Neural plate progenitors give rise to both anterior and posterior pituitary cells

The pituitary is the master neuroendocrine gland, which regulates body homeostasis. It consists of the anterior pituitary/adenohypophysis harboring hormones producing cells and the posterior pituitary/neurohypophysis, which relays the passage of hormones from the brain to the periphery. It is accepted that the adenohypophysis originates from the oral ectoderm (Rathke's pouch), whereas the neural ectoderm contributes to the neurohypophysis. Single-cell transcriptomics of the zebrafish pituitary showed that cyp26b1-positive astroglial pituicytes of the neurohypophysis and prop1-positive adenohypophyseal progenitors expressed common markers implying lineage relatedness. Genetic tracing identifies that, in contrast to the prevailing dogma, neural plate precursors of zebrafish (her4.3+) and mouse (Sox1+) contribute to both neurohypophyseal and a subset of adenohypophyseal cells. Pituicyte-derived retinoic-acid-degrading enzyme Cyp26b1 fine-tunes differentiation of prop1+ progenitors into hormone-producing cells. These results challenge the notion that adenohypophyseal cells are exclusively derived from non-neural ectoderm and demonstrate that crosstalk between neuro- and adeno-hypophyseal cells affects differentiation of pituitary cells.

Glucocorticoid-sensitive period of corticotroph development-Implications for mechanisms of early life stress

Corticotrophs are intermediaries in the hypothalamic-pituitary-adrenal (HPA) axis, which plays a crucial role in stress response in vertebrates. The HPA axis displays an intricate mode of negative feedback regulation, whereby the peripheral effector, cortisol inhibits the secretion of its upstream regulator, adrenocorticotropic hormone (ACTH) from proopiomelanocortin (POMC)-expressing cells in the pituitary. While the feedback regulation of the HPA axis is well characterized in the adult organism, the effect of feedback regulation on the development of corticotrophs is poorly understood. Here, we studied the effect of glucocorticoids on the development of POMC-expressing cells in the zebrafish pituitary. The development of POMC cells showed a steady increase in numbers between 2-6 days post fertilization. Inhibition of endogenous glucocorticoid synthesis resulted in an increase in POMC cell number due to reduced developmental feedback inhibition of cortisol on POMC cells. Conversely, addition of exogenous dexamethasone at a critical developmental window led to a decrease in corticotroph cell number, mimicking greater feedback control due to increased cortisol levels. Finally, developmental dysregulation of ACTH levels resulted in impaired anxiety-like and stress-coping behaviours. Hence, we identified a sensitive developmental window for the effect of glucocorticoids on corticotrophs and demonstrate the downstream effect on stress-responsive behaviour.

Local angiogenic interplay of Vegfc/d and Vegfa controls brain region-specific emergence of fenestrated capillaries

Fenestrated and blood-brain barrier (BBB)-forming endothelial cells constitute major brain capillaries, and this vascular heterogeneity is crucial for region-specific neural function and brain homeostasis. How these capillary types emerge in a brain region-specific manner and subsequently establish intra-brain vascular heterogeneity remains unclear. Here, we performed a comparative analysis of vascularization across the zebrafish choroid plexuses (CPs), circumventricular organs (CVOs), and retinal choroid, and show common angiogenic mechanisms critical for fenestrated brain capillary formation. We found that zebrafish deficient for Gpr124, Reck, or Wnt7aa exhibit severely-impaired BBB angiogenesis without any apparent defect in fenestrated capillary formation in the CPs, CVOs, and retinal choroid. Conversely, genetic loss of various Vegf combinations caused significant disruptions in Wnt7/Gpr124/Reck signaling-independent vascularization of these organs. The phenotypic variation and specificity revealed heterogeneous endothelial requirements for Vegfs-dependent angiogenesis during CP and CVO vascularization, identifying unexpected interplay of Vegfc/d and Vegfa in this process. Mechanistically, expression analysis and paracrine activity-deficient vegfc mutant characterization suggest that endothelial cells and non-neuronal specialized cell types present in the CPs and CVOs are major sources of Vegfs responsible for regionally-restricted angiogenic interplay. Thus, brain region-specific presentations and interplay of Vegfc/d and Vegfa control emergence of fenestrated capillaries, providing insight into the mechanisms driving intra-brain vascular heterogeneity and fenestrated vessel formation in other organs.

Evolutionarily conserved role of oxytocin in social fear contagion in zebrafish

Emotional contagion is the most ancestral form of empathy. We tested to what extent the proximate mechanisms of emotional contagion are evolutionarily conserved by assessing the role of oxytocin, known to regulate empathic behaviors in mammals, in social fear contagion in zebrafish. Using oxytocin and oxytocin receptor mutants, we show that oxytocin is both necessary and sufficient for observer zebrafish to imitate the distressed behavior of conspecific demonstrators. The brain regions associated with emotional contagion in zebrafish are homologous to those involved in the same process in rodents (e.g., striatum, lateral septum), receiving direct projections from oxytocinergic neurons located in the pre-optic area. Together, our results support an evolutionary conserved role for oxytocin as a key regulator of basic empathic behaviors across vertebrates.

