Neurone-specific expression and regulation of the pufferfish isotocin and vasotocin genes in transgenic mice

The Biology of Vasopressin

Vasopressins are evolutionarily conserved peptide hormones. Mammalian vasopressin functions systemically as an antidiuretic and regulator of blood and cardiac flow essential for adapting to terrestrial environments. Moreover, vasopressin acts centrally as a neurohormone involved in social and parental behavior and stress response. Vasopressin synthesis in several cell types, storage in intracellular vesicles, and release in response to physiological stimuli are highly regulated and mediated by three distinct G protein coupled receptors. Other receptors may bind or cross-bind vasopressin. Vasopressin is regulated spatially and temporally through transcriptional and post-transcriptional mechanisms, sex, tissue, and cell-specific receptor expression. Anomalies of vasopressin signaling have been observed in polycystic kidney disease, chronic heart failure, and neuropsychiatric conditions. Growing knowledge of the central biological roles of vasopressin has enabled pharmacological advances to treat these conditions by targeting defective systemic or central pathways utilizing specific agonists and antagonists.

Oxytocin, vasopressin and social behavior in the age of genome editing: A comparative perspective

Behavioral neuroendocrinology has a rich history of using diverse model organisms to elucidate general principles and evolution of hormone-brain-behavior relationships. The oxytocin and vasopressin systems have been studied in many species, revealing their role in regulating social behaviors. Oxytocin and vasopressin receptors show remarkable species and individual differences in distribution in the brain that have been linked to diversity in social behaviors. New technologies allow for unprecedented interrogation of the genes and neural circuitry regulating behaviors, but these approaches often require transgenic models and are most often used in mice. Here we discuss seminal findings relating the oxytocin and vasopressin systems to social behavior with a focus on non-traditional animal models. We then evaluate the potential of using CRISPR/Cas9 genome editing to examine the roles of genes and enable circuit dissection, manipulation and activity monitoring of the oxytocin and vasopressin systems. We believe that it is essential to incorporate these genetic and circuit level techniques in comparative behavioral neuroendocrinology research to ensure that our field remains innovative and attractive for the next generation of investigators and funding agencies.

Expression of isotocin is male-specifically up-regulated by gonadal androgen in the medaka brain

Oxytocin, a mammalian neuropeptide primarily synthesised in the supraoptic and paraventricular nuclei of the hypothalamus, mediates a variety of physiological and behavioural processes, ranging from parturition and lactation to affiliation and prosociality. Multiple studies in rodents have shown that the expression of the oxytocin gene (Oxt) is stimulated by oestrogen, whereas androgen has no apparent effect. However, this finding is not consistent across all studies, and no study has examined sex steroid regulation of Oxt or its orthologues in other animals. In the present study, we show that, in the teleost fish, medaka (Oryzias latipes), the expression of the isotocin gene (it), the teleost orthologue of Oxt, in the parvocellular preoptic nuclei (homologous to the mammalian supraoptic nucleus) is male-specifically up-regulated by gonadal androgen, whereas it expression in the magnocellular/gigantocellular preoptic nuclei (homologous to the mammalian paraventricular nucleus) is independent of sex steroids in both sexes. None of the it-expressing neurones appear to co-express androgen receptors, suggesting that the effect of androgen on it expression is indirect. We found that the expression of a kisspeptin gene, kiss2, in the male brain is dependent on gonadal androgen, raising the possibility that the androgen-dependent expression of it may be mediated by kiss2 neurones. Our data also show that the isotocin peptide synthesised in response to androgen is axonally transported to the posterior pituitary to act peripherally. Given that levels of it expression are higher in females than in males, androgen may serve to compensate for the female-biased it expression to ensure a role for isotocin that is equally important for both sexes. These results are unexpectedly quite different from those reported in rodents, indicating that the regulatory role of sex steroids in Oxt/it expression has diverged during evolution, possibly with accompanying changes in the role of oxytocin/isotocin.

Oxytocin and vasopressin neural networks: Implications for social behavioral diversity and translational neuroscience

Oxytocin- and vasopressin-related systems are present in invertebrate and vertebrate bilaterian animals, including humans, and exhibit conserved neuroanatomical and functional properties. In vertebrates, these systems innervate conserved neural networks that regulate social learning and behavior, including conspecific recognition, social attachment, and parental behavior. Individual and species-level variation in central organization of oxytocin and vasopressin systems has been linked to individual and species variation in social learning and behavior. In humans, genetic polymorphisms in the genes encoding oxytocin and vasopressin peptides and/or their respective target receptors have been associated with individual variation in social recognition, social attachment phenotypes, parental behavior, and psychiatric phenotypes such as autism. Here we describe both conserved and variable features of central oxytocin and vasopressin systems in the context of social behavioral diversity, with a particular focus on neural networks that modulate social learning, behavior, and salience of sociosensory stimuli during species-typical social contexts.

