Lymnaea

Characterization of corazonin signaling in a molluscan model species, Lymnaea stagnalis

In recent years, new concepts have emerged regarding the nomenclature, functions, and relationships of different peptide families of the gonadotropin-releasing hormone (GnRH) superfamily. One of the main driving forces for this originated from the emerging evidence that neuropeptides previously called molluscan GnRH are multifunctional and should be classified as corazonin (CRZ). However, research articles still appear that use incorrect nomenclature and attribute the same function to molluscan CRZs as vertebrate GnRHs. The aim of the present study was to further support the recent interpretation of the origin and function of the GnRH superfamily. Towards this goal, we report the characterization of CRZ signaling system in the molluscan model species, the great pond snail (Lymnaea stagnalis). We detected a CRZ-receptor-like sequence (Lym-CRZR) by homology-searching in the Lymnaea transcriptomes and the deduced amino acid sequence showed high sequence similarity to GnRH receptors and CRZ receptors. Molecular phylogenetic tree analysis demonstrated that Lym-CRZR is included in the cluster of molluscan CRZRs. Lym-CRZR transiently transfected into HEK293 cells was found to be localized at the plasma membrane, confirming that it functions as a membrane receptor, like other G protein-coupled receptors. The signaling assays revealed that the previously identified Lym-CRZ neuropeptide stimulated intracellular Ca2+ mobilization in a dose-dependent manner, but not cyclic AMP production, in HEK293 cells transfected with Lym-CRZR. Finally, we demonstrated a wide tissue distribution of Lym-CRZR. These results suggest that Lym-CRZ is a multifunctional peptide and provide further insights into the evolution of the GnRH neuropeptide superfamily. The present study also supports the notion that previously termed molluscan "GnRH" should be classified as "CRZ".

Lack of membrane sex steroid receptors for mediating rapid endocrine responses in molluscan nervous systems

Despite the lack of endogenous synthesis and relevant nuclear receptors, several papers have been published over the decades claiming that the physiology of mollusks is affected by natural and synthetic sex steroids. With scant evidence for the existence of functional steroid nuclear receptors in mollusks, some scientists have speculated that the effects of steroids might be mediated via membrane receptors (i.e. via non-genomic/non-classical actions) - a mechanism that has been well-characterized in vertebrates. However, no study has yet investigated the ligand-binding ability of such receptor candidates in mollusks. The aim of the present study was to further trace the evolution of the endocrine system by investigating the presence of functional membrane sex steroid receptors in a mollusk, the great pond snail (Lymnaea stagnalis). We detected sequences homologous to the known vertebrate membrane sex steroid receptors in the Lymnaea transcriptome and genome data: G protein-coupled estrogen receptor-1 (GPER1); membrane progestin receptors (mPRs); G protein-coupled receptor family C group 6 member A (GPRC6A); and Zrt- and Irt-like protein 9 (ZIP9). Sequence analyses, including conserved domain analysis, phylogenetics, and transmembrane domain prediction, indicated that the mPR and ZIP9 candidates appeared to be homologs, while the GPER1 and GPRC6A candidates seemed to be non-orthologous receptors. All candidates transiently transfected into HEK293MSR cells were found to be localized at the plasma membrane, confirming that they function as membrane receptors. However, the signaling assays revealed that none of the candidates interacted with the main vertebrate steroid ligands. Our findings strongly suggest that functional membrane sex steroid receptors which would be homologous to the vertebrate ones are not present in Lymnaea. Although further experiments are required on other molluscan model species as well, we propose that both classical and non-classical sex steroid signaling for endocrine responses are specific to chordates, confirming that molluscan and vertebrate endocrine systems are fundamentally different.

Neural mechanism of circadian clock-based photoperiodism in insects and snails

The photoperiodic mechanism distinguishes between long and short days, and the circadian clock system is involved in this process. Although the necessity of circadian clock genes for photoperiodic responses has been demonstrated in many species, how the clock system contributes to photoperiodic mechanisms remains unclear. A comprehensive study, including the functional analysis of relevant genes and physiology of their expressing cells, is necessary to understand the molecular and cellular mechanisms. Since Drosophila melanogaster exhibits a shallow photoperiodism, photoperiodic mechanisms have been studied in non-model species, starting with brain microsurgery and neuroanatomy, followed by genetic manipulation in some insects. Here, we review and discuss the involvement of the circadian clock in photoperiodic mechanisms in terms of neural networks in insects. We also review recent advances in the neural mechanisms underlying photoperiodic responses in insects and snails, and additionally circadian clock systems in snails, whose involvement in photoperiodism has hardly been addressed yet. Brain neurosecretory cells, insulin-like peptide/diuretic hormone44-expressing pars intercerebralis neurones in the bean bug Riptortus pedestris and caudo-dorsal cell hormone-expressing caudo-dorsal cells in the snail Lymnaea stagnalis, both promote egg laying under long days, and their electrical excitability is attenuated under short and medium days, which reduces oviposition. The photoperiodic responses of the pars intercerebralis neurones are mediated by glutamate under the control of the clock gene period. Thus, we are now able to assess the photoperiodic response by neurosecretory cell activity to investigate the upstream mechanisms, that is, the photoperiodic clock and counter.