Developmental Effects of Oxytocin Neurons on Social Affiliation and Processing of Social Information

Hormones regulate behavior either through activational effects that facilitate the acute expression of specific behaviors or through organizational effects that shape the development of the nervous system thereby altering adult behavior. Much research has implicated the neuropeptide oxytocin (OXT) in acute modulation of various aspects of social behaviors across vertebrate species, and OXT signaling is associated with the developmental social deficits observed in autism spectrum disorders (ASDs); however, little is known about the role of OXT in the neurodevelopment of the social brain. We show that perturbation of OXT neurons during early zebrafish development led to a loss of dopaminergic neurons, associated with visual processing and reward, and blunted the neuronal response to social stimuli in the adult brain. Ultimately, adult fish whose OXT neurons were ablated in early life, displayed altered functional connectivity within social decision-making brain nuclei both in naive state and in response to social stimulus and became less social. We propose that OXT neurons have an organizational role, namely, to shape forebrain neuroarchitecture during development and to acquire an affiliative response toward conspecifics.SIGNIFICANCE STATEMENT Social behavior is developed over the lifetime of an organism and the neuropeptide oxytocin (OXT) modulates social behaviors across vertebrate species, and is associated with neuro-developmental social deficits such as autism. However, whether OXT plays a role in the developmental maturation of neural systems that are necessary for social behavior remains poorly explored. We show that proper behavioral and neural response to social stimuli depends on a developmental process orchestrated by OXT neurons. Animals whose OXT system is ablated in early life show blunted neuronal and behavioral responses to social stimuli as well as wide ranging disruptions in the functional connectivity of the social brain. We provide a window into the mechanisms underlying OXT-dependent developmental processes that implement adult sociality.

Genetic variation in the social environment affects behavioral phenotypes of oxytocin receptor mutants in zebrafish

Oxytocin-like peptides have been implicated in the regulation of a wide range of social behaviors across taxa. On the other hand, the social environment, which is composed of conspecifics that may vary in their genotypes, also influences social behavior, creating the possibility for indirect genetic effects. Here, we used a zebrafish oxytocin receptor knockout line to investigate how the genotypic composition of the social environment (G_s) interacts with the oxytocin genotype of the focal individual (G_i) in the regulation of its social behavior. For this purpose, we have raised wild-type or knock-out zebrafish in either wild-type or knock-out shoals and tested different components of social behavior in adults. G_ixG_s effects were detected in some behaviors, highlighting the need to control for G_ixG_s effects when interpreting results of experiments using genetically modified animals, since the genotypic composition of the social environment can either rescue or promote phenotypes associated with specific genes.

Splice-specific deficiency of the PTSD-associated gene PAC1 leads to a paradoxical age-dependent stress behavior

The pituitary adenylate cyclase-activating polypeptide receptor (PAC1, also known as ADCYAP1R1) is associated with post-traumatic stress disorder and modulation of stress response in general. Alternative splicing of PAC1 results in multiple gene products, which differ in their mode of signalling and tissue distribution. However, the roles of distinct splice variants in the regulation of stress behavior is poorly understood. Alternative splicing of a short exon, which is known as the "hop cassette", occurs during brain development and in response to stressful challenges. To examine the function of this variant, we generated a splice-specific zebrafish mutant lacking the hop cassette, which we designated 'hopless'. We show that hopless mutant larvae display increased anxiety-like behavior, including reduced dark exploration and impaired habituation to dark exposure. Conversely, adult hopless mutants displayed superior ability to rebound from an acute stressor, as they exhibited reduced anxiety-like responses to an ensuing novelty stress. We propose that the developmental loss of a specific PAC1 splice variant mimics prolonged mild stress exposure, which in the long term, predisposes the organism's stress response towards a resilient phenotype. Our study presents a unique genetic model demonstrating how early-life state of anxiety paradoxically correlates with reduced stress susceptibility in adulthood.

Splice-specific deficiency of the PTSD-associated gene PAC1 leads to a paradoxical age-dependent stress behavior

The pituitary adenylate cyclase-activating polypeptide receptor (PAC1, also known as ADCYAP1R1) is associated with post-traumatic stress disorder and modulation of stress response in general. Alternative splicing of PAC1 results in multiple gene products, which differ in their mode of signalling and tissue distribution. However, the roles of distinct splice variants in the regulation of stress behavior is poorly understood. Alternative splicing of a short exon, which is known as the "hop cassette", occurs during brain development and in response to stressful challenges. To examine the function of this variant, we generated a splice-specific zebrafish mutant lacking the hop cassette, which we designated 'hopless'. We show that hopless mutant larvae display increased anxiety-like behavior, including reduced dark exploration and impaired habituation to dark exposure. Conversely, adult hopless mutants displayed superior ability to rebound from an acute stressor, as they exhibited reduced anxiety-like responses to an ensuing novelty stress. We propose that the developmental loss of a specific PAC1 splice variant mimics prolonged mild stress exposure, which in the long term, predisposes the organism's stress response towards a resilient phenotype. Our study presents a unique genetic model demonstrating how early-life state of anxiety paradoxically correlates with reduced stress susceptibility in adulthood.