Diversity of oxytocin neurons: beyond magno- and parvocellular cell types?

The hypothalamic neuropeptide oxytocin (OT), which is evolutionarily conserved among different species throughout the animal kingdom, is a key modulator of a variety of socio-emotional behaviors such as fear, trust and empathy. OT cells in the mammalian hypothalamus have been traditionally divided into two distinct types - magnocellular (magnOT) and parvocellular (parvOT) or preautonomic neurons. This distinction is based on OT cell sizes and shapes, projections, electrophysiological activity and functions. Indeed, while neuroendocrine magnOT neurons are known to primarily project their axons to the posterior pituitary and to a number of forebrain regions, non-neuroendocrine parvOT neurons have been seen as the main source of OT innervation of the brainstem and spinal cord to control autonomic functions and pain perception. However, very recent findings demonstrated distinct genetic profiles in OT neurons, allowing discrimination of at least four types of cells expressing OT. Furthermore, unexpected axonal projections of parvOT neurons to the forebrain and magnOT neurons to the midbrain have been newly reported. In this review, we focus on the detailed analysis of methods of distinction between OT cell types, in- and output sites, morphology as well as on the direct connectivity between OT neurons and its physiological significance. At the end, we propose a hypothesis that the central OT system is composed of more than just two OT cell types, which should be further verified by the application of available genetic and anatomical techniques. This article is protected by copyright. All rights reserved.

Examining the Role of Vasopressin in the Modulation of Parental and Sexual Behaviors

Vasopressin (VP) and VP-like neuropeptides are evolutionarily stable peptides found in all vertebrate species. In non-mammalian vertebrates, vasotocin (VT) plays a role similar to mammalian VP, whereas mesotocin and isotocin are functionally similar to mammalian oxytocin (OT). Here, we review the involvement of VP in brain circuits, synaptic plasticity, evolution, and function, highlighting the role of VP in social behavior. In all studied species, VP is encoded on chromosome 20p13, and in mammals, VP is produced in specific hypothalamic nuclei and released by the posterior pituitary. The role of VP is mediated by the stimulation of the V1a, V1b, and V2 receptors as well as the oxytocinergic and purinergic receptors. VT and VP functions are usually related to osmotic and cardiovascular homeostasis when acting peripherally. However, these neuropeptides are also critically involved in the central modulation of social behavior displays, such as pairing recognition, pair-bonding, social memory, sexual behavior, parental care, and maternal and aggressive behavior. Evidence suggests that these effects are primarily mediated by V1a receptor in specific brain circuits that provide important information for the onset and control of social behaviors in normal and pathological conditions.

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

Short Promoters in Viral Vectors Drive Selective Expression in Mammalian Inhibitory Neurons, but do not Restrict Activity to Specific Inhibitory Cell-Types

Short cell-type specific promoter sequences are important for targeted gene therapy and studies of brain circuitry. We report on the ability of short promoter sequences to drive fluorescent protein expression in specific types of mammalian cortical inhibitory neurons using adeno-associated virus (AAV) and lentivirus (LV) vectors. We tested many gene regulatory sequences derived from fugu (Takifugu rubripes), mouse, human, and synthetic composite regulatory elements. All fugu compact promoters expressed in mouse cortex, with only the somatostatin (SST) and the neuropeptide Y (NPY) promoters largely restricting expression to GABAergic neurons. However these promoters did not control expression in inhibitory cells in a subtype specific manner. We also tested mammalian promoter sequences derived from genes putatively coexpressed or coregulated within three major inhibitory interneuron classes (PV, SST, VIP). In contrast to the fugu promoters, many of the mammalian sequences failed to express, and only the promoter from gene A930038C07Rik conferred restricted expression, although as in the case of the fugu sequences, this too was not inhibitory neuron subtype specific. Lastly and more promisingly, a synthetic sequence consisting of a composite regulatory element assembled with PAX6 E1.1 binding sites, NRSE and a minimal CMV promoter showed markedly restricted expression to a small subset of mostly inhibitory neurons, but whose commonalities are unknown.

Oxytocin, vasopressin, and the neurogenetics of sociality

There is growing evidence that the neuropeptides oxytocin and vasopressin modulate complex social behavior and social cognition. These ancient neuropeptides display a marked conservation in gene structure and expression, yet diversity in the genetic regulation of their receptors seems to underlie natural variation in social behavior, both between and within species. Human studies are beginning to explore the roles of these neuropeptides in social cognition and behavior and suggest that variation in the genes encoding their receptors may contribute to variation in human social behavior by altering brain function. Understanding the neurobiology and neurogenetics of social cognition and behavior has important implications, both clinically and for society.