Same same, but different: exploring the enigmatic role of the pituitary adenylate cyclase-activating polypeptide (PACAP) in invertebrate physiology

Evidence has been accumulating that elements of the vertebrate pituitary adenylate cyclase-activating polypeptide (PACAP) system are missing in non-chordate genomes, which is at odds with the partial sequence-, immunohistochemical-, and physiological data in the literature. Multilevel experiments were performed on the great pond snail (Lymnaea stagnalis) to explore the role of PACAP in invertebrates. Screening of neuronal transcriptome and genome data did not reveal homologs to the elements of vertebrate PACAP system. Despite this, immunohistochemical investigations with an anti-human PAC1 receptor antibody yielded a positive signal in the neuronal elements in the heart. Although Western blotting of proteins extracted from the nervous system found a relevant band for PACAP-38, immunoprecipitation and mass spectrometric analyses revealed no corresponding peptide fragments. Similarly to the effects reported in vertebrates, PACAP-38 significantly increased cAMP synthesis in the heart and had a positive ionotropic effect on heart preparations. Moreover, it significantly modulated the effects of serotonin and acetylcholine. Homologs to members of Cluster B receptors, which have shared common evolutionary origin with the vertebrate PACAP receptors, PTHRs, and GCGRs, were identified and shown not to be expressed in the heart, which does not support a potential role in the mediation of PACAP-induced effects. Our findings support the notion that the PACAP system emerged after the protostome-deuterostome divergence. Using antibodies against vertebrate proteins is again highlighted to have little/no value in invertebrate studies. The physiological effects of vertebrate PACAP peptides in protostomes, no matter how similar they are to those in vertebrates, should be considered non-specific.

Copper-transporting ATPases throughout the animal evolution - From clinics to basal neuron-less animals

Copper-transporting ATPases are a group of heavy metal-transporting proteins and which can be found in all living organisms. In animals, they are generally referred to as ATP7 proteins and are involved in many different physiological processes including the maintaining of copper homeostasis and the supply of copper to cuproenzymes. A single ATP7 gene is present in non-chordate animals while it is divided into ATP7A and ATP7B in chordates. In humans, dysfunction of ATP7 proteins can lead to severe genetic disorders, such as, Menkes disease and Wilson's disease, which are characterized by abnormal copper transport and accumulation, causing significant health complications. Therefore, there is a substantial amount of research on ATP7 genes and ATP7 proteins in humans and mice to understand pathophysiological conditions and find potential therapeutic interventions. Copper-transporting ATPases have also been investigated in some non-mammalian vertebrates, protostomes, single-cellular eukaryotes, prokaryotes, and archaea to gain useful evolutionary insights. However, ATP7 function in many animals has been somewhat neglected, particularly in non-bilaterians. Previous reviews on this topic only broadly summarized the available information on the function and evolution of ATP7 genes and ATP7 proteins and included only the classic vertebrate and invertebrate models. Given this, and the fact that a considerable amount of new information on this topic has been published in recent years, the present study was undertaken to provide an up-to-date, comprehensive summary of ATP7s/ATP7s and give new insights into their evolutionary relationships. Additionally, this work provides a framework for studying these genes and proteins in non-bilaterians. As early branching animals, they are important to understand the evolution of function of these proteins and their important role in copper homeostasis and neurotransmission.

Unique morphology and photoperiodically regulated activity of neurosecretory canopy cells in the pond snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis exhibits clear photoperiodism in egg laying; it lays more eggs in long-day conditions than in medium-day conditions. A key regulator of egg laying is neurosecretory caudo-dorsal cells (CDCs) producing an ovulation hormone in the cerebral ganglia. Paired small budding structures of the cerebral ganglia (viz. the lateral lobe) also promote egg laying in addition to spermatogenesis and maturation of female accessory sex organs. However, it remains unknown which cells in the lateral lobe are responsible for these. Previous anatomical and physiological studies prompted us to hypothesize that canopy cells in the lateral lobe modulate activity of CDCs. However, double labeling of the canopy cell and CDCs revealed no sign of direct neural connections, suggesting that activity of CDCs is regulated either humorally or through a neural pathway independent of canopy cells. In addition, our detailed anatomical re-evaluation confirmed previous observations that the canopy cell bears fine neurites along the ipsilateral axon and extensions from the plasma membrane of the cell body, although the function of these extensions remains unexplored. Furthermore, comparison of electrophysiological properties between long-day and medium-day conditions indicated that the canopy cell's activity is moderately under photoperiodic regulation: resting membrane potentials of long-day snails are shallower than those of medium-day snails, and spontaneously spiking neurons are only observed in long-day conditions. Thus, canopy cells appear to receive photoperiodic information and regulate photoperiod-dependent phenomena, but not provide direct neural inputs to CDCs.

A novel and unique refraction-based optical recording system for pharmacological investigations on isolated muscle preparations

In the field of neuroscience and ecotoxicology, there is a great need for investigating the effect(s) of a variety of different chemicals (e.g., pharmacologically active compounds, pesticides, neurotransmitters, modulators) at different biological levels. Different contractile tissue preparations have provided excellent model systems for in vitro pharmacological experiments for a long time. However, such investigations usually apply mechanical force transducer-based approaches. Thus, a rapid, easy, cheap, digital, and reproducible in vitro pharmacological method based on an effective, 'non-invasive' (compared to the force-transducer approaches), refraction-based optical recording approach and isolated heart preparations was developed.•A versatile and unique refraction-based optical recording system with a Java application was developed.•The recording system was tested and validated on isolated heart preparations obtained from the widely used invertebrate model organism, the great pond snail (Lymnaea stagnalis).•The recording system illustrates the progression of technology from the mechanical force transducer system and can represent a suitable tool in ecotoxicology or neuroscience.