Oxytocin receptor signalling modulates novelty recognition but not social preference in zebrafish

Sociality is a complex phenomenon that involves the individual´s motivation to approach their conspecifics, along with social cognitive functions that enable individuals to interact and survive. The nonapeptide oxytocin (OXT) is known to regulate sociality in many species. However, the role of OXT in specific aspects of sociality is still not well understood. In the present study, we investigated the contribution of the OXT receptor (OXTR) signalling in two different aspects of zebrafish social behaviour: social preference, by measuring their motivation to approach a shoal of conspecifics, and social recognition, by measuring their ability to discriminate between a novel and familiar fish, using a mutant zebrafish lacking a functional OXTR. Although oxtr mutant zebrafish displayed normal attraction to a shoal of conspecifics, they exhibited reduced social recognition. We further investigated whether this effect would be social-domain specific by replacing conspecific fish by objects. Although no differences were observed in object approach, oxtr mutant fish also exhibited impaired object recognition. Our findings suggest that OXTR signalling regulates a more general memory recognition of familiar vs novel entities, not only in social but also in a non-social domain, in zebrafish.

Perceptual mechanisms of social affiliation in zebrafish

Social living animals need to recognize the presence of conspecifics in the environment in order to engage in adaptive social interactions. Social cues can be detected through different sensory modalities, including vision. Two main visual features can convey information about the presence of conspecifics: body form and biological motion (BM). Given the role that oxytocin plays in social behavior regulation across vertebrates, particularly in the salience and reward values of social stimuli, we hypothesized that it may also be involved in the modulation of perceptual mechanisms for conspecific detection. Here, using videoplaybacks, we assessed the role of conspecific form and BM in zebrafish social affiliation, and how oxytocin regulates the perception of these cues. We demonstrated that while each visual cue is important for social attraction, BM promotes a higher fish engagement than the static conspecific form alone. Moreover, using a mutant line for one of the two oxytocin receptors, we show that oxytocin signaling is involved in the regulation of BM detection but not conspecific form recognition. In summary, our results indicate that, apart from oxytocin role in the regulation of social behaviors through its effect on higher-order cognitive mechanisms, it may regulate social behavior by modulating very basic perceptual mechanisms underlying the detection of socially-relevant cues.

Single-Cell Molecular and Cellular Architecture of the Mouse Neurohypophysis

The neurohypophysis (NH), located at the posterior lobe of the pituitary, is a major neuroendocrine tissue, which mediates osmotic balance, blood pressure, reproduction, and lactation by means of releasing the neurohormones oxytocin (OXT) and arginine-vasopressin (AVP) from the brain into the peripheral blood circulation. The major cellular components of the NH are hypothalamic axonal termini, fenestrated endothelia and pituicytes, the resident astroglia. However, despite the physiological importance of the NH, the exact molecular signature defining neurohypophyseal cell types and in particular the pituicytes, remains unclear. Using single-cell RNA sequencing (scRNA-Seq), we captured seven distinct cell types in the NH and intermediate lobe (IL) of adult male mouse. We revealed novel pituicyte markers showing higher specificity than previously reported. Bioinformatics analysis demonstrated that pituicyte is an astrocytic cell type whose transcriptome resembles that of tanycyte. Single molecule in situ hybridization revealed spatial organization of the major cell types implying intercellular communications. We present a comprehensive molecular and cellular characterization of neurohypophyseal cell types serving as a valuable resource for further functional research.

The fenestrae-associated protein Plvap regulates the rate of blood-borne protein passage into the hypophysis

To maintain body homeostasis, endocrine systems must detect and integrate blood-borne peripheral signals. This is mediated by fenestrae, specialized permeable pores in the endothelial membrane. Plasmalemma vesicle-associated protein (Plvap) is located in the fenestral diaphragm and is thought to play a role in the passage of proteins through the fenestrae. However, this suggested function has yet to be demonstrated directly. We studied the development of fenestrated capillaries in the hypophysis, a major neuroendocrine interface between the blood and brain. Using a transgenic biosensor to visualize the vascular excretion of the genetically tagged plasma protein DBP-EGFP, we show that the developmental acquisition of vascular permeability coincides with differential expression of zebrafish plvap orthologs in the hypophysis versus brain. Ultrastructural analysis revealed that plvapb mutants display deficiencies in fenestral diaphragms and increased density of hypophyseal fenestrae. Measurements of DBP-EGFP extravasation in plvapb mutants provided direct proof that Plvap limits the rate of blood-borne protein passage through fenestrated endothelia. We present the regulatory role of Plvap in the development of blood-borne protein detection machinery at a neuroendocrine interface through which hormones are released to the general circulation.

Social creatures: Model animal systems for studying the neuroendocrine mechanisms of social behaviour

The interaction of animals with conspecifics, termed social behaviour, has a major impact on the survival of many vertebrate species. Neuropeptide hormones modulate the underlying physiology that governs social interactions, and many findings concerning the neuroendocrine mechanisms of social behaviours have been extrapolated from animal models to humans. Neurones expressing neuropeptides show similar distribution patterns within the hypothalamic nucleus, even when evolutionarily distant species are compared. During evolution, hypothalamic neuropeptides and releasing hormones have retained not only their structures, but also their biological functions, including their effects on behaviour. Here, we review the current understanding of the mechanisms of social behaviours in several classes of animals, such as worms, insects and fish, as well as laboratory, wild and domesticated mammals.

