FMRFamide-like neuropeptides and mechanosensory touch receptor neurons regulate male sexual turning behavior in Caenorhabditis elegans

A FMRFamide-like neuropeptide FLP-12 signaling regulates head locomotive behaviors in Caenorhabditis elegans

Neuropeptides play a critical role in regulating behaviors across organisms, but the precise mechanisms by which neuropeptides orchestrate complex behavioral programs are not fully understood. Here, we show that the FMRFamide-like neuropeptide FLP-12 signaling from the SMB head motor neurons modulates head locomotive behaviors, including stomatal oscillation in Caenorhabditis elegans. lim-4 mutants, in which the SMB neurons are not properly specified, exhibited various head and body locomotive defects, including stomatal oscillation. Chronic activation or inhibition of neuropeptidergic signaling in the SMB motor neurons resulted in a decrease or increase in stomatal oscillation, respectively. The flp-12 neuropeptide gene is expressed and acts in the SMB neurons to regulate head and body locomotion, including stomatal oscillation. Moreover, the frpr-8 G protein-couple receptor (GPCR) and gpa-7 Gα genes are expressed in the AVD command interneurons to relay the FLP-12 signal to mediate stomatal oscillation. Finally, heterologous expression of FRPR-8 either Xenopus oocytes or HEK293T cells conferred FLP-12 induced responses. Taken together, these results indicate that the C. elegans FMRFamide neuropeptide FLP-12 acts as a modulator of stomatal oscillation via the FRPR-8 GPCR and the GPA-7 G-protein.

Self-DNA in Caenorhabditis elegans Affects the Production of Specific Metabolites: Evidence from LC-MS and Chemometric Studies

The worm Caenorhabditis elegans, with its short lifecycle and well-known genetic and metabolic pathways, stands as an exemplary model organism for biological research. Its simplicity and genetic tractability make it an ideal system for investigating the effects of different conditions on its metabolism. The chemical analysis of this nematode was performed to identify specific metabolites produced by the worms when fed with either self- or nonself-DNA. A standard diet with OP50 feeding was used as a control. Different development stages were sampled, and their chemical composition was assessed by liquid chromatography-mass spectrometry combined with chemometrics, including both principal component analysis and orthogonal partial least squares discriminant analysis tools. The obtained data demonstrated that self-DNA-treated larvae, when arrested in their cycle, showed significant decreases in dynorphin, an appetite regulator of the nematode, and in N-formyl glycine, a known longevity promoter in C. elegans. Moreover, a substantial decrease was also recorded in the self-DNA-fed adults for the FMRF amide neuropeptide, an embryogenesis regulator, and for a dopamine derivative modulating nematode locomotion. In conclusion, this study allowed for the identification of key metabolites affected by the self-DNA diet, providing interesting hints on the main molecular pathways involved in its biological inhibitory effects.

The kpc-1 3'UTR facilitates dendritic transport and translation efficiency of mRNAs for dendrite arborization of a mechanosensory neuron important for male courtship

A recently reported Schizophrenia-associated genetic variant in the 3'UTR of the human furin gene, a homolog of C. elegans kpc-1, highlights an important role of the furin 3'UTR in neuronal development. We isolate three kpc-1 mutants that display abnormal dendrite arborization in PVD neurons and defective male mating behaviors. We show that the kpc-1 3'UTR participates in dendrite branching and self-avoidance. The kpc-1 3'UTR facilitates mRNA localization to branching points and contact points between sibling dendrites and promotes translation efficiency. A predicted secondary structural motif in the kpc-1 3'UTR is required for dendrite self-avoidance. Animals with over-expression of DMA-1, a PVD dendrite receptor, exhibit similar dendrite branching and self-avoidance defects that are suppressed with kpc-1 over-expression. Our results support a model in which KPC-1 proteins are synthesized at branching points and contact points to locally down-regulate DMA-1 receptors to promote dendrite branching and self-avoidance of a mechanosensory neuron important for male courtship.

Behavioral adjustment of C. elegans to mechanosensory loss requires intact mechanosensory neurons

Sensory neurons specialize in detecting and signaling the presence of diverse environmental stimuli. Neuronal injury or disease may undermine such signaling, diminishing the availability of crucial information. Can animals distinguish between a stimulus not being present and the inability to sense that stimulus in the first place? To address this question, we studied Caenorhabditis elegans nematode worms that lack gentle body touch sensation due to genetic mechanoreceptor dysfunction. We previously showed that worms can compensate for the loss of touch by enhancing their sense of smell, via an FLP-20 neuropeptide pathway. Here, we find that touch-deficient worms exhibit, in addition to sensory compensation, also cautious-like behavior, as if preemptively avoiding potential undetectable hazards. Intriguingly, these behavioral adjustments are abolished when the touch neurons are removed, suggesting that touch neurons are required for signaling the unavailability of touch information, in addition to their conventional role of signaling touch stimulation. Furthermore, we found that the ASE taste neurons, which similarly to the touch neurons, express the FLP-20 neuropeptide, exhibit altered FLP-20 expression levels in a touch-dependent manner, thus cooperating with the touch circuit. These results imply a novel form of neuronal signaling that enables C. elegans to distinguish between lack of touch stimulation and loss of touch sensation, producing adaptive behavioral adjustments that could overcome the inability to detect potential threats.

Roles of Progranulin and FRamides in Neural Versus Non-Neural Tissues on Dietary Restriction-Related Longevity and Proteostasis in C. elegans

Dietary Restriction (DR) mitigates loss of proteostasis associated with aging that underlies neurodegenerative conditions including Alzheimer's disease and related dementias. Previously, we observed increased translational efficiency of certain FMRFamide-Like neuro-Peptide (FLP) genes and the neuroprotective growth factor progranulin gene prgn-1 under dietary restriction in C. elegans. Here, we tested the effects of flp-5, flp-14, flp-15 and pgrn-1 on lifespan and proteostasis under both standard and dietary restriction conditions. We also tested and distinguished function based on their expression in either neuronal or non-neuronal tissue. Lowering the expression of pgrn-1 and flp genes selectively in neural tissue showed no difference in survival under normal feeding conditions nor under DR in two out of three experiments performed. Reduced expression of flp-14 in non-neuronal tissue showed decreased lifespan that was not specific to DR. With respect to proteostasis, a genetic model of DR from mutation of the eat-2 gene that showed increased thermotolerance compared to fully fed wild type animals demonstrated no change in thermotolerance in response to knockdown of pgrn-1 or flp genes. Finally, we tested effects on motility in a neural-specific model of proteotoxicity and found that neuronal knockdown of pgrn-1 and flp genes improved motility in early life regardless of diet. However, knocking these genes down in non-neuronal tissue had variable results. RNAi targeting flp-14 increased motility by day seven of adulthood regardless of diet. Interestingly, non-neuronal RNAi of pgrn-1 decreased motility under standard feeding conditions while DR increased motility for this gene knockdown by day seven (early mid-life). Results show that pgrn-1, flp-5, flp-14, and flp-15 do not have major roles in diet-related changes in longevity or whole-body proteostasis. However, reduced expression of these genes in neurons increases motility early in life in a neural-specific model of proteotoxicity, whereas knockdown of non-neuronal expression mostly increases motility in mid-life under the same conditions.

Sex-specific developmental gene expression atlas unveils dimorphic gene networks in C. elegans

Sex-specific traits and behaviors emerge during development by the acquisition of unique properties in the nervous system of each sex. However, the genetic events responsible for introducing these sex-specific features remain poorly understood. In this study, we create a comprehensive gene expression atlas of pure populations of hermaphrodites and males of the nematode Caenorhabditis elegans across development. We discover numerous differentially expressed genes, including neuronal gene families like transcription factors, neuropeptides, and G protein-coupled receptors. We identify INS-39, an insulin-like peptide, as a prominent male-biased gene expressed specifically in ciliated sensory neurons. We show that INS-39 serves as an early-stage male marker, facilitating the effective isolation of males in high-throughput experiments. Through complex and sex-specific regulation, ins-39 plays pleiotropic sexually dimorphic roles in various behaviors, while also playing a shared, dimorphic role in early life stress. This study offers a comparative sexual and developmental gene expression database for C. elegans. Furthermore, it highlights conserved genes that may underlie the sexually dimorphic manifestation of different human diseases.