Sublethal cadmium exposure in the freshwater snail Lymnaea stagnalis meets a deficient, poorly responsive metallothionein system while evoking oxidative and cellular stress

The Great Pond snail Lymnaea stagnalis (Gastropoda, Hygrophila) is a wide-spread freshwater gastropod, being considered as a model organism for research in many fields of biology, including ecotoxicology. The aim of the present study was to explore the Cd sensitivity of L. stagnalis through the measurement of a biomarker battery for oxidative, toxic and cellular stress. The interpretation of biomarker parameters occurred against the background of a truncated metallothionein protein with a limited Cd-binding capacity. Individuals of L. stagnalis were exposed through 14 days to uncontaminated water (controls) or to low (30 μg · L^-1) or high (50 μg · L^-1) Cd concentrations. The digestive gland of control and low-Cd exposed snails was processed for transcriptional analysis of the Metallothionein (MT) gene expression, and for determination of biomarkers for oxidative stress, toxicity and cellular stress. Digestive gland supernatants of high-Cd exposed snails were subjected to chromatography and subsequent analysis by spectrophotometry. It was shown that the MT system of L. stagnalis is functionally deficient, with a poor Cd responsiveness at both, the transcriptional and the protein expression levels. Instead, L. stagnalis appears to rely on alternative detoxification mechanisms such as Cd binding by phytochelatins and metal inactivation by compartmentalization within the lysosomal system. In spite of this, however, traces of Cd apparently leak out of the pre-determined detoxification pathways, leading to adverse effects, which is clearly indicated by biomarkers of oxidative and cellular stress.

Peptidergic modulation of a multi-functional central pattern generator in the pulmonate snail

Egg laying in pulmonate snails is a well-orchestrated process that involves a period of reduced locomotion, followed by substrate cleaning with rhythmic rasping of the surface to make tiny grooves, into which eggs are deposited. Although the neurohormonal control of initiating egg laying has been well established, the signals that modulate the buccal central pattern generator to substrate cleaning during egg laying are not known. Neuropeptides of the invertebrate gonadotropin-releasing hormone/corazonin family (invGnRH/CRZ) have been shown to be involved in reproduction and allied behaviors in many vertebrates and invertebrates. Here, we show that the buccal motor pattern underlying substrate cleaning during egg laying is altered by a vertebrate GnRH agonist. Signals from the intestinal nerve innervating reproductive structures, previously shown to be both necessary and sufficient for egg-laying behaviors, are blocked by a vertebrate GnRH antagonist. Further, the vertebrate GnRH-triggered response elicits rhythmic, phase 2 and non-phase 2 activity in the buccal motor pattern, with a shutdown of phase 3, indicative of repetitive rasping without accompanied swallowing behavior. Using immunohistochemistry, intracellular electrophysiology and extracellular nerve stimulation, we show that a member of the invGnRH/CRZ family of neuropeptides could be the signal that contextually switches the multifunctional buccal CPG to a biphasic rasping rhythm that underlies substrate cleaning behavior during egg laying in the pulmonate snail Planorbella (Helisoma) trivolvis.

From Biological Synapses to “Intelligent” Robots

This selective review explores biologically inspired learning as a model for intelligent robot control and sensing technology on the basis of specific examples. Hebbian synaptic learning is discussed as a functionally relevant model for machine learning and intelligence, as explained on the basis of examples from the highly plastic biological neural networks of invertebrates and vertebrates. Its potential for adaptive learning and control without supervision, the generation of functional complexity, and control architectures based on self-organization is brought forward. Learning without prior knowledge based on excitatory and inhibitory neural mechanisms accounts for the process through which survival-relevant or task-relevant representations are either reinforced or suppressed. The basic mechanisms of unsupervised biological learning drive synaptic plasticity and adaptation for behavioral success in living brains with different levels of complexity. The insights collected here point toward the Hebbian model as a choice solution for “intelligent” robotics and sensor systems.

Studies on a widely-recognized snail model species (Lymnaea stagnalis) provide further evidence that vertebrate steroids do not have a hormonal role in the reproduction of mollusks

Experiments were carried out to determine whether, as with other mollusks that have been studied, the snail, Lymnaea stagnalis, can absorb, esterify and store vertebrate steroids that are present in the water. We also carried out experiments to determine whether neural tissues of the snail could be immunohistochemically stained with an antibody to human aromatase (a key enzyme that catalyzes the conversion of testosterone [T] to 17β-estradiol [E₂]); and, if so, to determine the significance of such staining. Previous studies on other mollusks have reported such staining and have proposed this as decisive evidence that mollusks have the same steroid synthesis pathway as vertebrates. We found that snails absorb, esterify and retain esterified T, E₂, progesterone and ethinyl-estradiol (albeit with an absorption rate about four times slower, on a weight basis, than the mussel, Mytilus edulis). We also found that not only anti-human aromatase, but also anti-human nuclear progesterone receptor (nPR) and anti-human gonadotropin-releasing hormone antibodies immunohistochemically stained snail neural cells. However, further experiments, involving gel electrophoretic separation, followed by immunostaining, of proteins extracted from the neural tissue, found at least two positively-stained bands for each antibody, none of which had masses matching the human proteins to which the antibodies had been raised. The anti-aromatase antibody even stained the 140 kDA ladder protein used as a molecular weight marker on the gels. Mass spectrometric analysis of the bands did not find any peptide sequences that corresponded to the human proteins. Our findings confirm that the presence of vertebrate-like sex steroids in molluscan tissues is not necessarily evidence of endogenous origin. The results also show that immunohistochemical studies using antibodies against human proteins are grossly non-specific and likely to have little or no value in studying steroid synthesis or activity in mollusks. Our conclusions are consistent with the fact that genes for aromatase and nPR have not been found in the genome of the snail or of any other mollusk. Our overarching conclusion, from this and our previous studies, is that the endocrinology of mollusks is not the same as that of humans or any other vertebrates and that continuing to carry out physiological and ecotoxicological studies on mollusks on the basis of this false assumption, is an unconscionable waste of resources.

The Great Pond Snail (Lymnaea stagnalis) as a Model of Aging and Age-Related Memory Impairment: An Overview

With the increase of life span, normal aging and age-related memory decline are affecting an increasing number of people; however, many aspects of these processes are still not fully understood. Although vertebrate models have provided considerable insights into the molecular and electrophysiological changes associated with brain aging, invertebrates, including the widely recognized molluscan model organism, the great pond snail (Lymnaea stagnalis), have proven to be extremely useful for studying mechanisms of aging at the level of identified individual neurons and well-defined circuits. Its numerically simpler nervous system, well-characterized life cycle, and relatively long life span make it an ideal organism to study age-related changes in the nervous system. Here, we provide an overview of age-related studies on L. stagnalis and showcase this species as a contemporary choice for modeling the molecular, cellular, circuit, and behavioral mechanisms of aging and age-related memory impairment.

