Characterization of the neurohypophysial hormone gene loci in elephant shark and the Japanese lamprey: origin of the vertebrate neurohypophysial hormone genes

An evolutionary timeline of the oxytocin signaling pathway

Oxytocin is a neuropeptide associated with both psychological and somatic processes like parturition and social bonding. Although oxytocin homologs have been identified in many species, the evolutionary timeline of the entire oxytocin signaling gene pathway has yet to be described. Using protein sequence similarity searches, microsynteny, and phylostratigraphy, we assigned the genes supporting the oxytocin pathway to different phylostrata based on when we found they likely arose in evolution. We show that the majority (64%) of genes in the pathway are 'modern'. Most of the modern genes evolved around the emergence of vertebrates or jawed vertebrates (540 - 530 million years ago, 'mya'), including OXTR, OXT and CD38. Of those, 45% were under positive selection at some point during vertebrate evolution. We also found that 18% of the genes in the oxytocin pathway are 'ancient', meaning their emergence dates back to cellular organisms and opisthokonta (3500-1100 mya). The remaining genes (18%) that evolved after ancient and before modern genes were classified as 'medium-aged'. Functional analyses revealed that, in humans, medium-aged oxytocin pathway genes are highly expressed in contractile organs, while modern genes in the oxytocin pathway are primarily expressed in the brain and muscle tissue.

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.

A journey from speech to dance through the field of oxytocin

In this article, I am going through my scientific and personal journey using my work on oxytocin as a compass. I recount how my scientific questions were shaped over the years, and how I studied them through the lens of different fields ranging from linguistics and neuroscience to comparative and population genomics in a wide range of vertebrate species. I explain how my evolutionary findings and proposal for a universal gene nomenclature in the oxytocin-vasotocin ligand and receptor families have impacted relevant fields, and how my studies in the oxytocin and vasotocin system in songbirds, humans and non-human primates have led me to now be testing intranasal oxytocin as a candidate treatment for speech deficits. I also discuss my projects on the neurobiology of dance and where oxytocin fits in the picture of studying speech and dance in parallel. Lastly, I briefly communicate the challenges I have been facing as a woman and an international scholar in science and academia, and my personal ways to overcome them.

Vasopressin as Possible Treatment Option in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is rather common, presenting with prevalent early problems in social communication and accompanied by repetitive behavior. As vasopressin was implicated not only in salt-water homeostasis and stress-axis regulation, but also in social behavior, its role in the development of ASD might be suggested. In this review, we summarized a wide range of problems associated with ASD to which vasopressin might contribute, from social skills to communication, motor function problems, autonomous nervous system alterations as well as sleep disturbances, and altered sensory information processing. Beside functional connections between vasopressin and ASD, we draw attention to the anatomical background, highlighting several brain areas, including the paraventricular nucleus of the hypothalamus, medial preoptic area, lateral septum, bed nucleus of stria terminalis, amygdala, hippocampus, olfactory bulb and even the cerebellum, either producing vasopressin or containing vasopressinergic receptors (presumably V1a). Sex differences in the vasopressinergic system might underline the male prevalence of ASD. Moreover, vasopressin might contribute to the effectiveness of available off-label therapies as well as serve as a possible target for intervention. In this sense, vasopressin, but paradoxically also V1a receptor antagonist, were found to be effective in some clinical trials. We concluded that although vasopressin might be an effective candidate for ASD treatment, we might assume that only a subgroup (e.g., with stress-axis disturbances), a certain sex (most probably males) and a certain brain area (targeting by means of virus vectors) would benefit from this therapy.

A Putative Role of Vasopressin/Oxytocin-Type Neuropeptide in Osmoregulation and Feeding Inhibition of Apostichopus japonicus

Vasopressin/oxytocin (VP/OT)-type neuropeptide is an ancient neurophysin-associated neuropeptide and has been intensively studied to be involved in multiple physiological processes in protostomian and deuterostome vertebrates. However, little is known about the functions of VP/OT-type neuropeptide in deuterostome invertebrates especially in echinoderms. Here, we firstly report VP/OT-type neuropeptide signaling in an important economic species, Apostichopus japonicus, which is widely cultured in Asia, with high nutritional and medicinal values. Molecular characterization analysis of holotocin and its precursor revealed the highly conserved features of VP/OT family. The candidate receptor for holotocin (AjHOR) was confirmed to be able to activate the signaling via cAMP-PKA and possible Ca2+-PKC pathway, and further activated the downstream ERK1/2 cascade. Holotocin precursor expression profile showed that they were mainly concentrated in circumoral nerve ring. Furthermore, in vitro pharmacological experiments demonstrated that holotocin caused contractile responses in preparations from A. japonicus. And in vivo functional studies indicated that short-term injection of holotocin resulted in body bloat and long-term injection resulted in reduced body mass, suggesting potential roles of holotocin in osmoregulation and feeding co-inhibition with holotocin-CCK. Our findings provided a comprehensive description of AjHOR-holotocin signaling, revealed ancient roles of holotocin in osmoregulation and feeding inhibition by controlling muscle contractions.

Neurohypophysial and paracrine vasopressinergic signaling regulates aquaporin trafficking to hydrate marine teleost oocytes

The dual aquaporin (Aqp1ab1/Aqp1ab2)-mediated hydration of marine teleost eggs, which occurs during oocyte meiosis resumption (maturation), is considered a key adaptation underpinning their evolutionary success in the oceans. However, the endocrine signals controlling this mechanism are almost unknown. Here, we investigated whether the nonapeptides arginine vasopressin (Avp, formerly vasotocin) and oxytocin (Oxt, formerly isotocin) are involved in marine teleost oocyte hydration using the gilthead seabream (Sparus aurata) as a model. We show that concomitant with an increased systemic production of Avp and Oxt, the nonapeptides are also produced and accumulated locally in the ovarian follicles during oocyte maturation and hydration. Functional characterization of representative Avp and Oxt receptor subtypes indicates that Avpr1aa and Oxtrb, expressed in the postvitellogenic oocyte, activate phospholipase C and protein kinase C pathways, while Avpr2aa, which is highly expressed in the oocyte and in the follicular theca and granulosa cells, activates the cAMP-protein kinase A (PKA) cascade. Using ex vivo, in vitro and mutagenesis approaches, we determined that Avpr2aa plays a major role in the PKA-mediated phosphorylation of the aquaporin subdomains driving membrane insertion of Aqp1ab2 in the theca and granulosa cells, and of Aqp1ab1 and Aqp1ab2 in the distal and proximal regions of the oocyte microvilli, respectively. The data further indicate that luteinizing hormone, which surges during oocyte maturation, induces the synthesis of Avp in the granulosa cells via progestin production and the nuclear progestin receptor. Collectively, our data suggest that both the neurohypophysial and paracrine vasopressinergic systems integrate to differentially regulate the trafficking of the Aqp1ab-type paralogs via a common Avp-Avpr2aa-PKA pathway to avoid competitive occupancy of the same plasma membrane space and maximize water influx during oocyte hydration.