Roles of progranulin and FRamides in neural versus non-neural tissues on dietary restriction-related longevity and proteostasis in C. elegans

Dietary restriction (DR) mitigates loss of proteostasis associated with aging that underlies neurodegenerative conditions including Alzheimer's disease and related dementias. Previously, we observed increased translational efficiency of certain FMRFamide-like neuropeptide ( flp ) genes and the neuroprotective growth factor progranulin gene prgn-1 under dietary restriction in C. elegans . Here, we tested the effects of flp-5 , flp-14 , flp-15 and pgrn-1 on lifespan and proteostasis under both standard and dietary restriction conditions. We also tested and distinguished function based on their expression in either neuronal or non-neuronal tissue. Lowering the expression of pgrn-1 and flp genes selectively in neural tissue showed no difference in survival under normal feeding conditions nor under DR in two out of three experiments performed. Reduced expression of flp-14 in non-neuronal tissue showed decreased lifespan that was not specific to DR. With respect to proteostasis, a genetic model of DR from mutation of the eat-2 gene that showed increased thermotolerance compared to fully fed wild type animals demonstrated no change in thermotolerance in response to knockdown of pgrn-1 or flp genes. Finally, we tested effects on motility in a neural-specific model of proteotoxicity and found that neuronal knockdown of pgrn-1 and flp genes improved motility in early life regardless of diet. However, knocking these genes down in non-neuronal tissue had variable results. RNAi targeting flp-14 increased motility by day seven of adulthood regardless of diet. Interestingly, non-neuronal RNAi of pgrn-1 decreased motility under standard feeding conditions while DR increased motility for this gene knockdown by day seven (early mid-life). Results show that pgrn-1 , flp-5 , flp-14 , and flp-15 do not have major roles in diet-related changes in longevity or whole-body proteostasis. However, reduced expression of these genes in neurons increases motility early in life in a neural-specific model of proteotoxicity, whereas knockdown of non-neuronal expression mostly increases motility in mid-life under the same conditions.

Neural mechanisms involved in female mate choice in invertebrates

Mate choice is a critical decision with direct implications for fitness. Although it has been recognized for over 150 years, our understanding of its underlying mechanisms is still limited. Most studies on mate choice focus on the evolutionary causes of behavior, with less attention given to the physiological and molecular mechanisms involved. This is especially true for invertebrates, where research on mate choice has largely focused on male behavior. This review summarizes the current state of knowledge on the neural, molecular and neurohormonal mechanisms of female choice in invertebrates, including behaviors before, during, and after copulation. We identify areas of research that have not been extensively explored in invertebrates, suggesting potential directions for future investigation. We hope that this review will stimulate further research in this area.

Molecular and circuit mechanisms underlying avoidance of rapid cooling stimuli in C. elegans

The mechanisms by which animals respond to rapid changes in temperature are largely unknown. Here, we found that polymodal ASH sensory neurons mediate rapid cooling-evoked avoidance behavior within the physiological temperature range in C. elegans. ASH employs multiple parallel circuits that consist of stimulatory circuits (AIZ, RIA, AVA) and disinhibitory circuits (AIB, RIM) to respond to rapid cooling. In the stimulatory circuit, AIZ, which is activated by ASH, releases glutamate to act on both GLR-3 and GLR-6 receptors in RIA neurons to promote reversal, and ASH also directly or indirectly stimulates AVA to promote reversal. In the disinhibitory circuit, AIB is stimulated by ASH through the GLR-1 receptor, releasing glutamate to act on AVR-14 to suppress RIM activity. RIM, an inter/motor neuron, inhibits rapid cooling-evoked reversal, and the loop activities thus equally stimulate reversal. Our findings elucidate the molecular and circuit mechanisms underlying the acute temperature stimuli-evoked avoidance behavior.

Piezo-mediated mechanosensation contributes to stabilizing copulation posture and reproductive success in Drosophila males

In internal fertilization animals, reproductive success depends on maintaining copulation until gametes are transported from male to female. In Drosophila melanogaster, mechanosensation in males likely contributes to copulation maintenance, but its molecular underpinning remains to be identified. Here we show that the mechanosensory gene piezo and its' expressing neurons are responsible for copulation maintenance. An RNA-seq database search and subsequent mutant analysis revealed the importance of piezo for maintaining male copulation posture. piezo-GAL4-positive signals were found in the sensory neurons of male genitalia bristles, and optogenetic inhibition of piezo-expressing neurons in the posterior side of the male body during copulation destabilized posture and terminated copulation. Our findings suggest that the mechanosensory system of male genitalia through Piezo channels plays a key role in copulation maintenance and indicate that Piezo may increase male fitness during copulation in flies.

PEZO-1 and TRP-4 mechanosensors are involved in mating behavior in Caenorhabditis elegans

Male mating in Caenorhabditis elegans is a complex behavior with a strong mechanosensory component. C. elegans has several characterized mechanotransducer proteins, but few have been shown to contribute to mating. Here, we investigated the roles of PEZO-1, a piezo channel, and TRP-4, a mechanotransducing TRPN channel, in male mating behavior. We show that pezo-1 is expressed in several male-specific neurons with known roles in mating. We show that, among other neurons, trp-4 is expressed in the Post-Cloacal sensilla neuron type A (PCA) sensory neuron, which monitors relative sliding between the male and the hermaphrodite and inhibits neurons involved in vulva detection. Mutations in both genes compromise many steps of mating, including initial response to the hermaphrodite, scanning, turning, and vulva detection. We performed pan-neuronal imaging during mating between freely moving mutant males and hermaphrodites. Both pezo-1 and trp-4 mutants showed spurious activation of the sensory neurons involved in vulva detection. In trp-4 mutants, this spurious activation might be caused by PCA failure to inhibit vulva-detecting neurons during scanning. Indeed, we show that without functional TRP-4, PCA fails to detect the relative sliding between the male and hermaphrodite. Cell-specific TRP-4 expression restores PCA's mechanosensory function. Our results demonstrate new roles for both PEZO-1 and TRP-4 mechanotransducers in C. elegans mating behavior.

Neuropeptide signalling shapes feeding and reproductive behaviours in male Caenorhabditis elegans

LURY-1 peptides are expressed in distinct cells in different sexes and have sex-specific effects on feeding and mating, providing further evidence for the role of neuromodulators in sexual dimorphism.

Sexual dimorphism occurs where different sexes of the same species display differences in characteristics not limited to reproduction. For the nematode Caenorhabditis elegans, in which the complete neuroanatomy has been solved for both hermaphrodites and males, sexually dimorphic features have been observed both in terms of the number of neurons and in synaptic connectivity. In addition, male behaviours, such as food-leaving to prioritise searching for mates, have been attributed to neuropeptides released from sex-shared or sex-specific neurons. In this study, we show that the lury-1 neuropeptide gene shows a sexually dimorphic expression pattern; being expressed in pharyngeal neurons in both sexes but displaying additional expression in tail neurons only in the male. We also show that lury-1 mutant animals show sex differences in feeding behaviours, with pharyngeal pumping elevated in hermaphrodites but reduced in males. LURY-1 also modulates male mating efficiency, influencing motor events during contact with a hermaphrodite. Our findings indicate sex-specific roles of this peptide in feeding and reproduction in C. elegans, providing further insight into neuromodulatory control of sexually dimorphic behaviours.

