specific.

Distinct or shared actions of peptide family isoforms: II. Multiple pyrokinins exert similar effects in the lobster stomatogastric nervous system

Many neuropeptides are members of peptide families, with multiple structurally similar isoforms frequently found even within a single species. This raises the question of whether the individual peptides serve common or distinct functions. In the accompanying paper, we found high isoform specificity in the responses of the lobster (Homarus americanus) cardiac neuromuscular system to members of the pyrokinin peptide family: only one of five crustacean isoforms showed any bioactivity in the cardiac system. Because previous studies in other species had found little isoform specificity in pyrokinin actions, we examined the effects of the same five crustacean pyrokinins on the lobster stomatogastric nervous system (STNS). In contrast to our findings in the cardiac system, the effects of the five pyrokinin isoforms on the STNS were indistinguishable: they all activated or enhanced the gastric mill motor pattern, but did not alter the pyloric pattern. These results, in combination with those from the cardiac ganglion, suggest that members of a peptide family in the same species can be both isoform specific and highly promiscuous in their modulatory capacity. The mechanisms that underlie these differences in specificity have not yet been elucidated; one possible explanation, which has yet to be tested, is the presence and differential distribution of multiple receptors for members of this peptide family.

Consequences of acute and long-term removal of neuromodulatory input on the episodic gastric rhythm of the crab Cancer borealis

For decades, the episodic gastric rhythm of the crustacean stomatogastric nervous system (STNS) has served as an important model system for understanding the generation of rhythmic motor behaviors. Here we quantitatively describe many features of the gastric rhythm of the crab Cancer borealis under several conditions. First, we analyzed spontaneous gastric rhythms produced by freshly dissected preparations of the STNS, including the cycle frequency and phase relationships among gastric units. We find that phase is relatively conserved across frequency, similar to the pyloric rhythm. We also describe relationships between these two rhythms, including a significant gastric/pyloric frequency correlation. We then performed continuous, days-long extracellular recordings of gastric activity from preparations of the STNS in which neuromodulatory inputs to the stomatogastric ganglion were left intact and also from preparations in which these modulatory inputs were cut (decentralization). This allowed us to provide quantitative descriptions of variability and phase conservation within preparations across time. For intact preparations, gastric activity was more variable than pyloric activity but remained relatively stable across 4-6 days, and many significant correlations were found between phase and frequency within animals. Decentralized preparations displayed fewer episodes of gastric activity, with altered phase relationships, lower frequencies, and reduced coordination both among gastric units and between the gastric and pyloric rhythms. Together, these results provide insight into the role of neuromodulation in episodic pattern generation and the extent of animal-to-animal variability in features of spontaneously occurring gastric rhythms.

Molecular and pharmacological characterization of the Chelicerata pyrokinin receptor from the southern cattle tick, Rhipicephalus (Boophilus) microplus

We identified the first pyrokinin receptor (Rhimi-PKR) in Chelicerata and analyzed structure-activity relationships of cognate ligand neuropeptides and their analogs. Based on comparative and phylogenetic analyses, this receptor, which we cloned from larvae of the cattle tick Rhipicephalus microplus (Acari: Ixodidae), is the ortholog of the insect pyrokinin (PK)/pheromone biosynthesis activating neuropeptide (PBAN)/diapause hormone (DH) neuropeptide family receptor. Rhimi-PKR functional analyses using calcium bioluminescence were performed with a developed stable recombinant CHO-K1 cell line. Rhimi-PKR was activated by four endogenous PKs from the Lyme disease vector, the tick Ixodes scapularis (EC50s range: 85.4 nM-546 nM), and weakly by another tick PRX-amide peptide, periviscerokinin (PVK) (EC50 = 24.5 μM). PK analogs with substitutions of leucine, isoleucine or valine at the C-terminus for three tick PK peptides, Ixosc-PK1, Ixosc-PK2, and Ixosc-PK3, retained their potency on Rhimi-PKR. Therefore, Rhimi-PKR is less selective and substantially more tolerant than insect PK receptors of C-terminal substitutions of leucine to isoleucine or valine, a key structural feature that serves to distinguish insect PK from PVK/CAP2b receptors. In females, ovary and synganglion had the highest Rhimi-PKR relative transcript abundance followed by the rectal sac, salivary glands, Malpighian tubules, and midgut. This is the first pharmacological analysis of a PK/PBAN/DH-like receptor from the Chelicerata, which will now permit the discovery of the endocrinological roles of this neuropeptide family in vectors of vertebrate pathogens.