Phylogenetic and functional properties of hagfish neurohypophysial hormone receptors distinct from their jawed vertebrate counterparts

Vertebrate neurohypophysial hormones, i.e., vasopressin- and oxytocin-family peptides, exert versatile physiological actions via distinct G protein-coupled receptors. The neurohypophysial hormone receptor (NHR) family was classically categorized into four subtypes (V1aR, V1bR, V2R and OTR), while recent studies have identified seven subtypes (V1aR, V1bR, V2aR, V2bR, V2cR, V2dR and OTR; V2aR corresponds to the conventional V2R). The vertebrate NHR family were diversified via multiple gene duplication events at different scales. Despite intensive research effort in non-osteichthyes vertebrates such as cartilaginous fish and lamprey, the molecular phylogeny of the NHR family has not been fully understood. In the present study, we focused on the inshore hagfish (Eptatretus burgeri), another group of cyclostomes, and Arctic lamprey (Lethenteron camtschaticum) for comparison. Two putative NHR homologs, which were previously identified only in silico, were cloned from the hagfish and designated as ebV1R and ebV2R. In vitro, ebV1R, as well as two out of five Arctic lamprey NHRs, increased intracellular Ca2+ in response to exogenous neurohypophysial hormones. None of the examined cyclostome NHRs altered intracellular cAMP levels. Transcripts of ebV1R were detected in multiple tissues including the brain and gill, with intense hybridization signals in the hypothalamus and adenohypophysis, while ebV2R was predominantly expressed in the systemic heart. Similarly, Arctic lamprey NHRs showed distinct expression patterns, underscoring the multifunctionality of VT in the cyclostomes as in the gnathostomes. These results and exhaustive gene synteny comparisons provide new insights into the molecular and functional evolution of the neurohypophysial hormone system in vertebrates.

Evolution of Vertebrate Hormones and Their Receptors: Insights from Non-Osteichthyan Genomes

Homeostatic control and reproductive functions of humans are regulated at the molecular levels largely by peptide hormones secreted from endocrine and/or neuroendocrine cells in the central nervous system and peripheral organs. Homologs of those hormones and their receptors function similarly in many vertebrate species distantly related to humans, but the evolutionary history of the endocrine system involving those factors has been obscured by the scarcity of genome DNA sequence information of some taxa that potentially contain their orthologs. Focusing on non-osteichthyan vertebrates, namely jawless and cartilaginous fishes, this article illustrates how investigating genome sequence information assists our understanding of the diversification of vertebrate gene repertoires in four broad themes: (a) the presence or absence of genes, (b) multiplication and maintenance of paralogs, (c) differential fates of duplicated paralogs, and (d) the evolutionary timing of gene origins.

[Emergent role of astrocytes in oxytocin-mediated modulatory control of neuronal circuits and brain functions]

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity are critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, and give details of underlying intracellular cascades.

Dissecting social decision-making: A spotlight on oxytocinergic transmission

Social decision-making requires the ability to balance both the interests of the self and the interests of others to survive in social environments. Empathy is essential to the regulation of this type of interaction, and it often sustains relevant prosocial behaviors such as altruism and helping behavior. In the last decade, our capacity to assess affective and empathy-like behaviors in rodents has expanded our understanding of the neurobiological substrates that underly social decision-making processes such as prosocial behaviors. Within this context, oxytocinergic transmission is profoundly implicated in modulating some of the major components of social decision-making. Thus, this review will present evidence of the association between oxytocin and empathy-like and prosocial behaviors in nonhuman animals. Then, we will dissect the involvement of oxytocinergic transmission-across different brain regions and pathways-in some of the key elements of social decision-making such as emotional discrimination, social recognition, emotional contagion, social dominance, and social memory. Evidence of the modulatory role of oxytocin on social decision-making has raised considerable interest in its clinical relevance, therefore we will also discuss the controversial findings on intranasal oxytocin administration.

Reproductive roles of the vasopressin/oxytocin neuropeptide family in teleost fishes

The vertebrate nonapeptide families arginine vasopressin (AVP) and oxytocin (OXT) are considered to have evolved from a single vasopressin-like peptide present in invertebrates and termed arginine vasotocin in early vertebrate evolution. Unprecedented genome sequence availability has more recently allowed new insight into the evolution of nonapeptides and especially their receptor families in the context of whole genome duplications. In bony fish, nonapeptide homologues of AVP termed arginine vasotocin (Avp) and an OXT family peptide (Oxt) originally termed isotocin have been characterized. While reproductive roles of both nonapeptide families have historically been studied in several vertebrates, their roles in teleost reproduction remain much less understood. Taking advantage of novel genome resources and associated technological advances such as genetic modifications in fish models, we here critically review the current state of knowledge regarding the roles of nonapeptide systems in teleost reproduction. We further discuss sources of plasticity of the conserved nonapeptide systems in the context of diverse reproductive phenotypes observed in teleost fishes. Given the dual roles of preoptic area (POA) synthesized Avp and Oxt as neuromodulators and endocrine/paracrine factors, we focus on known roles of both peptides on reproductive behaviour and the regulation of the hypothalamic-pituitary-gonadal axis. Emphasis is placed on the identification of a gonadal nonapeptide system that plays critical roles in both steroidogenesis and gamete maturation. We conclude by highlighting key research gaps including a call for translational studies linking new mechanistic understanding of nonapeptide regulated physiology in the context of aquaculture, conservation biology and ecotoxicology.

Oxytocin and oxygen: the evolution of a solution to the ‘stress of life’

Oxytocin (OT) and the OT receptor occupy essential roles in our current understanding of mammalian evolution, survival, sociality and reproduction. This narrative review examines the hypothesis that many functions attributed to OT can be traced back to conditions on early Earth, including challenges associated with managing life in the presence of oxygen and other basic elements, including sulfur. OT regulates oxidative stress and inflammation especially through effects on the mitochondria. A related nonapeptide, vasopressin, as well as molecules in the hypothalamic–pituitary–adrenal axis, including the corticotropin-releasing hormone family of molecules, have a broad set of functions that interact with OT. Interactions among these molecules have roles in the causes and consequence of social behaviour and the management of threat, fear and stress. Here, we discuss emerging evidence suggesting that unique properties of the OT system allowed vertebrates, and especially mammals, to manage over-reactivity to the ‘side effects’ of oxygen, including inflammation, oxidation and free radicals, while also supporting high levels of sociality and a perception of safety.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

Evaluating the neuropeptide-social cognition link in ageing: the mediating role of basic cognitive skills

The roles of oxytocin (OT) and arginine-vasopressin (AVP) as crucial modulators of social cognition and related behaviours have been extensively addressed in the literature. The involvement of these neuropeptides in social cognition in ageing, however, and a potential mediating effect of basic cognitive capacities on this link, are not well understood. To fill these research gaps, this study assessed associations of plasma OT and AVP levels with dynamic emotion identification accuracy in generally healthy older men (aged 55-95 years) and probed the underlying roles of crystallized and fluid cognition in these associations. Higher plasma OT levels were associated with lower accuracy in dynamic emotion identification, with this negative relationship fully mediated by cognition. For plasma AVP levels, in contrast, there was no association with dynamic emotion identification accuracy. Integrated within existing theoretical accounts, results from this study advance understanding of the neuropeptide-social cognition link in ageing and support basic cognitive capacities as mediators in this association. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Emerging role of astrocytes in oxytocin-mediated control of neural circuits and brain functions

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors, among which sociality. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity is critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of specific gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, including detailed information about intracellular cascades, and speculate about future research directions on astrocytic oxytocin signaling.