Neuropeptides and Behaviors: How Small Peptides Regulate Nervous System Function and Behavioral Outputs

One of the reasons that most multicellular animals survive and thrive is because of the adaptable and plastic nature of their nervous systems. For an organism to survive, it is essential for the animal to respond and adapt to environmental changes. This is achieved by sensing external cues and translating them into behaviors through changes in synaptic activity. The nervous system plays a crucial role in constantly evaluating environmental cues and allowing for behavioral plasticity in the organism. Multiple neurotransmitters and neuropeptides have been implicated as key players for integrating sensory information to produce the desired output. Because of its simple nervous system and well-established neuronal connectome, C. elegans acts as an excellent model to understand the mechanisms underlying behavioral plasticity. Here, we critically review how neuropeptides modulate a wide range of behaviors by allowing for changes in neuronal and synaptic signaling. This review will have a specific focus on feeding, mating, sleep, addiction, learning and locomotory behaviors in C. elegans. With a view to understand evolutionary relationships, we explore the functions and associated pathophysiology of C. elegans neuropeptides that are conserved across different phyla. Further, we discuss the mechanisms of neuropeptidergic signaling and how these signals are regulated in different behaviors. Finally, we attempt to provide insight into developing potential therapeutics for neuropeptide-related disorders.

Signaling via the FLP-14/FRPR-19 neuropeptide pathway sustains nociceptive response to repeated noxious stimuli in C. elegans

In order to thrive in constantly changing environments, animals must adaptively respond to threatening events. Noxious stimuli are not only processed according to their absolute intensity, but also to their context. Adaptation processes can cause animals to habituate at different rates and degrees in response to permanent or repeated stimuli. Here, we used a forward genetic approach in Caenorhabditis elegans to identify a neuropeptidergic pathway, essential to prevent fast habituation and maintain robust withdrawal responses to repeated noxious stimuli. This pathway involves the FRPR-19A and FRPR-19B G-protein coupled receptor isoforms produced from the frpr-19 gene by alternative splicing. Loss or overexpression of each or both isoforms can impair withdrawal responses caused by the optogenetic activation of the polymodal FLP nociceptor neuron. Furthermore, we identified FLP-8 and FLP-14 as FRPR-19 ligands in vitro. flp-14, but not flp-8, was essential to promote withdrawal response and is part of the same genetic pathway as frpr-19 in vivo. Expression and cell-specific rescue analyses suggest that FRPR-19 acts both in the FLP nociceptive neurons and downstream interneurons, whereas FLP-14 acts from interneurons. Importantly, genetic impairment of the FLP-14/FRPR-19 pathway accelerated the habituation to repeated FLP-specific optogenetic activation, as well as to repeated noxious heat and harsh touch stimuli. Collectively, our data suggest that well-adjusted neuromodulation via the FLP-14/FRPR-19 pathway contributes to promote nociceptive signals in C. elegans and counteracts habituation processes that otherwise tend to rapidly reduce aversive responses to repeated noxious stimuli.

Transcription Factors That Control Behavior-Lessons From C. elegans

Behavior encompasses the physical and chemical response to external and internal stimuli. Neurons, each with their own specific molecular identities, act in concert to perceive and relay these stimuli to drive behavior. Generating behavioral responses requires neurons that have the correct morphological, synaptic, and molecular identities. Transcription factors drive the specific gene expression patterns that define these identities, controlling almost every phenomenon in a cell from development to homeostasis. Therefore, transcription factors play an important role in generating and regulating behavior. Here, we describe the transcription factors, the pathways they regulate, and the neurons that drive chemosensation, mechanosensation, thermosensation, osmolarity sensing, complex, and sex-specific behaviors in the animal model Caenorhabditis elegans. We also discuss the current limitations in our knowledge, particularly our minimal understanding of how transcription factors contribute to the adaptive behavioral responses that are necessary for organismal survival.

Distinct neuropeptide-receptor modules regulate a sex-specific behavioral response to a pheromone

Dioecious species are a hallmark of the animal kingdom, with opposing sexes responding differently to identical sensory cues. Here, we study the response of C. elegans to the small-molecule pheromone, ascr#8, which elicits opposing behavioral valences in each sex. We identify a novel neuropeptide-neuropeptide receptor (NP/NPR) module that is active in males, but not in hermaphrodites. Using a novel paradigm of neuropeptide rescue that we established, we leverage bacterial expression of individual peptides to rescue the sex-specific response to ascr#8. Concurrent biochemical studies confirmed individual FLP-3 peptides differentially activate two divergent receptors, NPR-10 and FRPR-16. Interestingly, the two of the peptides that rescued behavior in our feeding paradigm are related through a conserved threonine, suggesting that a specific NP/NPR combination sets a male state, driving the correct behavioral valence of the ascr#8 response. Receptor expression within pre-motor neurons reveals novel coordination of male-specific and core locomotory circuitries.

Exploring the Sexual Dimorphism of Crustacean Neuropeptide Expression Using Callinectes sapidus as a Model Organism

The impact of numerous diseases has been linked to differences in sex between organisms, including various neurological diseases. As neuropeptides are known to be key players in the nervous system, studying the variation of neuropeptidomic profiles between males and females in a crustacean model organism is of interest. By using high-resolution mass spectrometry with two complementary ionization sources in conjunction with quantitative chemical labeling (isotopic reductive dimethylation), differences were observed in five key neural tissues and hemolymph. Interestingly, while males and females possess numerous neuropeptide isoforms that are unique to their sex, the represented families of each sex remain largely consistent. However, some differences in familial isoforms were also observed, such as the relative numbers of neuropeptides belonging to RFamide and allatostatin A-type families. Additionally, >100 neuropeptides detected across five neural tissues and hemolymph were found to have statistically significant differences in abundance between male and female blue crab samples. Also, hundreds of putative peptide sequences were identified by de novo sequencing that may be indicative of previously undiscovered neuropeptides, highlighting the power of using a multifaceted MS approach.

Inferences of glia-mediated control in Caenorhabditis elegans

Astrocytes modulate synaptic transmission; yet, it remains unclear how glia influence complex behaviors. Here, we explore the effects of Caenorhabditis elegans astrocyte-like cephalic glia (CEPglia ) and the glia-specific bHLH transcription factor HLH-17 on mating behavior and the defecation motor program (DMP). In C. elegans, male mating has been explicitly described through the male tail circuit and is characterized by coordination of multiple independent behaviors to ensure that copulation is achieved. Furthermore, the sex-specific male mating circuitry shares similar components with the DMP, which is complex and rhythmic, and requires a fixed sequence of behaviors to be activated periodically. We found that loss of CEPglia reduced persistence in executing mating behaviors and hindered copulation, while males that lacked HLH-17 demonstrated repetitive prodding behavior that increased the time spent in mating but did not hinder copulation. During the DMP, we found that posterior body wall contractions (pBocs) and enteric muscle contractions (EMCs) were differentially affected by loss of HLH-17 or CEPglia in males and hermaphrodites. pBocs and EMCs required HLH-17 activity in both sexes, whereas loss of CEPglia alone did not affect DMP in males. Our data suggest that CEPglia mediate complex behaviors by signaling to the GABAergic DVB neuron, and that HLH-17 activity influences those discrete steps within those behaviors. Collectively, these data provide evidence of glia as a link in cooperative regulation of complex and rhythmic behavior that, in C. elegans links circuitry in the head and the tail.

Direct glia-to-neuron transdifferentiation gives rise to a pair of male-specific neurons that ensure nimble male mating

Sexually dimorphic behaviours require underlying differences in the nervous system between males and females. The extent to which nervous systems are sexually dimorphic and the cellular and molecular mechanisms that regulate these differences are only beginning to be understood. We reveal here a novel mechanism by which male-specific neurons are generated in Caenorhabditis elegans through the direct transdifferentiation of sex-shared glial cells. This glia-to-neuron cell fate switch occurs during male sexual maturation under the cell-autonomous control of the sex-determination pathway. We show that the neurons generated are cholinergic, peptidergic, and ciliated putative proprioceptors which integrate into male-specific circuits for copulation. These neurons ensure coordinated backward movement along the mate's body during mating. One step of the mating sequence regulated by these neurons is an alternative readjustment movement performed when intromission becomes difficult to achieve. Our findings reveal programmed transdifferentiation as a developmental mechanism underlying flexibility in innate behaviour.