Neuromodulation of circuits with variable parameters: single neurons and small circuits reveal principles of state-dependent and robust neuromodulation

Neuromodulation underlies many behavioral states and has been extensively studied in small circuits. This has allowed the systematic exploration of how neuromodulatory substances and the neurons that release them can influence circuit function. The physiological state of a network and its level of activity can have profound effects on how the modulators act, a phenomenon known as state dependence. We provide insights from experiments and computational work that show how state dependence can arise and the consequences it can have for cellular and circuit function. These observations pose a general unsolved question that is relevant to all nervous systems: How is robust modulation achieved in spite of animal-to-animal variability and degenerate, nonlinear mechanisms for the production of neuronal and network activity?

Mass spectrometric analysis of spatio-temporal dynamics of crustacean neuropeptides

Neuropeptides represent one of the largest classes of signaling molecules used by nervous systems to regulate a wide range of physiological processes. Over the past several years, mass spectrometry (MS)-based strategies have revolutionized the discovery of neuropeptides in numerous model organisms, especially in decapod crustaceans. Here, we focus our discussion on recent advances in the use of MS-based techniques to map neuropeptides in the spatial domain and monitoring their dynamic changes in the temporal domain. These MS-enabled investigations provide valuable information about the distribution, secretion and potential function of neuropeptides with high molecular specificity and sensitivity. In situ MS imaging and in vivo microdialysis are highlighted as key technologies for probing spatio-temporal dynamics of neuropeptides in the crustacean nervous system. This review summarizes the latest advancement in MS-based methodologies for neuropeptide analysis including typical workflow and sample preparation strategies as well as major neuropeptide families discovered in decapod crustaceans. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Convergent rhythm generation from divergent cellular mechanisms

Different modulatory inputs commonly elicit distinct rhythmic motor patterns from a central pattern generator (CPG), but they can instead elicit the same pattern. We are determining the rhythm-generating mechanisms in this latter situation, using the gastric mill (chewing) CPG in the crab (Cancer borealis) stomatogastric ganglion, where stimulating the projection neuron MCN1 (modulatory commissural neuron 1) or bath applying CabPK (C. borealis pyrokinin) peptide elicits the same gastric mill motor pattern, despite configuring different gastric mill circuits. In both cases, the core rhythm generator includes the same reciprocally inhibitory neurons LG (lateral gastric) and Int1 (interneuron 1), but the pyloric (food-filtering) circuit pacemaker neuron AB (anterior burster) is additionally necessary only for CabPK rhythm generation. MCN1 drives this rhythm generator by activating in the LG neuron the modulator-activated inward current (IMI), which waxes and wanes periodically due to phasic feedback inhibition of MCN1 transmitter release. Each buildup of IMI enables the LG neuron to generate a self-terminating burst and thereby alternate with Int1 activity. Here we establish that CabPK drives gastric mill rhythm generation by activating in the LG neuron IMI plus a slowly activating transient, low-threshold inward current (ITrans-LTS) that is voltage, time, and Ca(2+) dependent. Unlike MCN1, CabPK maintains a steady IMI activation, causing a subthreshold depolarization in LG that facilitates a periodic postinhibitory rebound burst caused by the regular buildup and decay of the availability of ITrans-LTS. Thus, different modulatory inputs can use different rhythm-generating mechanisms to drive the same neuronal rhythm. Additionally, the same ionic current (IMI) can play different roles under these different conditions, while different currents (IMI, ITrans-LTS) can play the same role.

Identification and characterization of the pyrokinin/pheromone biosynthesis activating neuropeptide family of G protein-coupled receptors from Ostrinia nubilalis

Insects have two closely related G protein-coupled receptors belonging to the pyrokinin/pheromone biosynthesis activating neuropeptide (pyrokinin/PBAN) family, one with the ligand PBAN or pyrokinin-2 and another with diapause hormone or pyrokinin-1 as a ligand. A related receptor is activated by products of the capa gene, periviscerokinins. Here we characterized the PBAN receptor and the diapause hormone receptor from the European corn borer, Ostrinia nubilalis. We also identified a partial sequence for the periviscerokinin receptor. Quantitative PCR of mRNA for all three receptors indicated differential expression in various life stages and tissues. All three splice variants of the PBAN receptor were identified with all variants found in pheromone gland tissue. Immunohistochemistry of V5 tags of expressed receptors indicated that all three variants and the diapause hormone receptor were expressed at similar levels in Spodoptera frugiperda 9 (Sf9) cells. However, the A- and B-variants were not active in our functional assay, which confirms studies from other moths. Functional expression of the C-variant indicated that it is has a 44 nM half effective concentration for activation by PBAN. The diapause hormone receptor was activated by diapause hormone with a 150 nM half effective concentration.

Neuromodulation of neuronal circuits: back to the future

All nervous systems are subject to neuromodulation. Neuromodulators can be delivered as local hormones, as cotransmitters in projection neurons, and through the general circulation. Because neuromodulators can transform the intrinsic firing properties of circuit neurons and alter effective synaptic strength, neuromodulatory substances reconfigure neuronal circuits, often massively altering their output. Thus, the anatomical connectome provides a minimal structure and the neuromodulatory environment constructs and specifies the functional circuits that give rise to behavior.