Oxytocin and vasopressin in the hippocampus

Oxytocin (OXT) and vasopressin (AVP) are related neuropeptides that exert a wide range of effects on general health, homeostasis, development, reproduction, adaptability, cognition, social and nonsocial behaviors. The two peptides are mainly of hypothalamic origin and execute their peripheral and central physiological roles via OXT and AVP receptors, which are members of the G protein-coupled receptor family. These receptors, largely distributed in the body, are abundantly expressed in the hippocampus, a brain region particularly vulnerable to stress exposure and various lesions. OXT and AVP have important roles in the hippocampus, by modulating important processes like neuronal excitability, network oscillatory activity, synaptic plasticity, and social recognition memory. This chapter includes an overview regarding OXT and AVP structure, synthesis, receptor distribution, and functions, focusing on their relationship with the hippocampus and mechanisms by which they influence hippocampal activity. Brief information regarding hippocampal structure and susceptibility to lesions is also provided. The roles of OXT and AVP in neurodevelopment and adult central nervous system function and disorders are highlighted, discussing their potential use as targeted therapeutic tools in neuropsychiatric diseases.

Oxytocin and love: Myths, metaphors and mysteries

Oxytocin is a peptide molecule with a multitude of physiological and behavioral functions. Based on its association with reproduction - including social bonding, sexual behavior, birth and maternal behavior - oxytocin also has been called "the love hormone." This essay specifically examines association and parallels between oxytocin and love. However, many myths and gaps in knowledge remain concerning both. A few of these are described here and we hypothesize that the potential benefits of both love and oxytocin may be better understood in light of interactions with more ancient systems, including specifically vasopressin and the immune system. Oxytocin is anti-inflammatory and is associated with recently evolved, social solutions to a variety of challenges necessary for mammalian survival and reproduction. The shared functions of oxytocin and love have profound implications for health and longevity, including the prevention and treatment of excess inflammation and related disorders, especially those occurring in early life and during periods of chronic threat or disease.

Identification of neurohypophysial hormones and the role of VT in the parturition of pregnant seahorses (Hippocampus erectus)

Neurohypophysial hormones regulate the reproductive behavior of teleosts; however, their role in the gestation and parturition of ovoviviparous fishes with male pregnancy (syngnathids) remains to be demonstrated. In the present study, the complementary DNA (cDNA) sequences of arginine vasotocin (VT) and isotocin (IT) from the lined seahorse (Hippocampus erectus) were cloned and identified. We observed that the mature core peptides of seahorse VT and IT were conserved among teleosts. In the phylogenic tree, seahorse VT and IT were clustered independently with teleost VT and IT. The tissue distribution patterns of VT and IT were similar, and both were highly expressed in the brain, gills, and gonads. Interestingly, they were also expressed to some extent in the brood pouch. In situ hybridization revealed that VT and IT messenger RNA (mRNA) signals in the brain were mainly located in the preoptic area region of the hypothalamus. Intraperitoneal administration of the VT core peptide to pregnant seahorses induced premature parturition, stimulated gonadotropin release, increased serum estrogen levels, and decreased prolactin secretion. Moreover, VT injection upregulated the mRNA expression of the membrane estrogen receptor in the brood pouch. In summary, neurohypophysial hormones promote premature parturition by regulating estrogen synthesis through the hypothalamus-pituitary-gonad axis.

Oxytocin-pathway polygenic scores for severe mental disorders and metabolic phenotypes in the UK Biobank

Oxytocin is a neuromodulator and hormone that is typically associated with social cognition and behavior. In light of its purported effects on social cognition and behavior, research has investigated its potential as a treatment for psychiatric illnesses characterized by social dysfunction, such as schizophrenia and bipolar disorder. While the results of these trials have been mixed, more recent evidence suggests that the oxytocin system is also linked with cardiometabolic conditions for which individuals with severe mental disorders are at a higher risk for developing. To investigate whether the oxytocin system has a pleiotropic effect on the etiology of severe mental illness and cardiometabolic conditions, we explored oxytocin's role in the shared genetic liability of schizophrenia, bipolar disorder, type-2 diabetes, and several phenotypes linked with cardiovascular disease and type 2 diabetes risk using a polygenic pathway-specific approach. Analysis of a large sample with about 480,000 individuals (UK Biobank) revealed statistically significant associations across the range of phenotypes analyzed. By comparing these effects to those of polygenic scores calculated from 100 random gene sets, we also demonstrated the specificity of many of these significant results. Altogether, our results suggest that the shared effect of oxytocin-system dysfunction could help partially explain the co-occurrence of social and cardiometabolic dysfunction in severe mental illnesses.

The oxytocin/vasopressin-like peptide receptor mRNA in the central nervous system and ovary of the blue swimming crab, Portunus pelagicus

The authors recently reported the presence and distribution of oxytocin/vasopressin-like peptide in Portunus pelagicus as well as demonstrated its function to inhibit ovarian steroid release (Saetan et al., 2018). Here, the full-length receptor of this peptide, namely oxytocin/vasopressin-like peptide receptor (PpelOT/VP-like peptide receptor) is reported. The coding region of the PpelOT/VP-like peptide receptor contained 1497 bp which translationally corresponded to 499 amino acids. Sequence analysis revealed its seven transmembrane characteristics, with -two N-linked glycosylation residues located before the first transmembrane domain (TM I). The phylogenetic tree revealed that the PpelOT/VP-like peptide receptor was placed in the group of invertebrate OT/VP-like receptors, and was clearly distinguishable from the V1R, V2R and OTR of vertebrates. Also, this receptor gene transcript was detected in several organs of the blue swimming crab with highest abundance found in brain tissue. In situ hybridization exhibited its distribution in all neuronal clusters of the eyestalk, brain, ventral nerve cord (VNC), as well as in the ovary. Comparative gene expressions between this receptor and its corresponding peptide in immature and mature female crabs revealed no significant difference of the PpelOT/VP-like peptide receptor gene expression in the central nervous system (CNS) and ovary. In contrast, the PpelOT/VP-like peptide gene was shown to significantly express higher in the VNC of immature crabs and in the ovary of mature crabs. Changes in expression of this peptide gene, but not its receptor, might result in ovarian steroid release inhibition. However, the detailed mechanism of this peptide in reproduction regulation will be included in our further studies.