Neuronal transcriptome analyses reveal novel neuropeptide modulators of excitation and inhibition imbalance in C. elegans

Neuropeptides are secreted molecules that have conserved roles modulating many processes, including mood, reproduction, and feeding. Dysregulation of neuropeptide signaling is also implicated in neurological disorders such as epilepsy. However, much is unknown about the mechanisms regulating specific neuropeptides to mediate behavior. Here, we report that the expression levels of dozens of neuropeptides are up-regulated in response to circuit activity imbalance in C. elegans. acr-2 encodes a homolog of human nicotinic receptors, and functions in the cholinergic motoneurons. A hyperactive mutation, acr-2(gf), causes an activity imbalance in the motor circuit. We performed cell-type specific transcriptomic analysis and identified genes differentially expressed in acr-2(gf), compared to wild type. The most over-represented class of genes are neuropeptides, with insulin-like-peptides (ILPs) the most affected. Moreover, up-regulation of neuropeptides occurs in motoneurons, as well as sensory neurons. In particular, the induced expression of the ILP ins-29 occurs in the BAG neurons, which were previously shown to function in gas-sensing. We also show that this up-regulation of ins-29 in acr-2(gf) animals is activity-dependent. Our genetic and molecular analyses support cooperative effects for ILPs and other neuropeptides in promoting motor circuit activity in the acr-2(gf) background. Together, this data reveals that a major transcriptional response to motor circuit dysregulation is in up-regulation of multiple neuropeptides, and suggests that BAG sensory neurons can respond to intrinsic activity states to feedback on the motor circuit.

Nerves innervating copulatory organs show common FMRFamide, FVRIamide, MIP and serotonin immunoreactivity patterns across Dinophilidae (Annelida) indicating their conserved role in copulatory behaviour

Background

Males of the microscopic annelid family Dinophilidae use their prominent glandomuscular copulatory organ (penis) to enzymatically dissolve the female’s epidermis and thereafter inject sperm. In order to test for putative conserved copulatory structures and neural orchestration across three dinophilid species, we reconstructed the reproductive myo- and neuroanatomy and mapped immunoreactivity patterns against two specific neurotransmitter markers with reported roles in invertebrate male mating behaviour (FVRIamide, MIP) and three general neural markers (acetylated α-tubulin, serotonin, FMRFamide).

Results

Seminal vesicles (one or two pairs), surrounded by a thin layer of longitudinal and circular muscles and innervated by neurites, are found between testes and copulatory organ in the larger males of Dinophilus vorticoides and Trilobodrilus axi, but are missing in the only 0.05 mm long D. gyrociliatus dwarf males. The midventral copulatory organ is in all species composed of an outer muscular penis sheath and an inner penis cone. Neurites encircle the organ equatorially, either as a ring-shaped circumpenial fibre mass or as dorsal and ventral commissures, which are connected to the ventrolateral nerve cords. All three examined dinophilids show similar immunoreactivity patterns against serotonin, FMRFamide, and FVRIamide in the neurons surrounding the penis, supporting the hypotheses about the general involvement of these neurotransmitters in copulatory behaviour in dinophilids. Immunoreactivity against MIP is restricted to the circumpenial fibre mass in D. gyrociliatus and commissures around the penis in T. axi (but not found in D. vorticoides), indicating its role in controlling the copulatory organ.

Conclusions

The overall myo- and neuroanatomy of the reproductive organs is rather similar in the three studied species, suggesting a common ancestry of the unpaired glandomuscular copulatory organ and its innervation in Dinophilidae. This is furthermore supported by the similar immunoreactivity patterns against the tested neurotransmitters around the penis. Smaller differences in the immunoreactivity patterns around the seminal vesicles and spermioducts might account for additional, individual traits. We thus show morphological support for the putatively conserved role of FMRFamide, FVRIamide, MIP and serotonin in dinophilid copulatory behaviour.

A Chemosensory GPCR as a Potential Target to Control the Root-Knot Nematode Meloidogyne incognita Parasitism in Plants

Root-knot nematodes (RKN), from the Meloidogyne genus, have a worldwide distribution and cause severe economic damage to many life-sustaining crops. Because of their lack of specificity and danger to the environment, most chemical nematicides have been banned from use. Thus, there is a great need for new and safe compounds to control RKN. Such research involves identifying beforehand the nematode proteins essential to the invasion. Since G protein-coupled receptors GPCRs are the target of a large number of drugs, we have focused our research on the identification of putative nematode GPCRs such as those capable of controlling the movement of the parasite towards (or within) its host. A datamining procedure applied to the genome of Meloidogyne incognita allowed us to identify a GPCR, belonging to the neuropeptide GPCR family that can serve as a target to carry out a virtual screening campaign. We reconstructed a 3D model of this receptor by homology modeling and validated it through extensive molecular dynamics simulations. This model was used for large scale molecular dockings which produced a filtered limited set of putative antagonists for this GPCR. Preliminary experiments using these selected molecules allowed the identification of an active compound, namely C260-2124, from the ChemDiv provider, which can serve as a starting point for further investigations.

Sentryn and SAD Kinase Link the Guided Transport and Capture of Dense Core Vesicles in Caenorhabditis elegans

Dense core vesicles (DCVs) can transmit signals by releasing neuropeptides from specialized synaptic regions called active zones. DCVs reach the active zone by motorized transport through a long axon. A reverse motor frequently interrupts progress by taking DCVs in the opposite direction. "Guided transport" refers to the mechanism by which outward movements ultimately dominate to bring DCVs to the synaptic region. After guided transport, DCVs alter their interactions with motors and enter a "captured" state. The mechanisms of guided transport and capture of DCVs are unknown. Here, we discovered two proteins that contribute to both processes in Caenorhabditis elegans SAD kinase and a novel conserved protein we named Sentryn are the first proteins found to promote DCV capture. By imaging DCVs moving in various regions of single identified neurons in living animals, we found that DCV guided transport and capture are linked through SAD kinase, Sentryn, and Liprin-α. These proteins act together to regulate DCV motorized transport in a region-specific manner. Between the cell body and the synaptic region, they promote forward transport. In the synaptic region, where all three proteins are highly enriched at active zones, they promote DCV pausing by inhibiting transport in both directions. These three proteins appear to be part of a special subset of active zone-enriched proteins because other active zone proteins do not share their unique functions.

Gene expression analysis of neuropeptides in oral mucosa during periodontal disease in non-human primates

BACKGROUND

Neuropeptides (NPs) are innate pivotal regulators of the immunoinflammatory response. Nevertheless, their role in the pathogenesis of periodontal disease remains unknown. Changes in gene expression of 10 NPs and 16 NP receptors (NPRs) coincident with the initiation, progression, and resolution of periodontitis were determined.

METHODS

The ligature-induced periodontitis model was used in rhesus monkeys (n = 18). Gingival tissue samples were taken at baseline (preligatures), at 2 weeks and at 1 month (initiation), and at 3 months (progression) postligation. Ligatures were removed and samples taken 2 months later (resolution). Total RNA was isolated from tissues and NP/NPR gene expression microarray analysis was performed. Gene expression changes were validated by quantitative polymerase chain reaction and immunohistochemistry.

RESULTS

Unexpectedly, the expression of pro-inflammatory NPs/NPRs did not change during periodontitis or with resolution. However, increased expression of the anti-inflammatory NPs adrenomedullin (ADM) and galanin (GAL), and the NPRs calcitonin receptor-like (CALCRL) and receptor activity-modifying protein-2 and -3 (RAMP2 and RAMP3) were observed during initiation and progression of disease. The expression of the same NPs/NPRs exhibited a significant positive correlation with both molecular (interleukin-1ß, matrix mettaloproteinase-9, and receptor activator of nuclear factor-kappa B ligand) and clinical measures of gingival inflammation and tissue destruction.

CONCLUSION

Initiation and progression of periodontitis involve significant overexpression of ADM, GAL, CALCRL, RAMP2, and RAMP3. These anti-inflammatory NPs/NPRs could play a role in the unresolved infection and inflammation that normally drives tissue destruction in periodontitis. Both ADM and GAL potentially are new candidates to consider as biomolecules associated with periodontal disease activity.