Variability in step training enhances locomotor recovery after a spinal cord injury

Performance of a motor task is improved by practicing a specific task with added 'challenges' to a training regimen. We tested the hypothesis that, in the absence of brain control, the performance of a motor task is enhanced by training using specific variations of that task. We utilized modifications of step performance training to improve the ability of spinal rats to forward step. After a complete thoracic spinal cord transection, 20 adult rats were divided randomly to bipedally step on a treadmill in the forward, sideward, or backward direction for 28 sessions (20 min, 5 days/week) and subsequently tested for their ability to step in the forward direction. Although the animals from all trained groups showed improvement, the rats in the sideward-trained and backward-trained groups had greater step consistency and coordination along with higher peak amplitudes and total integrated activity of the rectified electromyographic signals from selected hindlimb muscles per step during forward stepping than the rats in the forward-trained group. Our results demonstrate that, by retaining the fundamental features of a motor task (bipedal stepping), the ability to perform that motor task can be enhanced by the addition of specific contextual variations to the task (direction of stepping). Our data suggest that the forward stepping neuronal locomotor networks are partially complemented by synchronous activation of interneuronal/motoneuronal populations that are also a part of the sideward or backward stepping locomotor networks. Accordingly, the overlap and interaction of neuronal elements may play a critical role in positive task transference.

Feeding and the rhodopsin family g-protein coupled receptors in nematodes and arthropods

In vertebrates, receptors of the rhodopsin G-protein coupled superfamily (GPCRs) play an important role in the regulation of feeding and energy homeostasis and are activated by peptide hormones produced in the brain-gut axis. These peptides regulate appetite and energy expenditure by promoting or inhibiting food intake. Sequence and function homologs of human GPCRs involved in feeding exist in the nematode roundworm, Caenorhabditis elegans (C. elegans), and the arthropod fruit fly, Drosophila melanogaster (D. melanogaster), suggesting that the mechanisms that regulate food intake emerged early and have been conserved during metazoan radiation. Nematodes and arthropods are the most diverse and successful animal phyla on Earth. They can survive in a vast diversity of environments and have acquired distinct life styles and feeding strategies. The aim of the present review is to investigate if this diversity has affected the evolution of invertebrate GPCRs. Homologs of the C. elegans and D. melanogaster rhodopsin receptors were characterized in the genome of other nematodes and arthropods and receptor evolution compared. With the exception of bombesin receptors (BBR) that are absent from nematodes, a similar gene complement was found. In arthropods, rhodopsin GPCR evolution is characterized by species-specific gene duplications and deletions and in nematodes by gene expansions in species with a free-living stage and gene deletions in representatives of obligate parasitic taxa. Based upon variation in GPCR gene number and potentially divergent functions within phyla we hypothesize that life style and feeding diversity practiced by nematodes and arthropods was one factor that contributed to rhodopsin GPCR gene evolution. Understanding how the regulation of food intake has evolved in invertebrates will contribute to the development of novel drugs to control nematodes and arthropods and the pests and diseases that use them as vectors.

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

The G protein-coupled receptors in the silkworm, Bombyx mori

G protein-coupled receptors (GPCRs) are the largest and most versatile family of transmembrane receptors in the cell, occupying the highest hierarchical positions in the regulation of many physiological processes. Although they have been extensively studied in a number of model insects, there have been few investigations of GPCRs in large Lepidopterans, such as Bombyx mori, an organism that provides a means to perform detailed tissue expression analyses, which may help to characterize GPCRs and their ligands. In addition, B. mori, also known as the silkworm, is an insect of substantial economic importance, due to its use in silk production and traditional medicines. In this work, we computationally identified 90 putative GPCRs in B. mori, 33 of which represent novel proteins. These GPCRs were annotated and compared with their homologs in Drosophila melanogaster and Anopheles gambiae. Phylogenetics analyses of the GPCRs from these three insects showed that GPCRs may easily duplicate or disappear during insect evolution, especially in the neuropeptide and protein hormone receptor subfamily. Interestingly, we observed a decrease in the quantity and diversity of the stress-tolerance gene, Methuselah, in B. mori, which may be related to its long history of domestication. Moreover, the presence of many Bombyx-specific GPCRs suggests that neither Drosophila nor Anopheles is good representatives for the GPCRs in the Class Insecta.