Oxytocin, Neural Plasticity, and Social Behavior

Oxytocin regulates parturition, lactation, parental nurturing, and many other social behaviors in both sexes. The circuit mechanisms by which oxytocin modulates social behavior are receiving increasing attention. Here, we review recent studies on oxytocin modulation of neural circuit function and social behavior, largely enabled by new methods of monitoring and manipulating oxytocin or oxytocin receptor neurons in vivo. These studies indicate that oxytocin can enhance the salience of social stimuli and increase signal-to-noise ratios by modulating spiking and synaptic plasticity in the context of circuits and networks. We highlight oxytocin effects on social behavior in nontraditional organisms such as prairie voles and discuss opportunities to enhance the utility of these organisms for studying circuit-level modulation of social behaviors. We then discuss recent insights into oxytocin neuron activity during social interactions. We conclude by discussing some of the major questions and opportunities in the field ahead.

Universal nomenclature for oxytocin-vasotocin ligand and receptor families

Oxytocin (OXT; hereafter OT) and arginine vasopressin or vasotocin (AVP or VT; hereafter VT) are neurotransmitter ligands that function through specific receptors to control diverse functions^1,2. Here we performed genomic analyses on 35 species that span all major vertebrate lineages, including newly generated high-contiguity assemblies from the Vertebrate Genomes Project^3,4. Our findings support the claim⁵ that OT (also known as OXT) and VT (also known as AVP) are adjacent paralogous genes that have resulted from a local duplication, which we infer was through DNA transposable elements near the origin of vertebrates and in which VT retained more of the parental sequence. We identified six major oxytocin-vasotocin receptors among vertebrates. We propose that all six of these receptors arose from a single receptor that was shared with the common ancestor of invertebrates, through a combination of whole-genome and large segmental duplications. We propose a universal nomenclature based on evolutionary relationships for the genes that encode these receptors, in which the genes are given the same orthologous names across vertebrates and paralogous names relative to each other. This nomenclature avoids confusion due to differential naming in the pre-genomic era and incomplete genome assemblies, furthers our understanding of the evolution of these genes, aids in the translation of findings across species and serves as a model for other gene families.

The Evolution of Oxytocin and Vasotocin Receptor Genes in Jawed Vertebrates: A Clear Case for Gene Duplications Through Ancestral Whole-Genome Duplications

The neuronal and neuroendocrine peptides oxytocin (OT) and vasotocin (VT), including vasopressins, have six cognate receptors encoded by six receptor subtype genes in jawed vertebrates. The peptides elicit a broad range of responses that are specifically mediated by the receptor subtypes including neuronal functions regulating behavior and hormonal actions on reproduction and water/electrolyte balance. Previously, we have demonstrated that these six receptor subtype genes, which we designated VTR1A, VTR1B, OTR, VTR2A, VTR2B and VTR2C, arose from a syntenic ancestral gene pair, one VTR1/OTR ancestor and one VTR2 ancestor, through the early vertebrate whole-genome duplications (WGD) called 1R and 2R. This was supported by both phylogenetic and chromosomal conserved synteny data. More recently, other studies have focused on confounding factors, such as the OTR/VTR orthologs in cyclostomes, to question this scenario for the origin of the OTR/VTR gene family; proposing instead less parsimonious interpretations involving only one WGD followed by complex series of chromosomal or segmental duplications. Here, we have updated the phylogeny of the OTR/VTR gene family, including a larger number of vertebrate species, and revisited seven representative neighboring gene families from our previous conserved synteny analyses, adding chromosomal information from newer high-coverage genome assemblies from species that occupy key phylogenetic positions: the polypteriform fish reedfish (Erpetoichthys calabaricus), the cartilaginous fish thorny skate (Amblyraja radiata) and a more recent high-quality assembly of the Western clawed frog (Xenopus tropicalis) genome. Our analyses once again add strong support for four-fold symmetry, i.e., chromosome quadruplication in the same time window as the WGD events early in vertebrate evolution, prior to the jawed vertebrate radiation. Thus, the evolution of the OTR/VTR gene family can be most parsimoniously explained by two WGD events giving rise to the six ancestral genes, followed by differential gene losses of VTR2 genes in different lineages. We also argue for more coherence and clarity in the nomenclature of OT/VT receptors, based on the most parsimonious scenario.

The promiscuity of the oxytocin-vasopressin systems and their involvement in autism spectrum disorder

Oxytocin and vasopressin systems have been studied separately in autism spectrum disorder (ASD). Here, we provide evidence from an evolutionary and neuroscience perspective about the shared mechanisms and the common roles in regulating social behaviors. We first discuss findings on the evolutionary history of oxytocin and vasopressin ligands and receptors that highlight their common origin and clarify the evolutionary background of the crosstalk between them. Second, we conducted a comprehensive review of the increasing evidence for the role of both neuropeptides in regulating social behaviors. Third, we reviewed the growing evidence on the associations between the oxytocin/vasopressin systems and ASD, which includes oxytocin and vasopressin dysfunction in animal models of autism and in human patients, and the impact of treatments targeting the oxytocin or the vasopressin systems in children and in adults. Here, we highlight the potential of targeting the oxytocin/vasopressin systems to improve social deficits observed in ASD and the need for further investigations on how to transfer these research innovations into clinical applications.

Improving the precision of intranasal oxytocin research

The neuropeptide oxytocin has been popularized for its role in social behaviour and nominated as a candidate treatment for several psychiatric illnesses due to promising preclinical results. However, these results so far have failed to reliably translate from animal models to human research. In response, there have been justified calls to improve intranasal oxytocin delivery methodology in terms of verifying that intranasal administration increases central levels of oxytocin. Nonetheless, improved methodology needs to be coupled with a robust theory of the role of oxytocin in behaviour and physiology to ask meaningful research questions. Moreover, stringent methodology based on robust theory may yield interesting results, but such findings will have limited utility if they are not reproducible. We outline how the precision of intranasal oxytocin research can be improved by the complementary consideration of methodology, theory and reproducibility.