Sexual Dimorphism and Sex Differences in Caenorhabditis elegans Neuronal Development and Behavior

As fundamental features of nearly all animal species, sexual dimorphisms and sex differences have particular relevance for the development and function of the nervous system. The unique advantages of the nematode Caenorhabditis elegans have allowed the neurobiology of sex to be studied at unprecedented scale, linking ultrastructure, molecular genetics, cell biology, development, neural circuit function, and behavior. Sex differences in the C. elegans nervous system encompass prominent anatomical dimorphisms as well as differences in physiology and connectivity. The influence of sex on behavior is just as diverse, with biological sex programming innate sex-specific behaviors and modifying many other aspects of neural circuit function. The study of these differences has provided important insights into mechanisms of neurogenesis, cell fate specification, and differentiation; synaptogenesis and connectivity; principles of circuit function, plasticity, and behavior; social communication; and many other areas of modern neurobiology.

Sexual modulation of sex-shared neurons and circuits in Caenorhabditis elegans

Studies using the nematode C. elegans have provided unique insights into the development and function of sex differences in the nervous system. Enabled by the relative simplicity of this species, comprehensive studies have solved the complete cellular neuroanatomy of both sexes as well as the complete neural connectomes of the entire adult hermaphrodite and the adult male tail. This work, together with detailed behavioral studies, has revealed three aspects of sex differences in the nervous system: sex-specific neurons and circuits; circuits with sexually dimorphic synaptic connectivity; and sex differences in the physiology and functions of shared neurons and circuits. At all of these levels, biological sex influences neural development and function through the activity of a well-defined genetic hierarchy that acts throughout the body to translate chromosomal sex into the state of a master autosomal regulator of sexual differentiation, the transcription factor TRA-1A. This Review focuses on the role of genetic sex in implementing sex differences in shared neurons and circuits, with an emphasis on linking the sexual modulation of specific neural properties to the specification and optimization of sexually divergent and dimorphic behaviors. An important and unexpected finding from these studies is that chemosensory neurons are a primary focus of sexual modulation, with genetic sex adaptively shaping chemosensory repertoire to guide behavioral choice. Importantly, hormone-independent functions of genetic sex are the principal drivers of all of these sex differences, making nematodes an excellent model for understanding similar but poorly understood mechanisms that likely act throughout the animal kingdom. © 2016 Wiley Periodicals, Inc.

Caenorhabditis elegans Male Copulation Circuitry Incorporates Sex-Shared Defecation Components To Promote Intromission and Sperm Transfer

Sexual dimorphism can be achieved using a variety of mechanisms, including sex-specific circuits and sex-specific function of shared circuits, though how these work together to produce sexually dimorphic behaviors requires further investigation. Here, we explore how components of the sex-shared defecation circuitry are incorporated into the sex-specific male mating circuitry in Caenorhabditis elegans to produce successful copulation. Using behavioral studies, calcium imaging, and genetic manipulation, we show that aspects of the defecation system are coopted by the male copulatory circuitry to facilitate intromission and ejaculation. Similar to hermaphrodites, male defecation is initiated by an intestinal calcium wave, but circuit activity is coordinated differently during mating. In hermaphrodites, the tail neuron DVB promotes expulsion of gut contents through the release of the neurotransmitter GABA onto the anal depressor muscle. However, in the male, both neuron and muscle take on modified functions to promote successful copulation. Males require calcium-dependent activator protein for secretion (CAPS)/unc-31, a dense core vesicle exocytosis activator protein, in the DVB to regulate copulatory spicule insertion, while the anal depressor is remodeled to promote release of sperm into the hermaphrodite. This work shows how sex-shared circuitry is modified in multiple ways to contribute to sex-specific mating.

Ethological principles predict the neuropeptides co-opted to influence parenting

Ethologists predicted that parental care evolves by modifying behavioural precursors in the asocial ancestor. As a corollary, we predict that the evolved mechanistic changes reside in genetic pathways underlying these traits. Here we test our hypothesis in female burying beetles, Nicrophorus vespilloides, an insect where caring adults regurgitate food to begging, dependent offspring. We quantify neuropeptide abundance in brains collected from three behavioural states: solitary virgins, individuals actively parenting or post-parenting solitary adults and quantify 133 peptides belonging to 18 neuropeptides. Eight neuropeptides differ in abundance in one or more states, with increased abundance during parenting in seven. None of these eight neuropeptides have been associated with parental care previously, but all have roles in predicted behavioural precursors for parenting. Our study supports the hypothesis that predictable traits and pathways are targets of selection during the evolution of parenting and suggests additional candidate neuropeptides to study in the context of parenting.

cdc-25.4, a Caenorhabditis elegans Ortholog of cdc25, Is Required for Male Mating Behavior

Cell division cycle 25 (cdc25) is an evolutionarily conserved phosphatase that promotes cell cycle progression. Among the four cdc25 orthologs in Caenorhabditis elegans, we found that cdc-25.4 mutant males failed to produce outcrossed progeny. This was not caused by defects in sperm development, but by defects in male mating behavior. The cdc-25.4 mutant males showed various defects during male mating, including contact response, backing, turning, and vulva location. Aberrant turning behavior was the most prominent defect in the cdc-25.4 mutant males. We also found that cdc-25.4 is expressed in many neuronal cells throughout development. The turning defect in cdc-25.4 mutant males was recovered by cdc-25.4 transgenic expression in neuronal cells, suggesting that cdc-25.4 functions in neurons for male mating. However, the neuronal morphology of cdc-25.4 mutant males appeared to be normal, as examined with several neuronal markers. Also, RNAi depletion of wee-1.3, a C. elegans ortholog of Wee1/Myt1 kinase, failed to suppress the mating defects of cdc-25.4 mutant males. These findings suggest that, for successful male mating, cdc-25.4 does not target cell cycles that are required for neuronal differentiation and development. Rather, cdc-25.4 likely regulates noncanonical substrates in neuronal cells.

Neuropeptide-Driven Cross-Modal Plasticity following Sensory Loss in Caenorhabditis elegans

Sensory loss induces cross-modal plasticity, often resulting in altered performance in remaining sensory modalities. Whereas much is known about the macroscopic mechanisms underlying cross-modal plasticity, only scant information exists about its cellular and molecular underpinnings. We found that Caenorhabditis elegans nematodes deprived of a sense of body touch exhibit various changes in behavior, associated with other unimpaired senses. We focused on one such behavioral alteration, enhanced odor sensation, and sought to reveal the neuronal and molecular mechanisms that translate mechanosensory loss into improved olfactory acuity. To this end, we analyzed in mechanosensory mutants food-dependent locomotion patterns that are associated with olfactory responses and found changes that are consistent with enhanced olfaction. The altered locomotion could be reversed in adults by optogenetic stimulation of the touch receptor (mechanosensory) neurons. Furthermore, we revealed that the enhanced odor response is related to a strengthening of inhibitory AWC→AIY synaptic transmission in the olfactory circuit. Consistently, inserting in this circuit an engineered electrical synapse that diminishes AWC inhibition of AIY counteracted the locomotion changes in touch-deficient mutants. We found that this cross-modal signaling between the mechanosensory and olfactory circuits is mediated by neuropeptides, one of which we identified as FLP-20. Our results indicate that under normal function, ongoing touch receptor neuron activation evokes FLP-20 release, suppressing synaptic communication and thus dampening odor sensation. In contrast, in the absence of mechanosensory input, FLP-20 signaling is reduced, synaptic suppression is released, and this enables enhanced olfactory acuity; these changes are long lasting and do not represent ongoing modulation, as revealed by optogenetic experiments. Our work adds to a growing literature on the roles of neuropeptides in cross-modal signaling, by showing how activity-dependent neuropeptide signaling leads to specific cross-modal plastic changes in neural circuit connectivity, enhancing sensory performance.