Hormonal modulation of sensorimotor integration

Neuronal circuits commonly receive simultaneous inputs from descending, ascending, and hormonal systems. Thus far, however, most such inputs have been studied individually to determine their influence on a given circuit. Here, we examine the integrated action of the hormone crustacean cardioactive peptide (CCAP) and the gastropyloric receptor (GPR) proprioceptor neuron on the biphasic gastric mill (chewing) rhythm driven by the projection neuron modulatory commissural neuron 1 (MCN1) in the isolated crab stomatogastric ganglion. In control saline, GPR stimulation selectively prolongs the gastric mill retractor phase, via presynaptic inhibition of MCN1. In the absence of GPR stimulation, CCAP does not alter retraction duration and modestly prolongs protraction. Here, we show, using computational modeling and dynamic-clamp manipulations, that the presence of CCAP weakens or eliminates the GPR effect on the gastric mill rhythm. This CCAP action results from its ability to activate the same modulator-activated conductance (G(MI)) as MCN1 in the gastric mill circuit neuron lateral gastric (LG). Because GPR prolongs retraction by weakening MCN1 activation of G(MI) in LG, the parallel G(MI) activation by CCAP reduces the impact of GPR regulation of this conductance. The CCAP-activated G(MI) thus counteracts the GPR-mediated decrease in the MCN1-activated G(MI) in LG and reduces the GPR ability to regulate the gastric mill rhythm. Consequently, although CCAP neither changes retraction duration nor alters GPR inhibition of MCN1, its activation of a modulator-activated conductance in a pivotal downstream circuit neuron enables CCAP to weaken or eliminate sensory regulation of motor circuit output.

Comparative Neuropeptidomic Analysis of Food Intake via a Multi-faceted Mass Spectrometric Approach

Feeding behavior is a fundamental aspect of energy homeostasis and is crucial for animal survival. This process is regulated by a multitude of neurotransmitters including neuropeptides within a complex neuroendocrine system. Given the high chemical complexity and wide distribution of neuropeptides, the precise molecular mechanisms at the cellular and network levels remain elusive. Here we report comparative neuropeptidomic analysis of brain and major neuroendocrine organ in a crustacean model organism in response to feeding. A multi-faceted approach employing direct tissue matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), stable isotopic labeling of neuropeptide extracts for quantitation, and mass spectrometric imaging (MSI) has been employed to obtain complementary information on the expression changes of a large array of neuropeptides in the brain and the pericardial organ (PO) in the crab Cancer borealis. Multiple neuropeptides exhibited changes in abundance after feeding, including RFamides, Cancer borealis tachykinin related peptides (CabTRPs), RYamides, and pyrokinins. By combining quantitative analysis of neuropeptide changes via isotopic labeling of brain extract and MSI mapping of neuropeptides of brain slices, we identified the boundary of olfactory lobe (ON) and median protocerebrum (MPC) area as two potential feeding centers in the crab brain.

Combining in silico transcriptome mining and biological mass spectrometry for neuropeptide discovery in the Pacific white shrimp Litopenaeus vannamei

The shrimp Litopenaeus vannamei is arguably the most important aquacultured crustacean, being the subject of a multi-billion dollar industry worldwide. To extend our knowledge of peptidergic control in this species, we conducted an investigation combining transcriptomics and mass spectrometry to identify its neuropeptides. Specifically, in silico searches of the L. vannamei EST database were conducted to identify putative prepro-hormone-encoding transcripts, with the mature peptides contained within the deduced precursors predicted via online software programs and homology to known isoforms. MALDI-FT mass spectrometry was used to screen tissue fragments and extracts via accurate mass measurements for the predicted peptides, as well as for known ones from other species. ESI-Q-TOF tandem mass spectrometry was used to de novo sequence peptides from tissue extracts. In total 120 peptides were characterized using this combined approach, including 5 identified both by transcriptomics and by mass spectrometry (e.g. pQTFQYSRGWTNamide, Arg(7)-corazonin, and pQDLDHVFLRFamide, a myosuppressin), 49 predicted via transcriptomics only (e.g. pQIRYHQCYFNPISCF and pQIRYHQCYFIPVSCF, two C-type allatostatins, and RYLPT, authentic proctolin), and 66 identified solely by mass spectrometry (e.g. the orcokinin NFDEIDRAGMGFA). While some of the characterized peptides were known L. vannamei isoforms (e.g. the pyrokinins DFAFSPRLamide and ADFAFNPRLamide), most were novel, either for this species (e.g. pEGFYSQRYamide, an RYamide) or in general (e.g. the tachykinin-related peptides APAGFLGMRamide, APSGFNGMRamide and APSGFLDMRamide). Collectively, our data not only expand greatly the number of known L. vannamei neuropeptides, but also provide a foundation for future investigations of the physiological roles played by them in this commercially important species.

Monitoring neuropeptides in vivo via microdialysis and mass spectrometry

Neuropeptides are important signaling molecules that regulate many essential physiological processes. Microdialysis offers a way to sample neuropeptides in vivo. When combined with liquid chromatography-mass spectrometry detection, many known and unknown neuropeptides can be identified from a live organism. This chapter describes sample preparation techniques and general strategies for the mass spectral analysis of neuropeptides collected via microdialysis sampling. Methods for the in vitro microdialysis of a neuropeptide standard as well as the in vivo microdialysis sampling of neuropeptides from a live crab are described.