Robo2 regulates synaptic oxytocin content by affecting actin dynamics

The regulation of neuropeptide level at the site of release is essential for proper neurophysiological functions. We focused on a prominent neuropeptide, oxytocin (OXT) in the zebrafish as an in vivo model to visualize and quantify OXT content at the resolution of a single synapse. We found that OXT-loaded synapses were enriched with polymerized actin. Perturbation of actin filaments by either cytochalasin-D or conditional Cofilin expression resulted in decreased synaptic OXT levels. Genetic loss of robo2 or slit3 displayed decreased synaptic OXT content and robo2 mutants displayed reduced mobility of the actin probe Lifeact-EGFP in OXT synapses. Using a novel transgenic reporter allowing real-time monitoring of OXT-loaded vesicles, we show that robo2 mutants display slower rate of vesicles accumulation. OXT-specific expression of dominant-negative Cdc42, which is a key regulator of actin dynamics and a downstream effector of Robo2, led to a dose-dependent increase in OXT content in WT, and a dampened effect in robo2 mutants. Our results link Slit3-Robo2-Cdc42, which controls local actin dynamics, with the maintenance of synaptic neuropeptide levels.

Specific inhibition of splicing factor activity by decoy RNA oligonucleotides

Alternative splicing, a fundamental step in gene expression, is deregulated in many diseases. Splicing factors (SFs), which regulate this process, are up- or down regulated or mutated in several diseases including cancer. To date, there are no inhibitors that directly inhibit the activity of SFs. We designed decoy oligonucleotides, composed of several repeats of a RNA motif, which is recognized by a single SF. Here we show that decoy oligonucleotides targeting splicing factors RBFOX1/2, SRSF1 and PTBP1, can specifically bind to their respective SFs and inhibit their splicing and biological activities both in vitro and in vivo. These decoy oligonucleotides present an approach to specifically downregulate SF activity in conditions where SFs are either up-regulated or hyperactive.

Alternative splicing, critical for gene expression, is deregulated in many diseases. Here the authors develop decoy oligonucleotides to specifically downregulate splicing factors activity.

Pituicyte Cues Regulate the Development of Permeable Neuro-Vascular Interfaces

The hypothalamo-neurohypophyseal system (HNS) regulates homeostasis through the passage of neurohormones and blood-borne proteins via permeable blood capillaries that lack the blood-brain barrier (BBB). Why neurohypophyseal capillaries become permeable while the neighboring vasculature of the brain forms BBB remains unclear. We show that pituicytes, the resident astroglial cells of the neurohypophysis, express genes that are associated with BBB breakdown during neuroinflammation. Pituicyte-enriched factors provide a local microenvironment that instructs a permeable neurovascular conduit. Thus, genetic and pharmacological perturbations of Vegfa and Tgfβ3 affected HNS vascular morphogenesis and permeability and impaired the expression of the fenestral marker plvap. The anti-inflammatory agent dexamethasone decreased HNS permeability and downregulated the pituicyte-specific cyp26b gene, encoding a retinoic acid catabolic enzyme. Inhibition of Cyp26b activity led to upregulation of tight junction protein Claudin-5 and decreased permeability. We conclude that pituicyte-derived factors regulate the "decision" of endothelial cells to adopt a permeable endothelial fate instead of forming a BBB.

Genome Editing Reveals Idiosyncrasy of CNGA2 Ion Channel-Directed Antibody Immunoreactivity Toward Oxytocin

Presynaptic cGMP-gated ion (CNG) channels positively or negatively modulate neurotransmitter secretion as well as the strength of synaptic transmission. Zebrafish cGMP-gated ion channel, CNGA2a (a.k.a. CNGA5), was previously reported to be specifically enriched in synaptic terminals of zebrafish oxytocin (OXT) neurons. This conclusion was based on immunoreactivity of a monoclonal antibody (mAb) clone L55/54, which was directed against the carboxy terminal tail of the CNGA2a. To study the role of CNGA2a in oxytocin neurons function, we generated zebrafish mutants of cnga2a, cnga2b and oxt genes using clustered regularly interspaced short palindromic repeats (CRISPR)-mediated genome editing. We show that mAb L55/54 specifically recognizes CNGA2a protein when expressed in heterologous cell culture system. Surprisingly, anti-CNGA2a immunoreactivity was not eliminated following knockout of either cnga2a, cnga2b or both. However, knockout of oxt resulted in total loss of anti-CNGA2a mAb immunoreactivity despite the lack of sequence and structural similarities between OXT and CNGA2a proteins. Our results provide a noteworthy lesson of differences in antibody immunoreactivity, which could only be revealed using specific genetic tools.