Arginine Vasopressin Modulates Ion and Acid/Base Balance by Regulating Cell Numbers of Sodium Chloride Cotransporter and H+-ATPase Rich Ionocytes

Arginine vasopressin (Avp) is a conserved pleiotropic hormone that is known to regulate both water reabsorption and ion balance; however, many of the mechanisms underlying its effects remain unclear. Here, we used zebrafish embryos to investigate how Avp modulates ion and acid–base homeostasis. After incubating embryos in double-deionized water for 24 h, avp mRNA expression levels were significantly upregulated. Knockdown of Avp protein expression by an antisense morpholino oligonucleotide (MO) reduced the expression of ionocyte-related genes and downregulated whole-body Cl⁻ content and H⁺ secretion, while Na⁺ and Ca²⁺ levels were not affected. Incubation of Avp antagonist SR49059 also downregulated the mRNA expression of sodium chloride cotransporter 2b (ncc2b), which is a transporter responsible for Cl⁻ uptake. Correspondingly, avp morphants showed lower NCC and H⁺-ATPase rich (HR) cell numbers, but Na⁺/K⁺-ATPase rich (NaR) cell numbers remained unchanged. avp MO also downregulated the numbers of foxi3a- and p63-expressing cells. Finally, the mRNA expression levels of calcitonin gene-related peptide (cgrp) and its receptor, calcitonin receptor-like 1 (crlr1), were downregulated in avp morphants, suggesting that Avp might affect Cgrp and Crlr1 for modulating Cl⁻ balance. Together, our results reveal a molecular/cellular pathway through which Avp regulates ion and acid–base balance, providing new insights into its function.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

An Allostatic Theory of Oxytocin

Oxytocin has garnered considerable interest for its role in social behavior, as well as for the potential of intranasal administration to treat social difficulties. However, current theoretical models for the role of oxytocin in social behavior pay little consideration to its evolutionary and developmental history. This article aims to broaden our understanding of the role of oxytocin in social behavior by adopting an ethological approach through the lens of Nikolaas Tinbergen's 'four questions' - how does oxytocin work; how does the role of oxytocin change during development; how does oxytocin enhance survival; and how did the oxytocin system evolve? We argue that oxytocin is most accurately described as an allostatic hormone that modulates both social and non-social behavior by maintaining stability through changing environments.

Comparative and Evolutionary Physiology of Vasopressin/ Oxytocin-Type Neuropeptide Signaling in Invertebrates

The identification of structurally related hypothalamic hormones that regulate blood pressure and diuresis (vasopressin, VP; CYFQNCPRG-NH2) or lactation and uterine contraction (oxytocin, OT; CYIQNCPLG-NH2) was a major advance in neuroendocrinology, recognized in the award of the Nobel Prize for Chemistry in 1955. Furthermore, the discovery of central actions of VP and OT as regulators of reproductive and social behavior in humans and other mammals has broadened interest in these neuropeptides beyond physiology into psychology. VP/OT-type neuropeptides and their G-protein coupled receptors originated in a common ancestor of the Bilateria (Urbilateria), with invertebrates typically having a single VP/OT-type neuropeptide and cognate receptor. Gene/genome duplications followed by gene loss gave rise to variety in the number of VP/OT-type neuropeptides and receptors in different vertebrate lineages. Recent advances in comparative transcriptomics/genomics have enabled discovery of VP/OT-type neuropeptides in an ever-growing diversity of invertebrate taxa, providing new opportunities to gain insights into the evolution of VP/OT-type neuropeptide function in the Bilateria. Here we review the comparative physiology of VP/OT-type neuropeptides in invertebrates, with roles in regulation of reproduction, feeding, and water/salt homeostasis emerging as common themes. For example, we highlight recent reports of roles in regulation of oocyte maturation in the sea-squirt Ciona intestinalis, extraoral feeding behavior in the starfish Asterias rubens and energy status and dessication resistance in ants. Thus, VP/OT-type neuropeptides are pleiotropic regulators of physiological processes, with evolutionarily conserved roles that can be traced back to Urbilateria. To gain a deeper understanding of the evolution of VP/OT-type neuropeptide function it may be necessary to not only determine the actions of the peptides but also to characterize the transcriptomic/proteomic/metabolomic profiles of cells expressing VP/OT-type precursors and/or VP/OT-type receptors within the framework of anatomically and functionally identified neuronal networks. Furthermore, investigation of VP/OT-type neuropeptide function in a wider range of invertebrate species is now needed if we are to determine how and when this ancient signaling system was recruited to regulate diverse physiological and behavioral processes in different branches of animal phylogeny and in contrasting environmental contexts.

Oxytocin Receptors Regulate Social Preference in Zebrafish

With a strong tendency to socialise, the zebrafish is a useful model to study social behaviour, with implications for better treatments of social impairments, for instance in autism spectrum disorders. Although oxytocin is crucial for social behaviour in mammals, the importance of the fish orthologue – isotocin or zebrafish oxytocin (zOT) – for social behaviour in zebrafish is unclear. The aims of this study were firstly, to elucidate the receptor specificity of zOT and the related vasotocin or zebrafish vasopressin (zVP; the orthologue of mammalian vasopressin) and the nonpeptidergic oxytocin receptor antagonist L-368,899, and secondly to investigate if L-368,899 inhibits social preference in zebrafish. The potencies of ligands were evaluated for zOT/zVP family receptors in HEK293 cells. Adult and larval zebrafish were treated with L-368,899 or vehicle and subsequently assessed for social behaviour and anxiety (adults only). The antagonist L-368,899 specifically inhibited the two zOT receptors, but not the two zVP-1 receptors. The antagonist decreased social preference in adult and larval zebrafish. It did not affect anxiety in adults. These results indicate that endogenous zOT, and possibly zVP, is involved in social behaviour in zebrafish via either or both of the two zOT receptors, and show promise for future explorations of the anatomy and evolution of networks underlying social behaviour.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Inference of the ancestral vertebrate phenotype through vestiges of the whole-genome duplications

Inferring the phenotype of the last common ancestor of living vertebrates is a challenging problem because of several unresolvable factors. They include the lack of reliable out-groups of living vertebrates, poor information about less fossilizable organs and specialized traits of phylogenetically important species, such as lampreys and hagfishes (e.g. secondary loss of vertebrae in adult hagfishes). These factors undermine the reliability of ancestral reconstruction by traditional character mapping approaches based on maximum parsimony. In this article, we formulate an approach to hypothesizing ancestral vertebrate phenotypes using information from the phylogenetic and functional properties of genes duplicated by genome expansions in early vertebrate evolution. We named the conjecture as ‘chronological reconstruction of ohnolog functions (CHROF)’. This CHROF conjecture raises the possibility that the last common ancestor of living vertebrates may have had more complex traits than currently thought.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Molecular cloning and characterisation of an isotocin paralogue ([V8] isotocin) in catfishes (superorder Ostariophysi): Origin traced likely to the fish-specific whole genome duplication