Immunoaffinity-based mass spectrometric characterization of the FMRFamide-related peptide family in the pericardial organ of Cancer borealis

The tetrapeptide, FMRFamide, was first discovered in 1977 in the molluscan nervous system and was found to affect the contractile force of molluscan cardiac muscle and other muscles. Since then, numerous FMRFamide-related peptides (FaRPs) have been reported in both invertebrate and vertebrate species. We have previously reported the detection and identification of numerous FaRPs in Cancer borealis pericardial organs (POs), one of the major neurosecretory structures in the crustaceans. Here, we have developed two immunoaffinity-based methods, immunoprecipitation (IP) and immuno-dot blot screening assay, for the enrichment of FaRPs in C. borealis POs. A combined mass spectrometry (MS)-based approach involving both matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS) and nanoscale liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-QTOF MS/MS) is used for a more comprehensive characterization of the FaRP family by utilizing high mass accuracy measurement and efficient peptide sequencing. Overall, 17 FMRFamide-related peptides were identified using these two complementary immuno-based approaches. Among them, three novel peptides were reported for the first time in this study.

Reliable neuromodulation from circuits with variable underlying structure

Recent work argues that similar network performance can result from highly variable sets of network parameters, raising the question of whether neuromodulation can be reliable across individuals with networks with different sets of synaptic strengths and intrinsic membrane conductances. To address this question, we used the dynamic clamp to construct 2-cell reciprocally inhibitory networks from gastric mill (GM) neurons of the crab stomatogastric ganglion. When the strength of the artificial inhibitory synapses (g(syn)) and the conductance of an artificial I(h) (g(h)) were varied with the dynamic clamp, a variety of network behaviors resulted, including regions of stable alternating bursting. Maps of network output as a function of g(syn) and g(h) were constructed in normal saline and again in the presence of serotonin or oxotremorine. Both serotonin and oxotremorine depolarize and excite isolated individual GM neurons, but by different cellular mechanisms. Serotonin and oxotremorine each increased the size of the parameter regions that supported alternating bursting, and, on average, increased burst frequency. Nonetheless, in both cases some parameter sets within the sample space deviated from the mean population response and decreased in frequency. These data provide insight into why pharmacological treatments that work in most individuals can generate anomalous actions in a few individuals, and they have implications for understanding the evolution of nervous systems.

Expanding the Crustacean neuropeptidome using a multifaceted mass spectrometric approach

Jonah crab Cancer borealis is an excellent, long-served model organism for many areas of physiology, including the study of endocrinology and neurobiology. Characterizing the neuropeptides present in its nervous system provides the first critical step toward understanding the physiological roles of these complex molecules. Multiple mass spectral techniques were used to comprehensively characterize the neuropeptidome in C. borealis, including matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS), MALDI time-of-flight (TOF)/TOF MS and nanoflow liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-Q-TOF MS/MS). To enhance the detection signals and expand the dynamic range, direct tissue analysis, tissue extraction, capillary electrophoresis (CE) and off-line HPLC separation have also been employed. In total, 142 peptides were identified, including 85 previously known C. borealis peptides, 22 peptides characterized previously from other decapods, but new to this species, and 35 new peptides de novo sequenced for the first time in this study. Seventeen neuropeptide families were revealed including FMRFamide-related peptide (FaRP), allatostatin (A and B type), RYamide, orcokinin, orcomyotropin, proctolin, crustacean cardioactive peptide (CCAP), crustacean hyperglycemic hormone precursor-related peptide (CPRP), crustacean hyperglycemic hormone (CHH), corazonin, pigment-dispersing hormone (PDH), tachykinin, pyrokinin, SIFamide, red pigment concentrating hormone (RPCH) and HISGLYRamide. Collectively, our results greatly increase the number and expand the coverage of known C. borealis neuropeptides, and thus provide a stronger framework for future studies on the physiological roles played by these molecules in this important model organism.

Identification of novel neuropeptides in the ventral nerve cord ganglia and their targets in an annelid worm, Eisenia fetida