Genome Editing Reveals Idiosyncrasy of CNGA2 Ion Channel-Directed Antibody Immunoreactivity Toward Oxytocin

Presynaptic cGMP-gated ion (CNG) channels positively or negatively modulate neurotransmitter secretion as well as the strength of synaptic transmission. Zebrafish cGMP-gated ion channel, CNGA2a (a.k.a. CNGA5), was previously reported to be specifically enriched in synaptic terminals of zebrafish oxytocin (OXT) neurons. This conclusion was based on immunoreactivity of a monoclonal antibody (mAb) clone L55/54, which was directed against the carboxy terminal tail of the CNGA2a. To study the role of CNGA2a in oxytocin neurons function, we generated zebrafish mutants of cnga2a, cnga2b and oxt genes using clustered regularly interspaced short palindromic repeats (CRISPR)-mediated genome editing. We show that mAb L55/54 specifically recognizes CNGA2a protein when expressed in heterologous cell culture system. Surprisingly, anti-CNGA2a immunoreactivity was not eliminated following knockout of either cnga2a, cnga2b or both. However, knockout of oxt resulted in total loss of anti-CNGA2a mAb immunoreactivity despite the lack of sequence and structural similarities between OXT and CNGA2a proteins. Our results provide a noteworthy lesson of differences in antibody immunoreactivity, which could only be revealed using specific genetic tools.

The Dual Functional Reflecting Iris of the Zebrafish

Many marine organisms have evolved a reflective iris to prevent unfocused light from reaching the retina. The fish iris has a dual function, both to camouflage the eye and serving as a light barrier. Yet, the physical mechanism that enables this dual functionality and the benefits of using a reflective iris have remained unclear. Using synchrotron microfocused diffraction, cryo-scanning electron microscopy imaging, and optical analyses on zebrafish at different stages of development, it is shown that the complex optical response of the iris is facilitated by the development of high-order organization of multilayered guanine-based crystal reflectors and pigments. It is further demonstrated how the efficient light reflector is established during development to allow the optical functionality of the eye, already at early developmental stages.

Smells Familiar: Pheromone-Induced Neurotransmitter Switching Mediates Social Discrimination

Social discrimination is regulated by a variety of sensory inputs. In this issue of Neuron, Dulcis et al. (2017) show that chemosensory-mediated kin preference in Xenopus is determined by changes in neurotransmitter composition, which are regulated by specific microRNAs.

The not-so-long history of zebrafish research in Israel

The zebrafish has become a model of choice in fundamental and applied life sciences and is widely used in various fields of biomedical research as a human disease model for cancer, metabolic and neurodegenerative diseases, and regenerative medicine. The transparency of the zebrafish embryo allows real-time visualization of the development and morphogenesis of practically all of its tissues and organs. Zebrafish are amenable to genetic manipulation, for which innovative genetic and molecular techniques are constantly being introduced. These include the study of gene function and regulation using gene knockdown, knockout and knock-in, as well as transgenesis and tissue-specific genetic perturbations. Complementing this genetic toolbox, the zebrafish exhibits measurable behavioral and hormonal responses already at the larval stages, providing a viable vertebrate animal model for high-throughput drug screening and chemical genetics. With the available tools of the genomic era and the abundance of disease-associated human genes yet to be explored, the zebrafish model is becoming the preferred choice in many studies. Its advantages and potential are being increasingly recognized within the Israeli scientific community, and its use as a model system for basic and applied science has expanded in Israel in recent years. Since the first zebrafish-focused laboratory was introduced at Tel Aviv University 16 years ago, seven more zebrafish-centric research groups have been established, along with more than two dozen academic research groups and three bio-medical companies that are now utilizing this model.

Homeodomain protein Otp affects developmental neuropeptide switching in oxytocin neurons associated with a long-term effect on social behavior

Proper response to stress and social stimuli depends on orchestrated development of hypothalamic neuronal circuits. Here we address the effects of the developmental transcription factor orthopedia (Otp) on hypothalamic development and function. We show that developmental mutations in the zebrafish paralogous gene otpa but not otpb affect both stress response and social preference. These behavioral phenotypes were associated with developmental alterations in oxytocinergic (OXT) neurons. Thus, otpa and otpb differentially regulate neuropeptide switching in a newly identified subset of OXT neurons that co-express the corticotropin-releasing hormone (CRH). Single-cell analysis revealed that these neurons project mostly to the hindbrain and spinal cord. Ablation of this neuronal subset specifically reduced adult social preference without affecting stress behavior, thereby uncoupling the contribution of a specific OXT cluster to social behavior from the general otpa^-/- deficits. Our findings reveal a new role for Otp in controlling developmental neuropeptide balance in a discrete OXT circuit whose disrupted development affects social behavior.

Role of developmental factors in hypothalamic function

The hypothalamus is a brain region which regulates homeostasis by mediating endocrine, autonomic and behavioral functions. It is comprised of several nuclei containing distinct neuronal populations producing neuropeptides and neurotransmitters that regulate fundamental body functions including temperature and metabolic rate, thirst and hunger, sexual behavior and reproduction, circadian rhythm, and emotional responses. The identity, number and connectivity of these neuronal populations are established during the organism's development and are of crucial importance for normal hypothalamic function. Studies have suggested that developmental abnormalities in specific hypothalamic circuits can lead to obesity, sleep disorders, anxiety, depression and autism. At the molecular level, the development of the hypothalamus is regulated by transcription factors (TF), secreted growth factors, neuropeptides and their receptors. Recent studies in zebrafish and mouse have demonstrated that some of these molecules maintain their expression in the adult brain and subsequently play a role in the physiological functions that are regulated by hypothalamic neurons. Here, we summarize the involvement of some of the key developmental factors in hypothalamic development and function by focusing on the mouse and zebrafish genetic model organisms.