Maternal Serotonin: Shaping Developmental Patterns and Behavioral Strategy on Progeny in Molluscs

Serotonin is a well-known neurotransmitter and neurohormone regulating mood, sleep, feeding, and learning in high organisms. Serotonin also affects the embryonic events related to neurogenesis and maturation of hormonal systems, the underlying organism adaptation to a changing environment. Such serotonin-based mother-to-embryo signaling is realized via direct interactions in case of internal fertilization and embryonic development inside the mother body. However, the possibility of such signaling is less obvious in organisms with the ancestral type of embryogenesis and embryo development within the egg, outside the mother body. Our data, based on the investigation of freshwater gastropod molluscs (Lymnaea and Helisoma), demonstrated a correlation between seasonal variations of serotonin content within the female reproductive system, and developmental patterns and the behavioral characteristics of progeny. The direct action of serotonin via posttranslational protein modification—serotonylation—during early development, as well as classical receptor-mediated effects, underlies such serotonin-modulated developmental changes. In the present paper, I will shortly overview our results on freshwater molluscs and parallel the experimental data with the living strategy of these species occupying almost all Holarctic regions.

What can we teach Lymnaea and what can Lymnaea teach us?

This review describes the advantages of adopting a molluscan complementary model, the freshwater snail Lymnaea stagnalis, to study the neural basis of learning and memory in appetitive and avoidance classical conditioning; as well as operant conditioning of its aerial respiratory and escape behaviour. We firstly explored 'what we can teach Lymnaea' by discussing a variety of sensitive, solid, easily reproducible and simple behavioural tests that have been used to uncover the memory abilities of this model system. Answering this question will allow us to open new frontiers in neuroscience and behavioural research to enhance our understanding of how the nervous system mediates learning and memory. In fact, from a translational perspective, Lymnaea and its nervous system can help to understand the neural transformation pathways from behavioural output to sensory coding in more complex systems like the mammalian brain. Moving on to the second question: 'what can Lymnaea teach us?', it is now known that Lymnaea shares important associative learning characteristics with vertebrates, including stimulus generalization, generalization of extinction and discriminative learning, opening the possibility to use snails as animal models for neuroscience translational research.

Early Metazoan Origin and Multiple Losses of a Novel Clade of RIM Presynaptic Calcium Channel Scaffolding Protein Homologs

The precise localization of Ca_V2 voltage-gated calcium channels at the synapse active zone requires various interacting proteins, of which, Rab3-interacting molecule or RIM is considered particularly important. In vertebrates, RIM interacts with Ca_V2 channels in vitro via a PDZ domain that binds to the extreme C-termini of the channels at acidic ligand motifs of D/E-D/E/H-WC-_COOH, and knockout of RIM in vertebrates and invertebrates disrupts Ca_V2 channel synaptic localization and synapse function. Here, we describe a previously uncharacterized clade of RIM proteins bearing domain architectures homologous to those of known RIM homologs, but with some notable differences including key amino acids associated with PDZ domain ligand specificity. This novel RIM emerged near the stem lineage of metazoans and underwent extensive losses, but is retained in select animals including the early-diverging placozoan Trichoplax adhaerens, and molluscs. RNA expression and localization studies in Trichoplax and the mollusc snail Lymnaea stagnalis indicate differential regional/tissue type expression, but overlapping expression in single isolated neurons from Lymnaea. Ctenophores, the most early-diverging animals with synapses, are unique among animals with nervous systems in that they lack the canonical RIM, bearing only the newly identified homolog. Through phylogenetic analysis, we find that Ca_V2 channel D/E-D/E/H-WC-_COOH like PDZ ligand motifs were present in the common ancestor of cnidarians and bilaterians, and delineate some deeply conserved C-terminal structures that distinguish Ca_V1 from Ca_V2 channels, and Ca_V1/Ca_V2 from Ca_V3 channels.

Photoperiodic control of electrophysiological properties of the caudo-dorsal cells in the pond snail, Lymnaea stagnalis

Egg laying in the pond snail, Lymnaea stagnalis is regulated by the photoperiod; long-day conditions (16L8D) promote egg laying whereas medium-day conditions (12L12D) suppress it. In this snail, a caudo-dorsal cell hormone (CDCH) is produced by neurosecretory cells, CDCs in the cerebral ganglion (CG), and its release triggers ovulation and subsequent egg laying. However, the physiological basis for photoperiod-dependent egg laying remains unraveled. Here, we compared electrophysiological properties of CDCs between 16L8D and 12L12D using intracellular recording, and found that CDC excitability is higher in 16L8D than in 12L12D. Striking differences are as follows: (1) a shallower resting membrane potential in 16L8D than in 12L12D, and (2) a smaller threshold voltage (minimum depolarization from rest to elicit action potentials) in 16L8D than in 12L12D. Switching of the excitability can be a physiological basis of a photoperiod-dependent CDCH release. Simultaneous intracellular dye injection identified two morphological subtypes of CDCs, validating a previous report. Both types bear short lateral extensions in CG, some of which probably function as integration sites of photoperiodic inputs. In addition, we found two novel CDCH-immunoreactive cell groups (CDC_COM and SC_m ) in the CG besides conventional CDCs and small cells expressing CDCH. The CDC_COM with cell bodies and fibers in the neurohemal commissure may be involved in triggering ovulation. Notably, the total number of CDCs is larger than that previously reported, the right CDC cluster with more cells than the left. Our findings are instructive in following the neurophysiology of photoperiodism in L. stagnalis.