Periviscerokinins (PVKs) and pyrokinins (PKs) are neuropeptides known in several arthropod species. Sequence homology of these peptides with the molluscan small cardioactive peptides reveals that the occurrence of PVKs and PKs is not restricted to arthropods. Our study focuses on the biochemical and immunocytochemical identification of neuropeptides with sequence homology to PVKs and PKs in the central and peripheral nervous system of the earthworm Eisenia fetida. By means of affinity chromatography, nanoflow liquid chromatography, and high accuracy mass spectrometry, six peptides, SPFPR(L/I)amide, APFPR(L/I)amide, SPLPR(L/I)amide, SFVR(L/I)amide, AFVR(L/I)amide, and SPAFVR(L/I)amide, were identified in the central nervous system with the common -XR(L/I)amide C-terminal sequence. The exact anatomical position of 13 labeled XR(I/L)amide expressing neuron groups and numerous peptide-containing fibers were determined by means of immunocytochemistry and confocal laser scanning microscopy in whole-mount preparations of ventral nerve cord ganglia. The majority of the stained neurons were interneurons with processes joining the distinct fine-fibered polysegmental tracts in the central neuropil. Some stained fibers were seen running in each segmental nerve that innervated metanephridia and body wall. Distinct groups of neurosecretory cells characterized by small round soma and short processes were also identified. Based on immunoelectron microscopy six different types of labeled cells were described showing morphological heterogeneity of earthworm peptides containing elements. Our findings confirm that the sequence of the identified earthworm neuropeptides homologous to the insect PVKs and PKs suggesting that these peptides are phylogenetically conservative molecules and are expressed in sister-groups of animals such as annelids, mollusks, and insects.

Characterization of the Carcinus maenas neuropeptidome by mass spectrometry and functional genomics

Carcinus maenas, commonly known as the European green crab, is one of the best-known and most successful marine invasive species. While a variety of natural and anthropogenic mechanisms are responsible for the geographic spread of this crab, its ability to adapt physiologically to a broad range of salinities, temperatures and other environmental factors has enabled its successful establishment in new habitats. To extend our understanding of hormonal control in C. maenas, including factors that allow for its extreme adaptability, we have undertaken a mass spectral/functional genomics investigation of the neuropeptides used by this organism. Via a strategy combining MALDI-based high resolution mass profiling, biochemical derivatization, and nanoscale separation coupled to tandem mass spectrometric sequencing, 122 peptide paracrines/hormones were identified from the C. maenas central nervous system and neuroendocrine organs. These peptides include 31 previously described Carcinus neuropeptides (e.g. NSELINSILGLPKVMNDAamide [beta-pigment dispersing hormone] and PFCNAFTGCamide [crustacean cardioactive peptide]), 49 peptides only described in species other than the green crab (e.g. pQTFQYSRGWTNamide [Arg(7)-corazonin]), and 42 new peptides de novo sequenced here for the first time (e.g. the pyrokinins TSFAFSPRLamide and DTGFAFSPRLamide). Of particular note are large collections of FMRFamide-like peptides (25, including nine new isoforms sequenced de novo) and A-type allatostatin peptides (25, including 10 new sequences reported here for the first time) in this study. Also of interest is the identification of two SIFamide isoforms, GYRKPPFNGSIFamide and VYRKPPFNGSIFamide, the latter peptide known previously only from members of the astacidean genus Homarus. Using transcriptome analyses, 15 additional peptides were characterized, including an isoform of bursicon beta and a neuroparsin-like peptide. Collectively, the data presented in this study not only greatly expand the number of identified C. maenas neuropeptides, but also provide a framework for future investigations of the physiological roles played by these molecules in this highly adaptable species.

State dependence of network output: modeling and experiments

Emerging experimental evidence suggests that both networks and their component neurons respond to similar inputs differently, depending on the state of network activity. The network state is determined by the intrinsic dynamical structure of the network and may change as a function of neuromodulation, the balance or stochasticity of synaptic inputs to the network, and the history of network activity. Much of the knowledge on state-dependent effects comes from comparisons of awake and sleep states of the mammalian brain. Yet, the mechanisms underlying these states are difficult to unravel. Several vertebrate and invertebrate studies have elucidated cellular and synaptic mechanisms of state dependence resulting from neuromodulation, sensory input, and experience. Recent studies have combined modeling and experiments to examine the computational principles that emerge when network state is taken into account; these studies are highlighted in this article. We discuss these principles in a variety of systems (mammalian, crustacean, and mollusk) to demonstrate the unifying theme of state dependence of network output.

In silico analyses of peptide paracrines/hormones in Aphidoidea

The Aphidoidea is an insect superfamily comprising most of the known aphid species. While small in size, these animals are of considerable economic importance as many members of this taxon are serious agricultural pests, inflicting physical damage upon crop plants and serving as vectors in the transmission of viral plant diseases. In terms of identifying the paracrines/hormones used to modulate behavior, particularly peptides, members of the Aphidoidea have largely been ignored, as it is not tractable to isolate the large pools of tissue needed for standard biochemical investigations. Here, a bioinformatics approach to peptide discovery has been used to overcome this limitation of scale. Specifically, in silico searches of publicly accessible aphidoidean ESTs were conducted to identify transcripts encoding putative peptides precursors, with the mature peptides contained within them deduced using peptide processing software and homology to known arthropod sequences. In total, 39 ESTs encoding putative peptides precursors were identified from four aphid species: Acyrthosiphon pisum (14 ESTs), Aphis gossypii (four ESTs), Myzus persicae (20 ESTs) and Toxoptera citricida (one EST). These precursors included ones predicted to encode isoforms of B-type allatostatin, crustacean cardioactive peptide, FMRFamide-related peptide (both myosuppressin and short neuropeptide F subfamilies), insect kinin, orcokinin, proctolin, pyrokinin/periviscerokinin/pheromone biosynthesis activating neuropeptide, SIFamide and tachykinin-related peptide. In total, 83 peptides were characterized from the identified precursors, most novel, including two B-type allatostatins possessing the variant -WX(7)Wamide motif, two N-terminally extended proctolin isoforms and an N-terminally truncated and substituted SIFamide. Collectively, these results expand greatly the number of known/predicted aphid peptide paracrines/hormones, and provide a strong foundation for future molecular and physiological investigations of peptidergic control in this insect group.