Mechanosensation circuitry in Caenorhabditis elegans: A focus on gentle touch

Forward or reverse movement in Caenorhabditis elegans is the result of sequential contraction of muscle cells arranged along the body. In larvae, muscle cells are innervated by distinct classes of motorneurons. B motorneurons regulate forward movement and A motorneurons regulate backward movement. Ablation of the D motor neurons results in animals that are uncoordinated in either direction, which suggests that D motorneurons regulate the interaction between the two circuits. C. elegans locomotion is dictated by inputs from interneurons that regulate the activity of motorneurons which coordinate muscle contraction to facilitate forward or backwards movement. As C. elegans moves through the environment, sensory neurons interpret chemical and mechanical information which is relayed to the motor neurons that control locomotory direction. A mechanosensory input known as light nose touch can be simulated in the laboratory by touching the nose of the animal with a human eyebrow hair. The recoil reaction that follows from light nose touch appears to be primarily mediated by glutamate release from the polymodal sensory neuron ASH. Numerous glutamate receptor types are found in different neurons and interneurons which suggest that several pathways may regulate the aversive response. Based on the phenotypes of mutants in which neuropeptide processing is abolished, neuropeptides play a role in circuit regulation. The light touch response is also regulated by transient receptor channel proteins and degenerin/epithelial sodium channels which modulate the activity of sensory neurons involved in the nose touch response.

Sexual modulation of neural circuits and behavior in Caenorhabditis elegans

Sex differences in behavior-both sex-specific and shared behaviors-are fundamental to nearly all animal species. One often overlooked mechanism by which these behavioral differences can be generated is through sex-specific modulation of shared circuitry (i.e., circuits present in both sexes). In vertebrates this modulation is likely regulated by hormone-dependent mechanisms as well as by somatic sex itself; invertebrate models have particular promise for understanding the latter of these. Here we review molecular and behavioral evidence of sexual modulation of shared circuitry in the nematode Caenorhabditis elegans. Multiple behaviors in this species, both copulatory and not, are modulated by the genetic sex of shared neurons and circuit. These studies are close to uncovering the molecular mechanisms by which somatic sex modulates neural function in the worm, mechanisms which may be well conserved in more complex organisms. Improving our understanding of the modulation of neural circuit development and function by somatic sex may lend important insight into sex differences in the mammalian nervous system which, in turn, may have important implications for sex biases in disease.

A novel CaM kinase II pathway controls the location of neuropeptide release from Caenorhabditis elegans motor neurons

Neurons release neuropeptides via the regulated exocytosis of dense core vesicles (DCVs) to evoke or modulate behaviors. We found that Caenorhabditis elegans motor neurons send most of their DCVs to axons, leaving very few in the cell somas. How neurons maintain this skewed distribution and the extent to which it can be altered to control DCV numbers in axons or to drive release from somas for different behavioral impacts is unknown. Using a forward genetic screen, we identified loss-of-function mutations in UNC-43 (CaM kinase II) that reduce axonal DCV levels by ∼90% and cell soma/dendrite DCV levels by ∼80%, leaving small synaptic vesicles largely unaffected. Blocking regulated secretion in unc-43 mutants restored near wild-type axonal levels of DCVs. Time-lapse video microscopy showed no role for CaM kinase II in the transport of DCVs from cell somas to axons. In vivo secretion assays revealed that much of the missing neuropeptide in unc-43 mutants is secreted via a regulated secretory pathway requiring UNC-31 (CAPS) and UNC-18 (nSec1). DCV cargo levels in unc-43 mutants are similarly low in cell somas and the axon initial segment, indicating that the secretion occurs prior to axonal transport. Genetic pathway analysis suggests that abnormal neuropeptide function contributes to the sluggish basal locomotion rate of unc-43 mutants. These results reveal a novel pathway controlling the location of DCV exocytosis and describe a major new function for CaM kinase II.

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.

The interaction of octopamine and neuropeptides to slow aversive responses in C. elegans mimics the modulation of chronic pain in mammals

Octopamine (OA) appears to function as the invertebrate counterpart of norepinephrine (NE) in the modulation of a number of key behaviors. In C. elegans, OA signaling is complex, mediated by at least three distinct α-adrenergic-like receptors and appears to activate more global peptidergic signaling cascades that have the potential to dramatically amplify the octopaminergic signal. These OA-dependent peptidergic signaling cascades involve an array of neuropeptides that activate receptors throughout the nervous system and have the potential to both directly and indirectly modulate locomotory decision-making. In this commentary we highlight the use of C. elegans as a model to expand our understanding of noradrenergic signaling in mammals, specifically as it relates to the role of NE in anti-nociception.

Ancient neuromodulation by vasopressin/oxytocin-related peptides

Neuropeptidergic signaling is widely adopted by animals for the regulation of physiology and behavior in a rapidly changing environment. The vasopressin/oxytocin neuropeptide family originates from an ancestral peptide precursor in the antecedent of protostomian and deuterostomian animals. In vertebrates, vasopressin and oxytocin have both hormonal effects on peripheral target tissues, such as in the regulation of reproduction and water balance, and neuromodulatory actions in the central nervous system controlling social behavior and cognition. The recent identification of vasopressin/oxytocin-related signaling in C. elegans reveals that this peptidergic system is widespread among nematodes. Genetic analysis of the C. elegans nematocin system denotes vasopressin/oxytocin-like peptides as ancient neuromodulators of neuronal circuits involved in reproductive behavior and associative learning, whereas former invertebrate studies focused on conserved peripheral actions of this peptide family. Nematocin provides neuromodulatory input into the gustatory plasticity circuit as well as into distinct male mating circuits to generate a coherent mating behavior. Molecular interactions are comparable to those underlying vasopressin- and oxytocin-mediated effects in the mammalian brain. Understanding how the vasopressin/oxytocin family fine-tunes neuronal circuits for social behavior, learning and memory poses a major challenge. Functional conservation of these effects in nematodes and most likely in other invertebrates enables the development of future models to help answering this question.

Dauer-specific dendrite arborization in C. elegans is regulated by KPC-1/Furin

BACKGROUND

Dendrites often display remarkably complex and diverse morphologies that are influenced by developmental and environmental cues. Neuroplasticity in response to adverse environmental conditions entails both hypertrophy and resorption of dendrites. How dendrites rapidly alter morphology in response to unfavorable environmental conditions is unclear. The nematode Caenorhabditis elegans enters into a stress-resistant dauer larval stage in response to an adverse environment.

RESULTS

Here we show that the IL2 bipolar sensory neurons undergo dendrite arborization and axon remodeling during dauer development. When dauer larvae are returned to favorable environmental conditions, animals resume reproductive development and IL2 dendritic branches retract, leaving behind remnant branches in postdauer L4 and adult animals. The C. elegans furin homolog KPC-1 is required for dauer IL2 dendritic arborization and dauer-specific nictation behavior. KPC-1 is also necessary for dendritic arborization of PVD and FLP sensory neurons. In mammals, furin is essential, ubiquitously expressed, and associated with numerous pathologies, including neurodegenerative diseases. While broadly expressed in C. elegans neurons and epithelia, KPC-1 acts cell autonomously in IL2 neurons to regulate dauer-specific dendritic arborization and nictation.

CONCLUSIONS

Neuroplasticity of the C. elegans IL2 sensory neurons provides a paradigm to study stress-induced and reversible dendritic branching, and the role of environmental and developmental cues in this process. The newly discovered role of KPC-1 in dendrite morphogenesis provides insight into the function of proprotein convertases in nervous system development.

Neuropeptides function in a homeostatic manner to modulate excitation-inhibition imbalance in C. elegans

Neuropeptides play crucial roles in modulating neuronal networks, including changing intrinsic properties of neurons and synaptic efficacy. We previously reported a Caenorhabditis elegans mutant, acr-2(gf), that displays spontaneous convulsions as the result of a gain-of-function mutation in a neuronal nicotinic acetylcholine receptor subunit. The ACR-2 channel is expressed in the cholinergic motor neurons, and acr-2(gf) causes cholinergic overexcitation accompanied by reduced GABAergic inhibition in the locomotor circuit. Here we show that neuropeptides play a homeostatic role that compensates for this excitation-inhibition imbalance in the locomotor circuit. Loss of function in genes required for neuropeptide processing or release of dense core vesicles specifically modulate the convulsion frequency of acr-2(gf). The proprotein convertase EGL-3 is required in the cholinergic motor neurons to restrain convulsions. Electrophysiological recordings of neuromuscular junctions show that loss of egl-3 in acr-2(gf) causes a further reduction of GABAergic inhibition. We identify two neuropeptide encoding genes, flp-1 and flp-18, that together counteract the excitation-inhibition imbalance in acr-2(gf) mutants. We further find that acr-2(gf) causes an increased expression of flp-18 in the ventral cord cholinergic motor neurons and that overexpression of flp-18 reduces the convulsion of acr-2(gf) mutants. The effects of these peptides are in part mediated by two G-protein coupled receptors, NPR-1 and NPR-5. Our data suggest that the chronic overexcitation of the cholinergic motor neurons imposed by acr-2(gf) leads to an increased production of FMRFamide neuropeptides, which act to decrease the activity level of the locomotor circuit, thereby homeostatically modulating the excitation and inhibition imbalance.