The pond snail Lymnaea stagnalis

The freshwater snail Lymnaea stagnalis has a long research history, but only relatively recently has it emerged as an attractive model organism to study molecular mechanisms in the areas of developmental biology and translational medicine such as learning/memory and neurodegenerative diseases. The species has the advantage of being a hermaphrodite and can both cross- and self-mate, which greatly facilitates genetic approaches. The establishment of body-handedness, or chiromorphogenesis, is a major topic of study, since chirality is evident in the shell coiling. Chirality is maternally inherited, and only recently a gene-editing approach identified the actin-related gene Lsdia1 as the key handedness determinant. This short article reviews the natural habitat, life cycle, major research questions and interests, and experimental approaches.

Identification, presence, and possible multifunctional regulatory role of invertebrate gonadotropin-releasing hormone/corazonin molecule in the great pond snail (Lymnaea stagnalis)

In the last years, our interpretation of the origin and function of the gonadotropin-releasing hormone (GnRH) neuropeptide superfamily has changed substantially. A main driver for these conceptual changes came from increased investigations into functions and evolutionary lineage of previously identified molluscan GnRH molecules. Emerging evidence suggests not only reproductive, but also diverse biological effects of these molecules and proposes they should most likely be called corazonin (CRZ). Clearly, a more global understanding requires further exploration of species-specific functions and structure of invGnRH/CRZ peptides. Towards this goal, we have identified the full-length cDNA of invGnRH/CRZ peptide in an invertebrate model species, the great pond snail Lymnaea stagnalis, termed ly-GnRH/CRZ, and characterized the transcript and peptide distribution in the central nervous system (CNS) and peripheral organs. Our results are consistent with previous data that molluscan GnRHs are more related to CRZs and serve diverse functions. Hence, our findings support the notion that peptides originally termed molluscan GnRH are multifunctional modulators and that nomenclature change should be taken into consideration.

The unlimited potential of the great pond snail, Lymnaea stagnalis

Only a limited number of animal species lend themselves to becoming model organisms in multiple biological disciplines: one of these is the great pond snail, Lymnaea stagnalis. Extensively used since the 1970s to study fundamental mechanisms in neurobiology, the value of this freshwater snail has been also recognised in fields as diverse as host-parasite interactions, ecotoxicology, evolution, genome editing and 'omics', and human disease modelling. While there is knowledge about the natural history of this species, what is currently lacking is an integration of findings from the laboratory and the field. With this in mind, this article aims to summarise the applicability of L. stagnalis and points out that this multipurpose model organism is an excellent, contemporary choice for addressing a large range of different biological questions, problems and phenomena.

Aging and disease-relevant gene products in the neuronal transcriptome of the great pond snail (Lymnaea stagnalis): a potential model of aging, age-related memory loss, and neurodegenerative diseases

Modelling of human aging, age-related memory loss, and neurodegenerative diseases has developed into a progressive area in invertebrate neuroscience. Gold standard molluscan neuroscience models such as the sea hare (Aplysia californica) and the great pond snail (Lymnaea stagnalis) have proven to be attractive alternatives for studying these processes. Until now, A. californica has been the workhorse due to the enormous set of publicly available transcriptome and genome data. However, with growing sequence data, L. stagnalis has started to catch up with A. californica in this respect. To contribute to this and inspire researchers to use molluscan species for modelling normal biological aging and/or neurodegenerative diseases, we sequenced the whole transcriptome of the central nervous system of L. stagnalis and screened for the evolutionary conserved homolog sequences involved in aging and neurodegenerative/other diseases. Several relevant molecules were identified, including for example gelsolin, presenilin, huntingtin, Parkinson disease protein 7/Protein deglycase DJ-1, and amyloid precursor protein, thus providing a stable genetic background for L. stagnalis in this field. Our study supports the notion that molluscan species are highly suitable for studying molecular, cellular, and circuit mechanisms of the mentioned neurophysiological and neuropathological processes.

Gap Junction Coding Innexin in Lymnaea stagnalis: Sequence Analysis and Characterization in Tissues and the Central Nervous System

Connections between neurons called synapses are the key components underlying all nervous system functions of animals and humans. However, important genetic information on the formation and plasticity of one type, the electrical (gap junction-mediated) synapse, is understudied in many invertebrates. In the present study, we set forth to identify and characterize the gap junction-encoding gene innexin in the central nervous system (CNS) of the mollusk pond snail Lymnaea stagnalis. With PCR, 3′ and 5′ RACE, and BLAST searches, we identified eight innexin genes in the L. stagnalis genome, named Lst Inx1–Lst Inx8. Phylogenetic analysis revealed that the L. stagnalis innexin genes originated from a single copy in the common ancestor of molluskan species by multiple gene duplication events and have been maintained in L. stagnalis since they were generated. The paralogous innexin genes demonstrate distinct expression patterns among tissues. In addition, one paralog, Lst Inx1, exhibits heterogeneity in cells and ganglia, suggesting the occurrence of functional diversification after gene duplication. These results introduce possibilities to study an intriguing potential relationship between innexin paralog expression and cell-specific functional outputs such as heterogenic ability to form channels and exhibit synapse plasticity. The L. stagnalis CNS contains large neurons and functionally defined networks for behaviors; with the introduction of L. stagnalis in the gap junction gene field, we are providing novel opportunities to combine genetic research with direct investigations of functional outcomes at the cellular, synaptic, and behavioral levels.

Lymnaea stagnalis as model for translational neuroscience research: From pond to bench

The purpose of this review is to illustrate how a reductionistic, but sophisticated, approach based on the use of a simple model system such as the pond snail Lymnaea stagnalis (L. stagnalis), might be useful to address fundamental questions in learning and memory. L. stagnalis, as a model, provides an interesting platform to investigate the dialog between the synapse and the nucleus and vice versa during memory and learning. More importantly, the "molecular actors" of the memory dialogue are well-conserved both across phylogenetic groups and learning paradigms, involving single- or multi-trials, aversion or reward, operant or classical conditioning. At the same time, this model could help to study how, where and when the memory dialog is impaired in stressful conditions and during aging and neurodegeneration in humans and thus offers new insights and targets in order to develop innovative therapies and technology for the treatment of a range of neurological and neurodegenerative disorders.