Role of developmental factors in hypothalamic function

The hypothalamus is a brain region which regulates homeostasis by mediating endocrine, autonomic and behavioral functions. It is comprised of several nuclei containing distinct neuronal populations producing neuropeptides and neurotransmitters that regulate fundamental body functions including temperature and metabolic rate, thirst and hunger, sexual behavior and reproduction, circadian rhythm, and emotional responses. The identity, number and connectivity of these neuronal populations are established during the organism’s development and are of crucial importance for normal hypothalamic function. Studies have suggested that developmental abnormalities in specific hypothalamic circuits can lead to obesity, sleep disorders, anxiety, depression and autism. At the molecular level, the development of the hypothalamus is regulated by transcription factors (TF), secreted growth factors, neuropeptides and their receptors. Recent studies in zebrafish and mouse have demonstrated that some of these molecules maintain their expression in the adult brain and subsequently play a role in the physiological functions that are regulated by hypothalamic neurons. Here, we summarize the involvement of some of the key developmental factors in hypothalamic development and function by focusing on the mouse and zebrafish genetic model organisms.

Alternative Splicing of the Pituitary Adenylate Cyclase-Activating Polypeptide Receptor PAC1: Mechanisms of Fine Tuning of Brain Activity

Alternative splicing of the precursor mRNA encoding for the neuropeptide receptor PAC1/ADCYAP1R1 generates multiple protein products that exhibit pleiotropic activities. Recent studies in mammals and zebrafish have implicated some of these splice isoforms in control of both cellular and body homeostasis. Here, we review the regulation of PAC1 splice variants and their underlying signal transduction and physiological processes in the nervous system.

Homeodomain protein otp and activity-dependent splicing modulate neuronal adaptation to stress

Regulation of corticotropin-releasing hormone (CRH) activity is critical for the animal's adaptation to stressful challenges, and its dysregulation is associated with psychiatric disorders in humans. However, the molecular mechanism underlying this transcriptional response to stress is not well understood. Using various stress paradigms in mouse and zebrafish, we show that the hypothalamic transcription factor Orthopedia modulates the expression of CRH as well as the splicing factor Ataxin 2-Binding Protein-1 (A2BP1/Rbfox-1). We further show that the G protein coupled receptor PAC1, which is a known A2BP1/Rbfox-1 splicing target and an important mediator of CRH activity, is alternatively spliced in response to a stressful challenge. The generation of PAC1-hop messenger RNA isoform by alternative splicing is required for termination of CRH transcription, normal activation of the hypothalamic-pituitary-adrenal axis and adaptive anxiety-like behavior. Our study identifies an evolutionarily conserved biochemical pathway that modulates the neuronal adaptation to stress through transcriptional activation and alternative splicing.

The neurohypophysis: fishing for new insights

The neurohypophysis is a neurovascular interface through which the brain regulates peripheral organs to maintain homeostasis. The molecular mechanisms underlying its formation are poorly understood, although the emergence of new genetic and imaging tools has begun to yield new insights. In a recent study, researchers discovered that, in embryonic zebrafish, oxytocin secreted from hypophyseal axons serves as a local angiogenic cue that pulls in nearby blood vessels.

The hypothalamic neuropeptide oxytocin is required for formation of the neurovascular interface of the pituitary

The hypothalamo-neurohypophyseal system (HNS) is the neurovascular structure through which the hypothalamic neuropeptides oxytocin and arginine-vasopressin exit the brain into the bloodstream, where they go on to affect peripheral physiology. Here, we investigate the molecular cues that regulate the neurovascular contact between hypothalamic axons and neurohypophyseal capillaries of the zebrafish. We developed a transgenic system in which both hypothalamic axons and neurohypophyseal vasculature can be analyzed in vivo. We identified the cellular organization of the zebrafish HNS as well as the dynamic processes that contribute to formation of the HNS neurovascular interface. We show that formation of this interface is regulated during development by local release of oxytocin, which affects endothelial morphogenesis. This cell communication process is essential for the establishment of a tight axovasal interface between the neurons and blood vessels of the HNS. We present a unique example of axons affecting endothelial morphogenesis through secretion of a neuropeptide.

Visualization of mRNA expression in the zebrafish embryo

Examination of spatial and temporal gene expression pattern is a key step towards understanding gene function. Therefore, in situ hybridization of mRNA is one of the most powerful and widely used -techniques in biology. Recent advances allow the reliable and simultaneous detection of mRNA transcripts, or combinations of mRNA and protein, in zebrafish embryos.Here we describe a standard protocol for visualizing the precise expression pattern of a single transcript or multiple gene products. The procedure employs fixation and permeabilization of embryos, followed by hybridization with tagged antisense riboprobes. Excess probes are then washed and hybrids are detected by enzyme-mediated immunohistochemistry utilizing either chromogenic or fluorescent substrates.