Imbalanced neuronal circuit activity is considered a major underlying cause in many neurological disorders, such as epilepsy and autism. Neuropeptides are small polypeptides that are released from neurons. They are widely known to provide neuromodulatory functions and have diverse roles in the nervous system. By investigating a C. elegans mutant that exhibits convulsions as the result of an imbalanced excitation and inhibition in the locomotor circuit, we have identified a homeostatic mechanism involving two distinct neuropeptide genes. We find that the expression of the neuropeptides is up-regulated in response to over-excitation and that, in turn, they act to increase inhibitory transmission. While current treatment strategies for epilepsy have focused on targeting fast synaptic transmission, this work supports the general notion that manipulating slow neuropeptide neurotransmission can strongly influence neural excitation and inhibition imbalance.

A comparison of the FMRFamide-like peptide proteolytic activities of preparations from two plant-parasitic nematodes (Heterodera glycines and Meloidogyne incognita): possible targets for novel control

Proteolytic activities in extracts from the plant-parasitic nematodes Heterodera glycines and Meloidogyneincognita were examined for their abilities to digest three FRET-modified peptide substrates representing members of the large FMRFamide-like peptide (FLP) family in nematodes. Included were sequences distributed across all nematode species (KSAYMRFa and KHEYLRFa) and a sequence confined to a narrow range of plant-parasitic nematodes (KHEFVRFa). Species variations were observed among substrate affinities, reaction rates and effect of protease inhibitors. Km values for KHEYLRFa (1.48 ± 0.34 μm) and KSAYMRFa (2.13 ± 0.24 μm) in H. glycines were each lower (P< 0.05) than those for the same substrates in M. incognita (5.26 ± 1.30 μm and 3.90 ± 0.61 μm, respectively). The Km of KHEFVRFa was lower (P< 0.05) in M. incognita (5.83 ± 0.36 μm) than in H. glycines (11.01 ± 1.26 μm). Reaction rates (Vmax/min/μg) for KHEYLRFa were the same for both species, but KSAYMRFa and KHEFVRFa digestion rates were each nearly twofold higher (P< 0.05) in M. incognita than in H. glycines. Digestion of KSAYMRFa was strongly inhibited in both species by 4-(2-aminoethyl)-benzenesulfonyl-fluoride-HCl (AEBSF) and EDTA, but M. incognita was more sensitive (P< 0.05) to inhibition. AEBSF and EDTA (both at 1 mm) inhibited M. incognita activity 62.3% and 36.6% more, respectively, than H. glycines activity. Serine protease inhibition differed significantly (P< 0.05) between the two species. Maximum inhibition of M. incognita (76%) occurred at 1.85 mm AEBSF while maximum inhibition of H. glycines was 40% at 1.19 mm AEBSF.

The FMRFamide-related neuropeptide FLP-20 is required in the mechanosensory neurons during memory for massed training in C. elegans

Lasting memories are likely to result from a lasting change in neurotransmission. In the nematode Caenorhabditis elegans, spaced training with a tap stimulus induces habituation to the tap that lasts for >24 h and is dependent on glutamate transmission, postsynaptic AMPA receptors, and CREB. Here we describe a distinct, presynaptic mechanism for a shorter lasting memory for tap habituation induced by massed training. We report that a FMRFamide-related peptide (FMRF = Phe-Met-Arg-Phe-NH(2)), FLP-20, is critical for memory lasting 12 h following massed training, but is not required for other forms of memory. Massed training correlated with a flp-20-dependent increase in synaptobrevin tagged with green fluorescent protein in the presynaptic terminals of the PLM mechanosensory neurons that followed the timeline of the memory trace. We also demonstrated that flp-20 is required specifically in the mechanosensory neurons for memory 12 h after massed training. These findings show that within the same species and form of learning, memory is induced by distinct mechanisms to create a lasting alteration in neurotransmission that is dependent upon the temporal pattern of training: memory of spaced training results from postsynaptic changes in the interneurons of the neural circuit, whereas memory of massed training results from presynaptic changes in the mechanosensory neurons of the neural circuit.

C. elegans dopaminergic D2-like receptors delimit recurrent cholinergic-mediated motor programs during a goal-oriented behavior

Caenorhabditis elegans male copulation requires coordinated temporal-spatial execution of different motor outputs. During mating, a cloacal circuit consisting of cholinergic sensory-motor neurons and sex muscles maintains the male's position and executes copulatory spicule thrusts at his mate's vulva. However, distinct signaling mechanisms that delimit these behaviors to their proper context are unclear. We found that dopamine (DA) signaling directs copulatory spicule insertion attempts to the hermaphrodite vulva by dampening spurious stimulus-independent sex muscle contractions. From pharmacology and genetic analyses, DA antagonizes stimulatory ACh signaling via the D2-like receptors, DOP-2 and DOP-3, and Gα(o/i) proteins, GOA-1 and GPA-7. Calcium imaging and optogenetics suggest that heightened DA-expressing ray neuron activities coincide with the cholinergic cloacal ganglia function during spicule insertion attempts. D2-like receptor signaling also attenuates the excitability of additional mating circuits to reduce the duration of mating attempts with unproductive and/or inappropriate partners. This suggests that, during wild-type mating, simultaneous DA-ACh signaling modulates the activity threshold of repetitive motor programs, thus confining the behavior to the proper situational context.

Peptide neuromodulation in invertebrate model systems

Neuropeptides modulate neural circuits controlling adaptive animal behaviors and physiological processes, such as feeding/metabolism, reproductive behaviors, circadian rhythms, central pattern generation, and sensorimotor integration. Invertebrate model systems have enabled detailed experimental analysis using combined genetic, behavioral, and physiological approaches. Here we review selected examples of neuropeptide modulation in crustaceans, mollusks, insects, and nematodes, with a particular emphasis on the genetic model organisms Drosophila melanogaster and Caenorhabditis elegans, where remarkable progress has been made. On the basis of this survey, we provide several integrating conceptual principles for understanding how neuropeptides modulate circuit function, and also propose that continued progress in this area requires increased emphasis on the development of richer, more sophisticated behavioral paradigms.

In vitro proteolysis of nematode FMRFamide-like peptides (FLPs) by preparations from a free-living nematode (Panagrellus redivivus) and two plant-parasitic nematodes (Heterodera glycines and Meloidogyne incognita)

Proteolytic activities in extracts from three nematodes, the plant parasites Heterodera glycines and Meloidogyne incognita, and the free-living Panagrellus redivivus, were surveyed for substrate preferences using a battery of seven FRET-modified peptide substrates, all derived from members of the large FMRF-amide like peptide (FLP) family in nematodes. Overall protease activity in P. redivivus was four- to fivefold greater than in either of the parasites, a result that might reflect developmental differences. Digestion of the M. incognita FLP KHEFVRFa (substrate Abz-KHEFVRF-Y(3-NO2)a) by M. incognita extract was sevenfold greater than with H. glycines extract and twofold greater than P. redivivus, suggesting species-specific preferences. Additional species differences were revealed upon screening 12 different protease inhibitors. Two substrates were used in the screen, Abz-KHEFVRF-Y(3-NO2)a and Abz-KPSFVRF-Y(3-NO2)a), which was digested equally by all three species. The effects of various inhibitor, substrate and extract source combinations on substrate digestion suggest that M. incognita differs significantly from P. redivivus and H. glycines in its complement of cysteine proteases, particularly cathepsin L-type protease.