A new microfluidic device design for a defined positioning of neurons in vitro

A new triangle-shaped microfluidic channel system for defined cell trapping is presented. Different variants of the same basic geometry were produced to reveal the best fitting parameter combinations regarding efficiency and sensitivity. Variants with differences in the trap gap width and the inter-trap distance were analyzed in detail by Computational Fluid Dynamics simulations and in experiments with artificial beads of different sizes (30, 60, 80 μm). Simulation analysis of flow dynamics and pressure profiles revealed strongly reduced pressure conditions and balanced flow rates inside the microfluidic channels compared to commonly used systems with meandering channels. Quantitative experiments with beads showed very good trapping results in all channel types with slight variations due to geometrical differences. Highest efficiency in terms of fast trap filling and low particle loss was shown with channel types having a larger trap gap width (20 μm) and/or a larger inter-trap distance (400 μm). Here, experimental success was achieved in almost 85% to 100% of all cases. Particle loss appeared significantly more often with large beads than with small beads. A significantly reduced trapping efficiency of about 50% was determined by using narrow trap gaps and a small inter-trap distance in combination with large 80 μm beads. The combination of the same parameters with small and medium beads led to an only slight decrease in trapping efficiency (80%). All channel types were tested qualitatively with invertebrate neurons from the pond snail Lymnaea stagnalis. The systems were appropriate to trap those sensitive neurons and to keep their viability in the trapping area at the same time.

Susceptibility of memory consolidation during lapses in recall

Memories that can be recalled several hours after learning may paradoxically become inaccessible for brief periods after their formation. This raises major questions about the function of these early memory lapses in the structure of memory consolidation. These questions are difficult to investigate because of the lack of information on the precise timing of lapses. However, the use of a single-trial conditioning paradigm in Lymnaea solves this problem. Here we use electrophysiological and behavioural experiments to reveal lapses in memory recall at 30 min and 2 h post conditioning. We show that only during these lapses is consolidation of long-term memory susceptible to interruption by external disturbance. These shared time points of memory lapse and susceptibility correspond to transitions between different phases of memory that have different molecular requirements. We propose that during periods of molecular transition memory recall is weakened, allowing novel sensory cues to block the consolidation of long-term memory.

Memory lapses during memory consolidation are periods when the memory becomes briefly inaccessible after its formation. Marra and colleagues study memory lapses in the mollusc Lymnaea, and find that only during these lapses is consolidation of memories susceptible to interruption by external disturbances.

Involvement of insulin-like peptide in long-term synaptic plasticity and long-term memory of the pond snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis is capable of learning taste aversion and consolidating this learning into long-term memory (LTM) that is called conditioned taste aversion (CTA). Previous studies showed that some molluscan insulin-related peptides (MIPs) were upregulated in snails exhibiting CTA. We thus hypothesized that MIPs play an important role in neurons underlying the CTA-LTM consolidation process. To examine this hypothesis, we first observed the distribution of MIP II, a major peptide of MIPs, and MIP receptor and determined the amounts of their mRNAs in the CNS. MIP II was only observed in the light green cells in the cerebral ganglia, but the MIP receptor was distributed throughout the entire CNS, including the buccal ganglia. Next, when we applied exogenous mammalian insulin, secretions from MIP-containing cells or partially purified MIPs, to the isolated CNS, we observed a long-term change in synaptic efficacy (i.e., enhancement) of the synaptic connection between the cerebral giant cell (a key interneuron for CTA) and the B1 motor neuron (a buccal motor neuron). This synaptic enhancement was blocked by application of an insulin receptor antibody to the isolated CNS. Finally, injection of the insulin receptor antibody into the snail before CTA training, while not blocking the acquisition of taste aversion learning, blocked the memory consolidation process; thus, LTM was not observed. These data suggest that MIPs trigger changes in synaptic connectivity that may be correlated with the consolidation of taste aversion learning into CTA-LTM in the Lymnaea CNS.

Single electrode dynamic clamp with StdpC

Dynamic clamp is a powerful approach for electrophysiological investigations allowing researchers to introduce artificial electrical components into target neurons to simulate ionic conductances, chemical or electrotonic inputs or connections to other cells. Due to the rapidly changing and potentially large current injections during dynamic clamp, problematic voltage artifacts appear on the electrode used to inject dynamic clamp currents into a target neuron. Dynamic clamp experiments, therefore, typically use two separate electrodes in the same cell, one for recording membrane potential and one for injecting currents. The requirement for two independent electrodes has been a limiting factor for the use of dynamic clamp in applications where dual recordings of this kind are difficult or impossible to achieve. The recent development of an active electrode compensation (AEC) method has overcome some of these prior limitations, permitting artifact-free dynamic clamp experimentation with a single electrode. Here we describe an AEC method for the free dynamic clamp software StdpC. The AEC component of StdpC is the first such system implemented for the use of non-expert users and comes with a set of semi-automated configuration and calibration procedures that facilitate its use. We briefly introduce the AEC method and its implementation in StdpC and then validate it with an electronic model cell and in two different biological preparations.