The present study reports the molecular cloning of a previously uncharacterised neurohypophyseal nonapeptide precursor cDNA in two catfish species: Heteropneustes fossilis and Clarias batrachus. The deduced nonapeptide is CYISNCPVG ([V8] isotocin), which has not been reported in any vertebrate till date. Phylogenetic and conserved synteny analyses showed the gene to have originated from the isotocin precursor (pro-it) gene by fish-specific whole genome duplication (3R). The two isotocin lineages have been designated as pro-ita (new gene) and pro-itb (conventional it gene). All teleost groups may not possess both pro-ita and pro-itb and the pattern of losses/retention was found to be lineage-specific. Quantitative reverse transcriptase-polymerase chain reaction studies showed the expression of the pro-ita gene in the brain and ovary of H. fossilis. In situ hybridisation studies localised the pro-ita transcripts in the nucleus preopticus of the hypothalamus and the follicular layer (theca-granulosa) of oocytes, comprising tissues in which pro-itb and vasotocin precursor (pro-vt) mRNA expression was previously reported. The transcript levels varied with the reproductive stage and a high abundance was found in both brain and ovary during the breeding phase. The substitution of valine in place of isoleucine at the eighth position in Ita may have modified the ligand-receptor interaction, leading to sub-functionalisation and the retention of the gene in catfishes.

Characterization of Gonadotropin-Releasing Hormone (GnRH) Genes From Cartilaginous Fish: Evolutionary Perspectives

The neuropeptide gonadotropin-releasing hormone (GnRH) plays an important role in the control of reproductive functions. Vertebrates possess multiple GnRH forms that are classified into three main groups, namely GnRH1, GnRH2, and GnRH3. In order to gain more insights into the GnRH gene family in vertebrates, we sought to identify which paralogs of this family are present in cartilaginous fish. For this purpose, we searched the genomes and/or transcriptomes of three representative species of this group, the small-spotted catshark, Scyliorhinus canicula, the whale shark, Rhincodon typus and the elephant shark Callorhinchus milii. In each species, we report the identification of three GnRH genes. In catshark and whale shark, phylogenetic and synteny analysis showed that these three genes correspond to GnRH1, GnRH2, and GnRH3. In both species, GnRH1 was found to encode a novel form of GnRH whose primary structure was determined as follows: QHWSFDLRPG. In elephant shark, the three genes correspond to GnRH1a and GnRH1b, two copies of the GnRH1 gene, plus GnRH2. 3D structure prediction of the chondrichthyan GnRH-associated peptides (GAPs) revealed that catshark GAP1, GAP2, and elephant shark GAP2 peptides exhibit a helix-loop-helix (HLH) structure. This structure observed for many osteichthyan GAP1 and GAP2, may convey GAP biological activity. This HLH structure could not be observed for elephant shark GAP1a and GAP1b. As for all other GAP3 described so far, no typical 3D HLH structure was observed for catshark nor whale shark GAP3. RT-PCR analysis revealed that GnRH1, GnRH2, and GnRH3 genes are differentially expressed in the catshark brain. GnRH1 mRNA appeared predominant in the diencephalon while GnRH2 and GnRH3 mRNAs seemed to be most abundant in the mesencephalon and telencephalon, respectively. Taken together, our results show that the GnRH gene repertoire of the vertebrate ancestor was entirely conserved in the chondrichthyan lineage but that the GnRH3 gene was probably lost in holocephali. They also suggest that the three GnRH neuronal systems previously described in the brain of bony vertebrates are also present in cartilaginous fish.

Evolution of neuropeptide signalling systems

Neuropeptides are a diverse class of neuronal signalling molecules that regulate physiological processes and behaviour in animals. However, determining the relationships and evolutionary origins of the heterogeneous assemblage of neuropeptides identified in a range of phyla has presented a huge challenge for comparative physiologists. Here, we review revolutionary insights into the evolution of neuropeptide signalling that have been obtained recently through comparative analysis of genome/transcriptome sequence data and by ‘deorphanisation’ of neuropeptide receptors. The evolutionary origins of at least 30 neuropeptide signalling systems have been traced to the common ancestor of protostomes and deuterostomes. Furthermore, two rounds of genome duplication gave rise to an expanded repertoire of neuropeptide signalling systems in the vertebrate lineage, enabling neofunctionalisation and/or subfunctionalisation, but with lineage-specific gene loss and/or additional gene or genome duplications generating complex patterns in the phylogenetic distribution of paralogous neuropeptide signalling systems. We are entering a new era in neuropeptide research where it has become feasible to compare the physiological roles of orthologous and paralogous neuropeptides in a wide range of phyla. Moreover, the ambitious mission to reconstruct the evolution of neuropeptide function in the animal kingdom now represents a tangible challenge for the future.

Summary: A review of the revolutionary advances in our knowledge of the evolution of neuropeptide signalling systems that have been enabled by comparative genomics and neuropeptide receptor deorphanisation.

Isotocin Regulates Growth Hormone but Not Prolactin Release From the Pituitary of Ricefield Eels

The neurohypophyseal hormone oxytocin (Oxt) has been shown to stimulate prolactin (Prl) synthesis and release from the adenohypophysis in rats. However, little is known about the functional roles of Oxt-like neuropeptides in the adenohypophysis of non-mammalian vertebrates. In this study, cDNAs encoding ricefield eel oxytocin-like receptors (Oxtlr), namely isotocin (Ist) receptor 1 (Istr1) and 2 (Istr2), were isolated and specific antisera were generated, respectively. RT-PCR and Western blot analysis detected the presence of both Istr1 and Istr2 in the brain and pituitary, but differential expression in some peripheral tissues, including the liver and kidney, where only Istr1 was detected. In the pituitary, immunoreactive Istr1 and Istr2 were differentially distributed, with the former mainly in adenohypophyseal cell layers adjacent to the neurohypophysis, whereas the latter in peripheral areas of the adenohypophysis. Double immunofluorescent images showed that immunostaining of Istr1, but not Istr2 was localized to growth hormone (Gh) cells, but neither of them was expressed in Prl cells. Ist inhibited Gh release in primary pituitary cells of ricefield eels and increased Gh contents in the pituitary gland of ricefield eels at 6 h after in vivo administration. Ist inhibition of Gh release is probably mediated by cAMP, PKC/DAG, and IP3/Ca²⁺ pathways. In contrast, Ist did not affect either prl gene expression or Prl contents in primary pituitary cells. Results of this study demonstrated that Ist may not be involved in the regulation of Prl, but inhibit Gh release via Istr1 rather than Istr2 in ricefield eels, and provided evidence for the direct regulation of Gh cells by oxytocin-like neuropeptides in the pituitary of non-mammalian vertebrates.

Oxytocin - The Sweet Hormone?