UNC-73/trio RhoGEF-2 activity modulates Caenorhabditis elegans motility through changes in neurotransmitter signaling upstream of the GSA-1/Galphas pathway

Rho-family GTPases play regulatory roles in many fundamental cellular processes. Caenorhabditis elegans UNC-73 RhoGEF isoforms function in axon guidance, cell migration, muscle arm extension, phagocytosis, and neurotransmission by activating either Rac or Rho GTPase subfamilies. Multiple differentially expressed UNC-73 isoforms contain a Rac-specific RhoGEF-1 domain, a Rho-specific RhoGEF-2 domain, or both domains. The UNC-73E RhoGEF-2 isoform is activated by the G-protein subunit Gαq and is required for normal rates of locomotion; however, mechanisms of UNC-73 and Rho pathway regulation of locomotion are not clear. To better define UNC-73 function in the regulation of motility we used cell-specific and inducible promoters to examine the temporal and spatial requirements of UNC-73 RhoGEF-2 isoform function in mutant rescue experiments. We found that UNC-73E acts within peptidergic neurons of mature animals to regulate locomotion rate. Although unc-73 RhoGEF-2 mutants have grossly normal synaptic morphology and weak resistance to the acetylcholinesterase inhibitor aldicarb, they are significantly hypersensitive to the acetylcholine receptor agonist levamisole, indicating alterations in acetylcholine neurotransmitter signaling. Consistent with peptidergic neuron function, unc-73 RhoGEF-2 mutants exhibit a decreased level of neuropeptide release from motor neuron dense core vesicles (DCVs). The unc-73 locomotory phenotype is similar to those of rab-2 and unc-31, genes with distinct roles in the DCV-mediated secretory pathway. We observed that constitutively active Gαs pathway mutations, which compensate for DCV-mediated signaling defects, rescue unc-73 RhoGEF-2 and rab-2 lethargic movement phenotypes. Together, these data suggest UNC-73 RhoGEF-2 isoforms are required for proper neurotransmitter signaling and may function in the DCV-mediated neuromodulatory regulation of locomotion rate.

Schistosome I/Lamides--a new family of bioactive helminth neuropeptides

Here we report the identification of a new family of helminth neuropeptides with members in both nematodes and flatworms, and include preliminary cell biological and functional characterisation of one of the peptides from the trematode parasite of humans, Schistosoma mansoni. Bioinformatics and Rapid Amplification of cDNA Ends (RACE)-PCR were used to identify the complete S. mansoni neuropeptide precursor gene Sm-npp-1, which encodes three pentapeptides bearing the motif (A/G)FVR(I/L).NH(2). Similar peptides were identified in three other flatworm species and in 15 nematode species. Quantitative PCR (qPCR) and immunocytochemical (ICC) analyses showed that Sm-npp-1 is constitutively expressed in larval and adult worms. ICC and confocal microscopy were employed to localise one of the schistosome NPP-1 peptides (GFVRIamide) in adult worms and schistosomules; antibodies labelled a pair of neurones in the cerebral ganglia that extend posteriorly along the main nerve cords. GFVRIamide displayed no detectable co-localisation with FMRFamide-like peptides (FLPs), nor was it detectable in muscle innervation. Exogenously applied peptide had a significant inhibitory effect on the mobility of whole adult worm pairs at 10(-5)M (n = 9). Finally, we explored Sm-npp-1 function in schistosomules using RNA interference (RNAi); we successfully achieved specific knockdown of the Sm-npp-1 transcript (54.46 ± 10.41% knockdown, n = 3), but did not detect any clear, aberrant mobility or morphological phenotypes. NPP-1-like peptides are a new family of helminth peptides with a cell-specific expression pattern distinct from FLPs and a modulatory effect on schistosome muscular activity.

The robustness of Caenorhabditis elegans male mating behavior depends on the distributed properties of ray sensory neurons and their output through core and male-specific targets

Many evolutionarily significant behaviors, such as mating, involve dynamic interactions with animate targets. This raises the question of what features of neural circuit design are essential to support these complex types of behavior. The Caenorhabditis elegans male uses 18 ray sensilla of the tail to coordinate mate apposition behavior, which facilitates a systematic search of the hermaphrodite surface for the vulva. Precisely how ray neuron types, A and B, robustly endow the male with a high degree of spatial and temporal precision is unknown. We show that the appositional postures that drive the search trajectory reflect the complex interplay of ray neuron type-induced motor outputs. Cell-type-specific ablations reveal that the A-neurons are required for all appositional postures. Their activity is instructive because the A-neurons can induce scanning- and turning-like appositional postures when artificially activated with channel rhodopsin (ChR2). B-neurons are essential only for initiation of the behavior in which they enhance male responsiveness to hermaphrodite contact. When artificially activated using ChR2, A- and B-neurons produce different tail ventral curl postures. However, when coactivated, A-neuron posture dominates, limiting B-neuron contributions to initiation or subsequent postures. Significantly, males lacking the majority of rays retain a high degree of postural control, indicating significant functional resilience in the system. Furthermore, eliminating a large number of male-specific ray neuron targets only partially attenuates tail posture control revealing that gender-common cells make an important contribution to the behavior. Thus, robustness may be a crucial feature of circuits underlying complex behaviors, such as mating, even in simple animals.

A cholinergic-regulated circuit coordinates the maintenance and bi-stable states of a sensory-motor behavior during Caenorhabditis elegans male copulation

Penetration of a male copulatory organ into a suitable mate is a conserved and necessary behavioral step for most terrestrial matings; however, the detailed molecular and cellular mechanisms for this distinct social interaction have not been elucidated in any animal. During mating, the Caenorhabditis elegans male cloaca is maintained over the hermaphrodite's vulva as he attempts to insert his copulatory spicules. Rhythmic spicule thrusts cease when insertion is sensed. Circuit components consisting of sensory/motor neurons and sex muscles for these steps have been previously identified, but it was unclear how their outputs are integrated to generate a coordinated behavior pattern. Here, we show that cholinergic signaling between the cloacal sensory/motor neurons and the posterior sex muscles sustains genital contact between the sexes. Simultaneously, via gap junctions, signaling from these muscles is transmitted to the spicule muscles, thus coupling repeated spicule thrusts with vulval contact. To transit from rhythmic to sustained muscle contraction during penetration, the SPC sensory-motor neurons integrate the signal of spicule's position in the vulva with inputs from the hook and cloacal sensilla. The UNC-103 K(+) channel maintains a high excitability threshold in the circuit, so that sustained spicule muscle contraction is not stimulated by fewer inputs. We demonstrate that coordination of sensory inputs and motor outputs used to initiate, maintain, self-monitor, and complete an innate behavior is accomplished via the coupling of a few circuit components.

Conserved genes act as modifiers of invertebrate SMN loss of function defects

Spinal Muscular Atrophy (SMA) is caused by diminished function of the Survival of Motor Neuron (SMN) protein, but the molecular pathways critical for SMA pathology remain elusive. We have used genetic approaches in invertebrate models to identify conserved SMN loss of function modifier genes. Drosophila melanogaster and Caenorhabditis elegans each have a single gene encoding a protein orthologous to human SMN; diminished function of these invertebrate genes causes lethality and neuromuscular defects. To find genes that modulate SMN function defects across species, two approaches were used. First, a genome-wide RNAi screen for C. elegans SMN modifier genes was undertaken, yielding four genes. Second, we tested the conservation of modifier gene function across species; genes identified in one invertebrate model were tested for function in the other invertebrate model. Drosophila orthologs of two genes, which were identified originally in C. elegans, modified Drosophila SMN loss of function defects. C. elegans orthologs of twelve genes, which were originally identified in a previous Drosophila screen, modified C. elegans SMN loss of function defects. Bioinformatic analysis of the conserved, cross-species, modifier genes suggests that conserved cellular pathways, specifically endocytosis and mRNA regulation, act as critical genetic modifiers of SMN loss of function defects across species.