De novo sequencing and transcriptome analysis of the central nervous system of mollusc Lymnaea stagnalis by deep RNA sequencing

The pond snail Lymnaea stagnalis is among several mollusc species that have been well investigated due to the simplicity of their nervous systems and large identifiable neurons. Nonetheless, despite the continued attention given to the physiological characteristics of its nervous system, the genetic information of the Lymnaea central nervous system (CNS) has not yet been fully explored. The absence of genetic information is a large disadvantage for transcriptome sequencing because it makes transcriptome assembly difficult. We here performed transcriptome sequencing for Lymnaea CNS using an Illumina Genome Analyzer IIx platform and obtained 81.9 M of 100 base pair (bp) single end reads. For de novo assembly, five programs were used: ABySS, Velvet, OASES, Trinity and Rnnotator. Based on a comparison of the assemblies, we chose the Rnnotator dataset for the following blast searches and gene ontology analyses. The present dataset, 116,355 contigs of Lymnaea transcriptome shotgun assembly (TSA), contained longer sequences and was much larger compared to the previously reported Lymnaea expression sequence tag (EST) established by classical Sanger sequencing. The TSA sequences were subjected to blast analyses against several protein databases and Aplysia EST data. The results demonstrated that about 20,000 sequences had significant similarity to the reported sequences using a cutoff value of 1e-6, and showed the lack of molluscan sequences in the public databases. The richness of the present TSA data allowed us to identify a large number of new transcripts in Lymnaea and molluscan species.

Multi-neuronal refractory period adapts centrally generated behaviour to reward

Oscillating neuronal circuits, known as central pattern generators (CPGs), are responsible for generating rhythmic behaviours such as walking, breathing and chewing. The CPG model alone however does not account for the ability of animals to adapt their future behaviour to changes in the sensory environment that signal reward. Here, using multi-electrode array (MEA) recording in an established experimental model of centrally generated rhythmic behaviour we show that the feeding CPG of Lymnaea stagnalis is itself associated with another, and hitherto unidentified, oscillating neuronal population. This extra-CPG oscillator is characterised by high population-wide activity alternating with population-wide quiescence. During the quiescent periods the CPG is refractory to activation by food-associated stimuli. Furthermore, the duration of the refractory period predicts the timing of the next activation of the CPG, which may be minutes into the future. Rewarding food stimuli and dopamine accelerate the frequency of the extra-CPG oscillator and reduce the duration of its quiescent periods. These findings indicate that dopamine adapts future feeding behaviour to the availability of food by significantly reducing the refractory period of the brain's feeding circuitry.

Functional organization and adaptability of a decision-making network in aplysia

Whereas major insights into the neuronal basis of adaptive behavior have been gained from the study of automatic behaviors, including reflexive and rhythmic motor acts, the neural substrates for goal-directed behaviors in which decision-making about action selection and initiation are crucial, remain poorly understood. However, the mollusk Aplysia is proving to be increasingly relevant to redressing this issue. The functional properties of the central circuits that govern this animal’s goal-directed feeding behavior and particularly the neural processes underlying the selection and initiation of specific feeding actions are becoming understood. In addition to relying on the intrinsic operation of central networks, goal-directed behaviors depend on external sensory inputs that through associative learning are able to shape decision-making strategies. Here, we will review recent findings on the functional design of the central network that generates Aplysia’s feeding-related movements and the sensory-derived plasticity that through learning can modify the selection and initiation of appropriate action. The animal’s feeding behavior and the implications of decision-making will be briefly described. The functional design of the underlying buccal network will then be used to illustrate how cellular diversity and the coordination of neuronal burst activity provide substrates for decision-making. The contribution of specific synaptic and neuronal membrane properties within the buccal circuit will also be discussed in terms of their role in motor pattern selection and initiation. The ability of learning to “rigidify” these synaptic and cellular properties so as to regularize network operation and lead to the expression of stereotyped rhythmic behavior will then be described. Finally, these aspects will be drawn into a conceptual framework of how Aplysia’s goal-directed circuitry compares to the central pattern generating networks for invertebrate rhythmic behaviors.

In vitro aversion conditioning in Lymnaea. Short communication

In an in vitro semi-intact Lymnaea preparation we were successful in using a training procedure to bring about Conditioning Taste Aversion (CTA ). Following paired presentation of the CS (sucrose) and US (tactile), the CS no longer elicits feeding. We can use sucrose to the lips in the semi-intact preparation as the CS; while we use direct current injection to depolarize RPeD11 as the US. Following pairing of these stimuli, the CS no longer elicits fictive feeding. We can determine the changes in synaptic input to neurons that play key roles in controlling feeding behavior.

Nitric oxide as a regulator of behavior: new ideas from Aplysia feeding

Nitric oxide (NO) regulates Aplysia feeding by novel mechanisms, suggesting new roles for NO in controlling the behavior of higher animals. In Aplysia, (1) NO helps maintain arousal when produced by neurons responding to attempts to swallow food; (2) NO biases the motor system to reject and reposition food that resists swallowing; (3) if mechanically resistant food is not successfully swallowed, NO mediates the formation and expression of memories of food inedibility; (4) NO production at rest inhibits feeding, countering the effects of food stimuli exciting feeding. At a cellular level, NO-dependent channels contribute to the resting potential of neurons controlling food finding and food consumption. Increases in L-arginine after animals eat act as a post-feeding inhibitory signal, presumably by modulating NO production at rest. NO also signals non-feeding behaviors that are associated with feeding inhibition. Thus, depending on context, NO may enhance or inhibit feeding behavior. The different functions of NO may reflect the evolution of NO signaling from a response to tissue damage that was then elaborated and used for additional functions. These results suggest that in higher animals (1) elicited and background transmitter release may have similar effects; (2) NO may be produced by neurons without firing, influencing adjacent neurons; (3) background NO production may contribute to a neuron's resting potential; (4) circulating factors affecting background NO production may regulate spatially separated neurons; (5) L-arginine can be used to regulate neural activity; (6) L-arginine may be an effective post-ingestion metabolic signal to regulate feeding.