Genomic imprinting and the evolution of sex differences in mammalian reproductive strategies

Two major developments have occurred that have influenced the evolution of sexually dimorphic reproductive strategies of mammals. Viviparity and development of a placenta is one such development, especially in small-brained rodent lineages, where there has been a major impact of placental hormones on the maternal brain. In the Old World primate/hominoid lineages, the massive expansion of the brain through growth of the neocortex has radically changed how reproductive strategies are determined. Genomic imprinting has played a significant part in both of these developments. Most of the imprinted genes investigated to date are expressed in the placenta and a subset are expressed in both placenta and hypothalamus. Based on phenotypes derived from targeted mutagenesis, a hypothesis is developed for the coadaptive evolution of placenta and hypothalamus, particularly in the context of neurohormonal regulation of maternalism. In small-brained mammals, maternalism places a severe restriction on sexual activity, which in the case of a female rodent is little more than several hours in a lifetime compared with the several weeks given over to maternalism. The consequent sparsity of oestrous, sexually receptive females imposes a rigorous competitive reproductive strategy in males, with the onus being on the male's ability to find oestrous females. This has resulted in a marked sex difference in the chemosensory system, particularly the VNO accessory olfactory system, for the engagement of male sexual behavior in response to oestrous females. Genomic imprinting, together with neonatal androgens, has also played a role in the developing accessory olfactory system and its role in detecting oestrous females. With the evolutionary expansion of the neocortex seen in Old World primates and hominids, reproductive strategies are complex and embedded in the social structure and hierarchies which characterize primate societies. Reproductive strategies depend far more on intelligent behavioral determinants than they do on hormonal determinants. In females, sexual activity is not restricted to oestrous periods, indeed most of the sexual activity is not reproductive. Male Old World primates continue to mate for years after castration, but loss of dominance status leads to a loss of sexual interest within days. The genetic basis for the expansion of neocortical development is complex, but those parts of the brain which have expanded are undoubtedly under the influence of imprinted genes, as studies using parthenogenetic and androgenetic chimeras and allometric analysis of brains across comparative phylogenies have shown. Sex differences in behavior owe much to social structure, social learning, and the deployment of intelligent behavioral strategies. The epigenetic effects of social learning on brain development have become equally as important as the epigenetic effects of hormones on brain development and both contribute to sex differences in behavior in large-brained primates.

Transcriptional regulation of pituitary POMC is conserved at the vertebrate extremes despite great promoter sequence divergence

The stress response involves complex physiological mechanisms that maximize behavioral efficacy during attack or defense and is highly conserved in all vertebrates. Key mediators of the stress response are pituitary hormones encoded by the proopiomelanocortin gene (POMC). Despite conservation of physiological function and expression pattern of POMC in all vertebrates, phylogenetic footprinting analyses at the POMC locus across vertebrates failed to detect conserved noncoding sequences with potential regulatory function. To investigate whether ortholog POMC promoters from extremely distant vertebrates are functionally conserved, we used 5'-flanking sequences of the teleost fish Tetraodon nigroviridis POMCalpha gene to produce transgenic mice. Tetraodon POMCalpha promoter targeted reporter gene expression exclusively to mouse pituitary cells that normally express Pomc. Importantly, transgenic expression in mouse corticotrophs was increased after adrenalectomy. To understand how conservation of precise gene expression mechanisms coexists with great sequence divergence, we investigated whether very short elements are still conserved in all vertebrate POMC promoters. Multiple local sequence alignments that consider phylogenetic relationships of ortholog regions identified a unique 10-bp motif GTGCTAA(T/G)CC that is usually present in two copies in POMC 5'-flanking sequences of all vertebrates. Underlined nucleotides represent totally conserved sequences. Deletion of these paired motifs from Tetraodon POMCalpha promoter markedly reduced its transcriptional activity in a mouse corticotropic cell line and in pituitary POMC cells of transgenic mice. In mammals, the conserved motifs correspond to reported binding sites for pituitary-specific nuclear proteins that participate in POMC transcriptional regulation. Together, these results demonstrate that mechanisms that participate in pituitary-specific and hormonally regulated expression of POMC have been preserved since mammals and teleosts diverged from a common ancestor 450 million years ago despite great promoter sequence divergence.