Mammalian neurons that produce oxytocin and vasopressin apparently evolved from an ancient cell type with both sensory and neurosecretory properties that probably linked reproductive functions to energy status and feeding behavior. Oxytocin in modern mammals is an autocrine/paracrine regulator of cell function, a systemic hormone, a neuromodulator released from axon terminals within the brain, and a 'neurohormone' that acts at receptors distant from its site of release. In the periphery oxytocin is involved in electrolyte homeostasis, gastric motility, glucose homeostasis, adipogenesis, and osteogenesis, and within the brain it is involved in food reward, food choice, and satiety. Oxytocin preferentially suppresses intake of sweet-tasting carbohydrates while improving glucose tolerance and supporting bone remodeling, making it an enticing translational target.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Oxytocin mediated behavior in invertebrates: An evolutionary perspective

The molecular and functional conservation of oxytocin-related neuropeptides in behavior is striking. In animals separated by at least 600 million years of evolution, from roundworms to humans, oxytocin homologs play critical roles in the modulation of reproductive behavior and other biological functions. Here, we review the roles of oxytocin in invertebrate behavior from an evolutionary perspective. We begin by tracing the evolution of oxytocin through the invertebrate animal lineages, and then describe common themes in invertebrate behaviors that are mediated by oxytocin-related peptides, including reproductive behavior, learning and memory, food arousal, and predator/prey relationships. Finally, we discuss interesting future directions that have recently become experimentally tractable. Studying oxytocin in invertebrates offers precise insights into the activity of neuropeptides on well-defined neural circuits; the principles that emerge may also be represented in the more complex vertebrate brain. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 128-142, 2017.

Oxytocin and arginine vasopressin receptor evolution: implications for adaptive novelties in placental mammals

Oxytocin receptor (OXTR) and arginine vasopressin receptors (AVPR1a, AVPR1b, and AVPR2) are paralogous genes that emerged through duplication events; along the evolutionary timeline, owing to speciation, numerous orthologues emerged as well. In order to elucidate the evolutionary forces that shaped these four genes in placental mammals and to reveal specific aspects of their protein structures, 35 species were selected. Specifically, we investigated their molecular evolutionary history and intrinsic protein disorder content, and identified the presence of short linear interaction motifs. OXTR seems to be under evolutionary constraint in placental mammals, whereas AVPR1a, AVPR1b, and AVPR2 exhibit higher evolutionary rates, suggesting that they have been under relaxed or experienced positive selection. In addition, we describe here, for the first time, that the OXTR, AVPR1a, AVPR1b, and AVPR2 mammalian orthologues preserve their disorder content, while this condition varies among the paralogues. Finally, our results reveal the presence of short linear interaction motifs, indicating possible functional adaptations related to physiological and/or behavioral taxa-specific traits.

A combined proteomic and transcriptomic analysis of slime secreted by the southern bottletail squid, Sepiadarium austrinum (Cephalopoda)

Sepiadarium austrinum, the southern bottletail squid, is a small squid that inhabits soft sediments along Australia's south-east coast. When provoked, it rapidly secretes large volumes of slime, presumably as a form of chemical defense. We analyzed the proteomic composition of this slime using tandem mass spectrometry and transcriptomics and found that it was remarkably complex with 1735 identified protein groups (FDR:0.01). To investigate the chemical defense hypothesis we performed an Artemia toxicity assay and used sequence analysis to search for toxin-like molecules. Although the slime did not appear to be toxic to Artemia we found 13 proteins in slime with the hallmarks of toxins, namely cysteine richness, short length, a signal peptide and/or homology to known toxins. These included three short (80-130AA) cysteine rich secreted proteins with no homology to proteins on the NCBI or UniProt databases. Other protein families found included, CAP, phospholipase-B, ShKT-like peptides, peptidase S10, Kunitz BPTI and DNase II. Quantitative analysis using intensity based absolute quantification (iBAQ via MaxQuant) revealed 20 highly abundant proteins, accounting for 67% of iBAQ signal, and three of these were toxin-like. No mucin homologues were found suggesting that the structure of the slime gel may be formed by an unknown mechanism.

BIOLOGICAL SIGNIFICANCE

This study is the first known instance of a slime secretion from a cephalopod to be analyzed by proteomics methods and is the first investigation of a member of the family Sepiadariidae using proteomic methods. 1735 proteins were identified with 13 of these fitting criteria established for the identification of putative toxins. The slime is dominated by 20 highly abundant proteins with secreted, cysteine rich proteins. The study highlights the importance of 'omics approaches in understanding novel organisms.

Neuropeptide diversity and the regulation of social behavior in New World primates

Oxytocin (OT) and vasopressin (AVP) are important hypothalamic neuropeptides that regulate peripheral physiology, and have emerged as important modulators of brain function, particularly in the social realm. OT structure and the genes that ultimately determine structure are highly conserved among diverse eutherian mammals, but recent discoveries have identified surprising variability in OT and peptide structure in New World monkeys (NWM), with five new OT variants identified to date. This review explores these new findings in light of comparative OT/AVP ligand evolution, documents coevolutionary changes in the oxytocin and vasopressin receptors (OTR and V1aR), and highlights the distribution of neuropeptidergic neurons and receptors in the primate brain. Finally, the behavioral consequences of OT and AVP in regulating NWM sociality are summarized, demonstrating important neuromodulatory effects of these compounds and OT ligand-specific influences in certain social domains.

The emergence of the vasopressin and oxytocin hormone receptor gene family lineage: Clues from the characterization of vasotocin receptors in the sea lamprey (Petromyzon marinus)

The sea lamprey (Petromyzon marinus) is a jawless vertebrate at an evolutionary nexus between invertebrates and jawed vertebrates. Lampreys are known to possess the arginine vasotocin (AVT) hormone utilized by all non-mammalian vertebrates. We postulated that the lamprey would possess AVT receptor orthologs of predecessors to the arginine vasopressin (AVP)/oxytocin (OXT) family of G protein-coupled receptors found in mammals, providing insights into the origins of the mammalian V1A, V1B, V2 and OXT receptors. Among the earliest animals to diverge from the vertebrate lineage in which these receptors are characterized is the jawed, cartilaginous elephant shark, which has genes orthologous to all four mammalian receptor types. Therefore, our work was aimed at helping resolve the critical gap concerning the outcomes of hypothesized large-scale (whole-genome) duplication events. We sequenced one partial and four full-length putative lamprey AVT receptor genes and determined their mRNA expression patterns in 15 distinct tissues. Phylogenetically, three of the full-coding genes possess structural characteristics of the V1 clade containing the V1A, V1B and OXT receptors. Another full-length coding gene and the partial sequence are part of the V2 clade and appear to be most closely related to the newly established V2B and V2C receptor subtypes. Our synteny analysis also utilizing the Japanese lamprey (Lethenteron japonicum) genome supports the recent proposal that jawless and jawed vertebrates shared one-round (1R) of WGD as the most likely scenario.