Neuro-endocrine control of reproduction in hermaphroditic freshwater snails: mechanisms and evolution

Invertebrates are used extensively as model species to investigate neuro-endocrine processes regulating behaviors, and many of these processes may be extrapolated to vertebrates. However, when it comes to reproductive processes, many of these model species differ notably in their mode of reproduction. A point in case are simultaneously hermaphroditic molluscs. In this review I aim to achieve two things. On the one hand, I provide a comprehensive overview of the neuro-endocrine control of male and female reproductive processes in freshwater snails. Even though the focus will necessarily be on Lymnaea stagnalis, since this is the best-studied species in this respect, extensions to other species are made wherever possible. On the other hand, I will place these findings in the actual context of the whole animal, after all these are simultaneous hermaphrodites. By considering the hermaphroditic situation, I uncover a numbers of possible links between the regulation of the two reproductive systems that are present within this animal, and suggest a few possible mechanisms via which this animal can effectively switch between the two sexual roles in the flexible way that it does. Evidently, this opens up a number of new research questions and areas that explicitly integrate knowledge about behavioral decisions (e.g., mating, insemination, egg laying) and sexual selection processes (e.g., mate choice, sperm allocation) with the actual underlying neuronal and endocrine mechanisms required for these processes to act and function effectively.

Pituitary adenylate cyclase activating polypeptide (PACAP) and its receptors are present and biochemically active in the central nervous system of the pond snail Lymnaea stagnalis

PACAP is a highly conserved adenylate cyclase (AC) activating polypeptide, which, along with its receptors (PAC1-R, VPAC1, and VPAC2), is expressed in both vertebrate and invertebrate nervous systems. In vertebrates, PACAP has been shown to be involved in associative learning, but it is not known if it plays a similar role in invertebrates. To prepare the way for a detailed investigation into the possible role of PACAP and its receptors in a suitable invertebrate model of learning and memory, here, we undertook a study of their expression and biochemical role in the central nervous system of the pond snail Lymnaea stagnalis. Lymnaea is one of the best established invertebrate model systems to study the molecular mechanisms of learning and memory, including the role of cyclic AMP-activated signaling mechanisms, which crucially depend on the learning-induced activation of AC. However, there was no information available on the expression of PACAP and its receptors in sensory structures and central ganglia of the Lymnaea nervous system known to be involved in associative learning or whether or not PACAP can actually activate AC in these ganglia. Here, using matrix-assisted laser desorption ionization time of flight (MALDI-TOF) and immunohistochemistry, we established the presence of PACAP-like peptides in the cerebral ganglia and the lip region of Lymnaea. The MALDI-TOF data indicated an identity with mammalian PACAP-27 and the presence of a squid-like PACAP-38 highly homologous to vertebrate PACAP-38. We also showed that PACAP, VIP, and maxadilan stimulated the synthesis of cAMP in Lymnaea cerebral ganglion homogenates and that this effect was blocked by the appropriate general and selective PACAP receptor antagonists.

Contrary effects of octopamine receptor ligands on behavioral and neuronal changes in locomotion of lymnaea

The pond snail Lymnaea stagnalis moves along the sides and bottom of an aquarium, but it can also glide upside down on its back below the water's surface. We have termed these two forms of locomotion "standard locomotion" and "upside-down gliding," respectively. Previous studies showed that standard locomotion is produced by both cilia activity on the foot and peristaltic contraction of the foot muscles, whereas upside-down gliding is mainly caused by cilia activity. The pedal A neurons are thought to receive excitatory octopaminergic input, which ultimately results in increased cilia beating. However, the relationship between locomotory speed and the responses of these neurons to octopamine is not known. We thus examined the effects of both an agonist and an antagonist of octopamine receptors on locomotory speed and the firing rate of the pedal A neurons. We also examined, at the electron and light-microscopic levels, whether structural changes occur in cilia following the application of either an agonist or an antagonist of octopamine receptors to the central nervous system (CNS). We found that the application of an octopamine antagonist to the CNS increased the speed of both forms of locomotion, whereas application of octopamine increased only the firing rate of the pedal A neurons. Microscopic examination of the cilia proved that there were no changes in their morphology after application of octopamine ligands. These data suggest that there is an unidentified octopaminergic neuronal network in the CNS whose activation reduces cilia movement and thus locomotory speed.

Transcriptome analysis of the central nervous system of the mollusc Lymnaea stagnalis

Background

The freshwater snail Lymnaea stagnalis (L. stagnalis) has served as a successful model for studies in the field of Neuroscience. However, a serious drawback in the molecular analysis of the nervous system of L. stagnalis has been the lack of large-scale genomic or neuronal transcriptome information, thereby limiting the use of this unique model.

Results

In this study, we report 7,712 distinct EST sequences (median length: 847 nucleotides) of a normalized L. stagnalis central nervous system (CNS) cDNA library, resulting in the largest collection of L. stagnalis neuronal transcriptome data currently available. Approximately 42% of the cDNAs can be translated into more than 100 consecutive amino acids, indicating the high quality of the library. The annotated sequences contribute 12% of the predicted transcriptome size of 20,000. Surprisingly, approximately 37% of the L. stagnalis sequences only have a tBLASTx hit in the EST library of another snail species Aplysia californica (A. californica) even using a low stringency e-value cutoff at 0.01. Using the same cutoff, approximately 67% of the cDNAs have a BLAST hit in the NCBI non-redundant protein and nucleotide sequence databases (nr and nt), suggesting that one third of the sequences may be unique to L. stagnalis. Finally, using the same cutoff (0.01), more than half of the cDNA sequences (54%) do not have a hit in nematode, fruitfly or human genome data, suggesting that the L. stagnalis transcriptome is significantly different from these species as well. The cDNA sequences are enriched in the following gene ontology functional categories: protein binding, hydrolase, transferase, and catalytic enzymes.

Conclusion

This study provides novel molecular insights into the transcriptome of an important molluscan model organism. Our findings will contribute to functional analyses in neurobiology, and comparative evolutionary biology. The L. stagnalis CNS EST database is available at .