Copeptin, derived from isotocin precursor, is a probable prolactin releasing factor in carp

Control of prolactin (PRL) release is of crucial importance for the multiple functions exerted by PRL in vertebrates. Recently identified hypothalamic PRL-releasing peptides displayed additional neuromodulatory activities and in fish only few could be detected close to lactotrophs. Here we describe the C-terminal peptide processed from the carp isotocin precursor as probable physiologically relevant regulator of PRL release in carp. The amino acid sequence derived from the complete isotocin precursor gene of Cyprinus carpio, predicted a C-terminal peptide uncleaved between the neurophysin (Np) and copeptin (Cp) domain. Accordingly, antibodies against synthetic Np- and Cp-specific oligopeptides both immunodetected a 13kDa protein (cNpCp) in total pituitary proteins and showed abundant immunoreaction in hypothalamic axons in direct contact with lactotrophs in the rostral pars distalis of carp pituitary gland sections. Finally, incubation of cultured carp pituitary explants with purified carp cNpCp resulted in a potent stimulation of PRL release.

Zebrafish orthopedia (otp) is required for isotocin cell development

Several behavioral and physiological processes such as social, sexual, and maternal behaviors, learning and memory, and parturition are influenced by the neurohypophysial peptide oxytocin. Studies in knockout mice have identified four transcriptional regulatory genes that are required for oxytocin neuronal development in the hypothalamus. These are the basic helix-loop-helix PAS genes Single-minded 1 (Sim1) and Arylhydrocarbon receptor nuclear translocator 2 (Arnt2), the POU homeobox gene Pou3f2, and the paired homeobox gene Orthopedia (Otp). Overall, however, the molecular control of oxytocin cell development is poorly understood. Studies in zebrafish provide a complementary view to mouse knockout experiments and facilitate understanding of neuroendocrine cell development. Isotocin, which is orthologous to oxytocin, is expressed early in the developing zebrafish brain. In this paper we show that zebrafish otp mRNA expression in the embryonic forebrain is dynamic and complex, and that it overlaps with isotocin expression in the dorsal preoptic area. Additionally, these studies demonstrate that otp is required for isotocin cell development. Evidence is also provided that otp and sim1 function in parallel to direct the differentiation of isotocin cells, and that otp is unlikely to affect brain patterning. Overall, these studies support the hypothesis that the role of otp in zebrafish neuroendocrine cell development is evolutionarily conserved with that of mammals.

The zebrafish bHLH PAS transcriptional regulator, single-minded 1 (sim1), is required for isotocin cell development

A wide range of physiological and behavioral processes, such as social, sexual, and maternal behaviors, learning and memory, and osmotic homeostasis are influenced by the neurohypophysial peptides oxytocin and vasopressin. Disruptions of these hormone systems have been linked to several neurobehavioral disorders, including autism, Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder. Studies in zebrafish promise to reveal the complex network of regulatory genes and signaling pathways that direct the development of oxytocin- and vasopressin-like neurons, and provide insight into factors involved in brain disorders associated with disruption of these systems. Isotocin, which is homologous to oxytocin, is expressed early, in a simple pattern in the developing zebrafish brain. Single-minded 1 (sim1), a member of the bHLH-PAS family of transcriptional regulatory genes, is required for terminal differentiation of mammalian oxytocin cells and is a master regulator of neurogenesis in Drosophila. Here we show that sim1 is expressed in the zebrafish forebrain and is required for isotocin cell development. The expression pattern of sim1 mRNA in the embryonic forebrain is dynamic and complex, and overlaps with isotocin expression in the preoptic area. We provide evidence that the role of sim1 in zebrafish neuroendocrine cell development is evolutionarily conserved with that of mammals.

Comparative genomics using fugu: a tool for the identification of conserved vertebrate cis-regulatory elements

With the imminent completion of the whole genome sequence of humans, increasing attention is being focused on the annotation of cis-regulatory elements in the human genome. Comparative genomics approaches based on evolutionary conservation have proved useful in the detection of conserved cis-regulatory elements. The pufferfish, Fugu rubripes, is an attractive vertebrate model for comparative genomics, by virtue of its compact genome and maximal phylogenetic distance from mammals. Fugu has lost a large proportion of nonessential DNA, and retained single orthologs for many duplicate genes that arose in the fish lineage. Non-coding sequences conserved between fugu and mammals have been shown to be functional cis-regulatory elements. Thus, fugu is a model fish genome of choice for discovering evolutionarily conserved regulatory elements in the human genome. Such evolutionarily conserved elements are likely to be shared by all vertebrates, and related to regulatory interactions fundamental to all vertebrates. The functions of these conserved vertebrate elements can be rapidly assayed in mammalian cell lines or in transgenic systems such as zebrafish/medaka and Xenopus, followed by validation of crucial elements in transgenic rodents.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.