Two-photon-based photoactivation in live zebrafish embryos

Photoactivation of target compounds in a living organism has proven a valuable approach to investigate various biological processes such as embryonic development, cellular signaling and adult physiology. In this respect, the use of multi-photon microscopy enables quantitative photoactivation of a given light responsive agent in deep tissues at a single cell resolution. As zebrafish embryos are optically transparent, their development can be monitored in vivo. These traits make the zebrafish a perfect model organism for controlling the activity of a variety of chemical agents and proteins by focused light. Here we describe the use of two-photon microscopy to induce the activation of chemically caged fluorescein, which in turn allows us to follow cell's destiny in live zebrafish embryos. We use embryos expressing a live genetic landmark (GFP) to locate and precisely target any cells of interest. This procedure can be similarly used for precise light induced activation of proteins, hormones, small molecules and other caged compounds.

Cxcl12a-Cxcr4b signaling is important for proper development of the forebrain GnRH system in zebrafish

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons control pituitary gonadotropin secretion and gametogenesis. In the course of development, these neurons migrate from the olfactory placode to the hypothalamus. The precise molecular mechanism of this neuronal migration is unclear. Here, we investigated whether the chemokine receptor, Cxcr4b, and its cognate ligand, Cxcl12a, are required for proper migration of GnRH3 neurons in zebrafish. Deviated GnRH3 axonal projections and neuronal migration were detected in larvae that carry a homozygote cxcr4b mutation. Similarly, knockdown of Cxcr4b or Cxcl12a led to the appearance of abnormal GnRH3 axonal projections and cell migration, including absence of the characteristic lateral crossing of GnRH3 axons at the anterior commissure and optic chiasm. Double-labeling analysis has shown that cxcr4b and cxcl12a are expressed along the GnRH3 migration pathway (i.e. olfactory placode, terminal nerve and the optic chiasm). The results of this study suggest that the Cxcl12a-Cxcr4b ligand-receptor pair are involved in the migration of GnRH3 neurons in zebrafish, and are therefore crucial for the development of this system.

High resolution fate map of the zebrafish diencephalon

The diencephalon acts as an interactive site between the sensory, central, and endocrine systems and is one of the most elaborate structures in the vertebrate brain. To better understand the embryonic development and morphogenesis of the diencephalon, we developed an improved photoactivation (uncaging)-based lineage tracing strategy. To determine the exact position of a given diencephalic progenitor domain, we used a transgenic line driving green fluorescent protein (GFP) in cells expressing the proneural protein, Neurogenin1 (Neurog1), which was used as a visible neural plate landmark. This approach facilitated precise labeling of defined groups of cells in the prospective diencephalon of the zebrafish neural plate. In this manner, we labeled multiple overlapping areas of the diencephalon, thereby ensuring both accuracy and reproducibility of our lineage tracing regardless of the dynamic changes of the developing neural plate. We present a fate map of the zebrafish diencephalon at a higher spatial resolution than previously described.

Neural protein Olig2 acts upstream of the transcriptional regulator Sim1 to specify diencephalic dopaminergic neurons

Neural factors are expressed in neural progenitors and regulate neurogenesis and gliogenesis. Recent studies suggested that these factors are also involved in determining specific neuronal fates by regulating the expression of their target genes, thereby creating transcriptional codes for neuronal subtype specification. In the present study, we show that in the zebrafish the neural gene Olig2 and the transcriptional regulator Sim1 are co-expressed in a subset of diencephalic progenitors destined towards the dopaminergic (DA) neuronal fate. While sim1 mRNA is also detected in mature DA neurons, the expression of olig2 is extinguished prior to terminal DA differentiation. Loss of function of either Olig2 or Sim1 leads to impaired DA development. Finally, Olig2 regulates the expression of Sim1 and gain of function of Sim1 rescues the deficits in DA differentiation caused by targeted knockdown of Olig2. Our findings demonstrate for the first time that commitment of basal diencephalic DA neurons is regulated by the combined action of the neural protein Olig2 and its downstream neuronal specific effector Sim1.

Dopaminergic neuronal cluster size is determined during early forebrain patterning

We have explored the effects of robust neural plate patterning signals, such as canonical Wnt, on the differentiation and configuration of neuronal subtypes in the zebrafish diencephalon at single-cell resolution. Surprisingly, perturbation of Wnt signaling did not have an overall effect on the specification of diencephalic fates, but selectively affected the number of dopaminergic (DA) neurons. We identified the DA progenitor zone in the diencephalic anlage of the neural plate using a two-photon-based uncaging method and showed that the number of non-DA neurons derived from this progenitor zone is not altered by Wnt attenuation. Using birthdating analysis, we determined the timing of the last cell division of DA progenitors and revealed that the change in DA cell number following Wnt inhibition is not due to changes in cell cycle exit kinetics. Conditional inhibition of Wnt and of cell proliferation demonstrated that Wnt restricts the number of DA progenitors during a window of plasticity, which occurs at primary neurogenesis. Finally, we demonstrated that Wnt8b is a modulator of DA cell number that acts through the Fz8a (Fzd8a) receptor and its downstream effector Lef1, and which requires the activity of the Fezl (Fezf2) transcription factor for this process. Our data show that the differential response of distinct neuronal populations to the Wnt signal is not a simple interpretation of their relative anteroposterior position. This study also shows, for the first time, that diencephalic DA population size is modulated inside the neural plate much earlier than expected, concomitant with Wnt-mediated regional patterning events.