Combining microdialysis, NanoLC-MS, and MALDI-TOF/TOF to detect neuropeptides secreted in the crab, Cancer borealis

Microdialysis is a useful technique for sampling neuropeptides in vivo, and decapod crustaceans are important model organisms for studying how these peptides regulate physiological processes. However, to date, no microdialysis procedure has been reported for sampling neuropeptides from crustaceans. Here we report the first application of microdialysis to sample neuropeptides from the hemolymph of the crab, Cancer borealis. Microdialysis probes were implanted into the pericardial region of live crabs, and the resulting dialysates were desalted, concentrated, and analyzed by LC-ESI-QTOF and MALDI-TOF/TOF mass spectrometry. Analysis of in vitro microdialysates of hemolymph revealed more neuropeptides and fewer protein fragments than hemolymph prepared by typical analysis methods. Mass spectra of in vivo dialysates displayed neuropeptides from 10 peptide families, including the RFamide, allatostatin, and orcokinin families. In addition, GAHKNYLRFa, SDRNFLRFa, and TNRNFLRFa were sequenced from hemolymph dialysates. The detection of these neuropeptides in the hemolymph suggests that they are functioning as hormones as well as neuromodulators. In vivo microdialysis offers the capability to further study these and other neuropeptides in crustacean hemolymph, complementing current tissue-based studies and extending our knowledge of hormonal regulation of physiological states.

A newly identified extrinsic input triggers a distinct gastric mill rhythm via activation of modulatory projection neurons

Neuronal network flexibility enables animals to respond appropriately to changes in their internal and external states. We are using the isolated crab stomatogastric nervous system to determine how extrinsic inputs contribute to network flexibility. The stomatogastric system includes the well-characterized gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the stomatogastric ganglion. Projection neurons with somata in the commissural ganglia (CoGs) regulate these rhythms. Previous work characterized a unique gastric mill rhythm that occurred spontaneously in some preparations, but whose origin remained undetermined. This rhythm includes a distinct protractor phase activity pattern, during which a key gastric mill circuit neuron (LG neuron) and the projection neurons MCN1 and CPN2 fire in a pyloric rhythm-timed activity pattern instead of the tonic firing pattern exhibited by these neurons during previously studied gastric mill rhythms. Here we identify a new extrinsic input, the post-oesophageal commissure (POC) neurons, relatively brief stimulation (30 s) of which triggers a long-lasting (tens of minutes) activation of this novel gastric mill rhythm at least in part via its lasting activation of MCN1 and CPN2. Immunocytochemical and electrophysiological data suggest that the POC neurons excite MCN1 and CPN2 by release of the neuropeptide Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). These data further suggest that the CoG arborization of the POC neurons comprises the previously identified anterior commissural organ (ACO), a CabTRP Ia-containing neurohemal organ. This endocrine organ thus appears to also have paracrine actions, including activation of a novel and lasting gastric mill rhythm.

Neuropeptide discovery in Ixodoidea: an in silico investigation using publicly accessible expressed sequence tags

The Ixodoidea (ticks) are important vectors in the transmission of many human diseases; for example, the blacklegged tick Ixodes scapularis is the major vector in the transmission of Lyme disease, the most frequently reported vector-borne illness in the United States. The development of expressed sequence tags (ESTs) for ixodoidean cDNA libraries, and their public deposition, has generated a rich resource for protein discovery in members of this taxon, thereby providing an opportunity for better understanding the physiology and behavior of these disease vectors. Here, in silico searches of publicly accessible ESTs were conducted to identify transcripts encoding putative ixodoidean neuropeptide precursors, with the mature peptides contained within them predicted using online peptide processing programs and homology to known arthropod sequences. In total, 37 putative neuropeptide-encoding ESTs were identified from three ixodoidean species: I. scapularis (29 ESTs), Rhipicephalus microplus (seven ESTs) and Amblyomma americanum (one EST). Among those identified from I. scapularis were ones predicted to encode isoforms of corazonin, crustacean hyperglycemic hormone/ion transport peptide, diuretic hormone (both calcitonin- and corticotropin-releasing factor-like), FMRFamide-related peptide (both short neuropeptide F and sulfakinin subfamilies) orcokinin, proctolin, pyrokinin/periviscerokinin/pheromone biosynthesis activating neuropeptide, SIFamide, and tachykinin-related peptide. Collectively, 80 distinct ixodoidean neuropeptides were characterized from the identified precursors. These results not only expand greatly the number of known/predicted ixodoidean neuropeptides, but also provide a strong foundation for future molecular and physiological investigations of peptidergic control in this important group of disease-transmitting arthropods.