Molecular Evidence for Convergence and Parallelism in Evolution of Complex Brains of Cephalopod Molluscs: Insights from Visual Systems

Coleoid cephalopods show remarkable evolutionary convergence with vertebrates in their neural organization, including (1) eyes and visual system with optic lobes, (2) specialized parts of the brain controlling learning and memory, such as vertical lobes, and (3) unique vasculature supporting such complexity of the central nervous system. We performed deep sequencing of eye transcriptomes of pygmy squids (Idiosepius paradoxus) and chambered nautiluses (Nautilus pompilius) to decipher the molecular basis of convergent evolution in cephalopods. RNA-seq was complemented by in situ hybridization to localize the expression of selected genes. We found three types of genomic innovations in the evolution of complex brains: (1) recruitment of novel genes into morphogenetic pathways, (2) recombination of various coding and regulatory regions of different genes, often called "evolutionary tinkering" or "co-option", and (3) duplication and divergence of genes. Massive recruitment of novel genes occurred in the evolution of the "camera" eye from nautilus' "pinhole" eye. We also showed that the type-2 co-option of transcription factors played important roles in the evolution of the lens and visual neurons. In summary, the cephalopod convergent morphological evolution of the camera eyes was driven by a mosaic of all types of gene recruitments. In addition, our analysis revealed unexpected variations of squids' opsins, retinochromes, and arrestins, providing more detailed information, valuable for further research on intra-ocular and extra-ocular photoreception of the cephalopods.

Molecular cloning, sequencing and tissue expression of vasotocin and isotocin precursor genes from Ostariophysian catfishes: phylogeny and evolutionary considerations in teleosts

Basic and neutral neurohypophyseal (NH) nonapeptides have evolved from vasotocin (VT) by a gene duplication at the base of the gnathostome lineage. In teleosts, VT and IT are the basic and neutral peptides, respectively. In the present study, VT and IT precursor genes of Heteropneustes fossilis and Clarias batrachus (Siluriformes, Ostariophysi) were cloned and sequenced. The channel catfish Icatalurus punctatus NH precursor sequences were obtained from EST database. The catfish NH sequences were used along with the available Acanthopterygii and other vertebrate NH precursor sequences to draw phylogenetic inference on the evolutionary history of the teleost NH peptides. Synteny analysis of the NH gene loci in various teleost species was done to complement the phylogenetic analysis. In H. fossilis, the NH transcripts were also sequenced from the ovary. The cloned genes and the deduced precursor proteins showed conserved characteristics of the NH nonapeptide precursors. The genes are expressed in brain and ovary (follicular envelope) of H. fossilis with higher transcript abundance in the brain. The addition of the catfish sequences in the phylogenetic analysis revealed that the VT and IT precursors of the species-rich superorders of teleosts have a distinct phylogenetic history with the Acanthopterygii VT and IT precursors sharing a less evolutionary distance and the Ostariophysi VT and IT having a greater evolutionary distance. The genomic location of VT and IT precursors, and synteny analysis of the NH loci lend support to the phylogenetic inference and suggest a footprint of fish- specific whole genome duplication (3R) and subsequent diploidization in the NH loci. The VT and IT precursor genes are most likely lineage-specific paralogs resulting from differential losses of the 3R NH paralogs in the two superorders. The independent yet consistent retention of VT and IT in the two superorders might be directed by a stringent ligand-receptor selectivity.

GnRH receptors and peptides: skating backward

Gonadotropin-releasing hormone (GnRH) and its receptor are essential for reproduction in vertebrates. Although there are three major types of GnRH peptides and two major types of receptors in vertebrates, the pattern of distribution is unusual. Evidence is presented from genome mining that type I GnRHRs are not restricted to mammals, but can be found in the lobe-finned and cartilaginous fishes. This implies that this tail-less GnRH receptor emerged early in vertebrate evolution, followed by several independent losses in different lineages. Also, we have identified representatives from the three major GnRH peptide types (mammalian GnRH1, vertebrate GnRH2 and dogfish GnRH3) in a single cartilaginous fish, the little skate. Skate and coelacanth are the only examples of animals with both type I and II GnRH receptors and all three peptide types, suggesting this was the ancestral condition in vertebrates. Our analysis of receptor synteny in combination with phylogeny suggests that there were three GnRH receptor types present before the two rounds of whole genome duplication in early vertebrates. To further understand the origin of the GnRH peptide-receptor system, the relationship of vertebrate and invertebrate homologs was examined. Our evidence supports the hypothesis of a GnRH superfamily with a common ancestor for the vertebrate GnRHs, invertebrate (inv)GnRHs, corazonins and adipokinetic hormones. The invertebrate deuterostomes (echinoderms, hemichordates and amphioxus) have derived GnRH-like peptides, although one amphioxus GnRH with a syntenic relationship to human GnRHs has been shown to be functional. Phylogenetic analysis suggests that gene duplications in the ancestral bilaterian produced two receptor types, one of which became adipokinetic hormone receptor/GnRHR and the other corazonin receptor/invGnRHR. It appears that the ancestral deuterostome had both a GnRHR and invGnRHR, and this is still the case in amphioxus. During the transition to vertebrates both the invertebrate-type peptide and receptor were lost, leaving only the vertebrate-type system that presently exists.

The lineage-specific evolution of aquaporin gene clusters facilitated tetrapod terrestrial adaptation

A major physiological barrier for aquatic organisms adapting to terrestrial life is dessication in the aerial environment. This barrier was nevertheless overcome by the Devonian ancestors of extant Tetrapoda, but the origin of specific molecular mechanisms that solved this water problem remains largely unknown. Here we show that an ancient aquaporin gene cluster evolved specifically in the sarcopterygian lineage, and subsequently diverged into paralogous forms of AQP2, -5, or -6 to mediate water conservation in extant Tetrapoda. To determine the origin of these apomorphic genomic traits, we combined aquaporin sequencing from jawless and jawed vertebrates with broad taxon assembly of >2,000 transcripts amongst 131 deuterostome genomes and developed a model based upon Bayesian inference that traces their convergent roots to stem subfamilies in basal Metazoa and Prokaryota. This approach uncovered an unexpected diversity of aquaporins in every lineage investigated, and revealed that the vertebrate superfamily consists of 17 classes of aquaporins (Aqp0 - Aqp16). The oldest orthologs associated with water conservation in modern Tetrapoda are traced to a cluster of three aqp2-like genes in Actinistia that likely arose >500 Ma through duplication of an aqp0-like gene present in a jawless ancestor. In sea lamprey, we show that aqp0 first arose in a protocluster comprised of a novel aqp14 paralog and a fused aqp01 gene. To corroborate these findings, we conducted phylogenetic analyses of five syntenic nuclear receptor subfamilies, which, together with observations of extensive genome rearrangements, support the coincident loss of ancestral aqp2-like orthologs in Actinopterygii. We thus conclude that the divergence of sarcopterygian-specific aquaporin gene clusters was permissive for the evolution of water conservation mechanisms that facilitated tetrapod terrestrial adaptation.