Serotonin transduction cascades mediate variable changes in pyloric network cycle frequency in response to the same modulatory challenge

A fundamental question in systems biology addresses the issue of how flexibility is built into modulatory networks such that they can produce context-dependent responses. Here we examine flexibility in the serotonin (5-HT) response system that modulates the cycle frequency (cf) of a rhythmic motor output. We found that depending on the preparation, the same 5-min bath application of 5-HT to the pyloric network of the California spiny lobster, Panulirus interruptus, could produce a significant increase, decrease, or no change in steady-state cf relative to baseline. Interestingly, the mean circuit output was not significantly different among preparations prior to 5-HT application. We developed pharmacological tools to examine the preparation-to-preparation variability in the components of the 5-HT response system. We found that the 5-HT response system consisted of at least three separable components: a 5-HT(2betaPan)-like component mediated a rapid decrease followed by a sustained increase in cf; a 5-HT(1alphaPan)-like component produced a small and usually gradual increase in cf; at least one other component associated with an unknown receptor mediated a sustained decrease in cf. The magnitude of the change in cf produced by each component was highly variable, so that when summed they could produce either a net increase, decrease, or no change in cf depending on the preparation. Overall, our research demonstrates that the balance of opposing components of the 5-HT response system determines the direction and magnitude of 5-HT-induced change in steady-state cf relative to baseline.

Mass spectral characterization of peptide transmitters/hormones in the nervous system and neuroendocrine organs of the American lobster Homarus americanus

The American lobster Homarus americanus is a decapod crustacean with both high economic and scientific importance. To facilitate physiological investigations of peptide transmitter/hormone function in this species, we have used matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nanoscale liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-Q-TOF MS/MS) to elucidate the peptidome present in its nervous system and neuroendocrine organs. In total, 84 peptides were identified, including 27 previously known H. americanus peptides (e.g., VYRKPPFNGSIFamide [Val(1)-SIFamide]), 23 peptides characterized previously from other decapods, but new to the American lobster (e.g., pQTFQYSRGWTNamide [Arg(7)-corazonin]), and 34 new peptides de novo sequenced/detected for the first time in this study. Of particular note are a novel B-type allatostatin (TNWNKFQGSWamide) and several novel FMRFamide-related peptides, including an unsulfated analog of sulfakinin (GGGEYDDYGHLRFamide), two myosuppressins (QDLDHVFLRFamide and pQDLDHVFLRFamide), and a collection of short neuropeptide F isoforms (e.g., DTSTPALRLRFamide and FEPSLRLRFamide). Our data also include the first detection of multiple tachykinin-related peptides in a non-brachyuran decapod, as well as the identification of potential individual-specific variants of orcokinin and orcomyotropin-related peptide. Taken collectively, our results not only expand greatly the number of known H. americanus neuropeptides, but also provide a framework for future studies on the physiological roles played by these molecules in this commercially and scientifically important species.

Mechanosensory gating of proprioceptor input to modulatory projection neurons

Sensorimotor gating commonly occurs at sensory neuron synapses onto motor circuit neurons and motor neurons. Here, using the crab stomatogastric nervous system, we show that sensorimotor gating also occurs at the level of the projection neurons that activate motor circuits. We compared the influence of the gastro-pyloric receptor (GPR) muscle stretch-sensitive neuron on two projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), with and without a preceding activation of the mechanosensory ventral cardiac neurons (VCNs). MCN1 and CPN2 project from the paired commissural ganglia (CoGs) to the stomatogastric ganglion (STG), where they activate the gastric mill (chewing) motor circuit. When stimulated separately, the GPR and VCN neurons each elicit the gastric mill rhythm by coactivating MCN1 and CPN2. When GPR is instead stimulated during the VCN-gastric mill rhythm, it slows this rhythm. This effect results from a second GPR synapse onto MCN1 that presynaptically inhibits its STG terminals. Here, we show that, during the VCN-triggered rhythm, the GPR excitation of MCN1 and CPN2 in the CoGs is gated out, leaving only its influence in the STG. This gating effect appears to occur within the CoG and does not result from a ceiling effect on projection neuron firing frequency. Additionally, this gating action enables GPR to either activate rhythmic motor activity or act as a phasic sensorimotor feedback system. These results also indicate that the site of sensorimotor gating can occur at the level of the projection neurons that activate a motor circuit.