Specification of hypothalamic neurons by dual regulation of the homeodomain protein Orthopedia

In the developing hypothalamus, a variety of neurons are generated adjacent to each other in a highly coordinated, but poorly understood process. A critical question that remains unanswered is how coordinated development of multiple neuronal types is achieved in this relatively narrow anatomical region. We focus on dopaminergic (DA) and oxytocinergic (OT) neurons as a paradigm for development of two prominent hypothalamic cell types. We report that the development of DA and OT-like neurons in the zebrafish is orchestrated by two novel pathways that regulate the expression of the homeodomain-containing protein Orthopedia (Otp), a key determinant of hypothalamic neural differentiation. Genetic analysis showed that the G-protein-coupled receptor PAC1 and the zinc finger-containing transcription factor Fezl act upstream to Otp. In vivo and in vitro experiments demonstrated that Fezl and PAC1 regulate Otp at the transcriptional and the post-transcriptional levels, respectively. Our data reveal a new genetic network controlling the specification of hypothalamic neurons in vertebrates, and places Otp as a critical determinant underlying Fezl- and PAC1-mediated differentiation.

Passive amyloid immunotherapy clears amyloid and transiently activates microglia in a transgenic mouse model of amyloid deposition

The role of microglia in the removal of amyloid deposits after systemically administered anti-Abeta antibodies remains unclear. In the current study, we injected Tg2576 APP transgenic mice weekly with an anti-Abeta antibody for 1, 2, or 3 months such that all mice were 22 months at the end of the study. In mice immunized for 3 months, we found an improvement in alternation performance in the Y maze. Histologically, we were able to detect mouse IgG bound to congophilic amyloid deposits in those mice treated with the anti-Abeta antibody but not in those treated with a control antibody. We found that Fcgamma receptor expression on microglia was increased after 1 month of treatment, whereas CD45 was increased after 2 months of treatment. Associated with these microglial changes was a reduction in both diffuse and compact amyloid deposits after 2 months of treatment. Interestingly, the microglia markers were reduced to control levels after 3 months of treatment, whereas amyloid levels remained reduced. Serum Abeta levels and anti-Abeta antibody levels were elevated to similar levels at all three survival times in mice given anti-Abeta injections rather than control antibody injections. These data show that the antibody is able to enter the brain and bind to the amyloid deposits, likely opsonizing the Abeta and resulting in Fcgamma receptor-mediated phagocytosis. Together with our earlier work, our data argue that all proposed mechanisms of anti-Abeta antibody-mediated amyloid removal can be simultaneously active.

Zinc finger protein too few controls the development of monoaminergic neurons

The mechanism controlling the development of dopaminergic (DA) and serotonergic (5HT) neurons in vertebrates is not well understood. Here we characterized a zebrafish mutant--too few (tof)--that develops hindbrain 5HT and noradrenergic neurons, but does not develop hypothalamic DA and 5HT neurons. tof encodes a forebrain-specific zinc finger transcription repressor that is homologous to the mammalian Fezl (forebrain embryonic zinc finger-like protein). Mosaic and co-staining analyses showed that fezl was not expressed in DA or 5HT neurons and instead controlled development of these neurons non-cell-autonomously. Both the eh1-related repressor motif and the second zinc finger domain were necessary for tof function. Our results indicate that tof/fezl is a key component in regulating the development of monoaminergic neurons in the vertebrate brain.

c-Cbl is a suppressor of the neu oncogene

A rodent oncogenic mutant of the Neu receptor tyrosine kinase is a useful experimental model because overexpression of the respective receptor, namely HER2/ErbB-2, in human malignancies is associated with relatively aggressive diseases. Here we show that the oncogenic form of Neu is constitutively associated with the product of the c-cbl proto-oncogene and is part of a large complex that includes the phosphoinositide 3-kinase and Shc. Ectopic expression of c-Cbl, a ubiquitin-protein isopeptide ligase specific to activated tyrosine kinases, causes rapid removal of Neu from the cell surface and severely reduces signaling downstream of oncogenic Neu. c-Cbl-induced down-regulation of Neu involves covalent attachment of ubiquitin molecules and requires the carboxyl-terminal domain of Neu. The negative effect of c-Cbl is antagonized by v-Cbl, a virus-encoded oncogenic truncated form of c-Cbl. In an in vivo model, infection of a Neu-transformed neuroblastoma with a c-Cbl-encoding retrovirus caused enhanced down-regulation of Neu and correlated with tumor retardation. Our results implicate c-Cbl in negative regulation of Neu and offer a potential target for treatment of HER2/ErbB-2-positive human malignancies.

Ubiquitin ligase activity and tyrosine phosphorylation underlie suppression of growth factor signaling by c-Cbl/Sli-1

Receptor desensitization is accomplished by accelerated endocytosis and degradation of ligand-receptor complexes. An in vitro reconstituted system indicates that Cbl adaptor proteins directly control downregulation of the receptor for the epidermal growth factor (EGFR) by recruiting ubiquitin-activating and -conjugating enzymes. We infer a sequential process initiated by autophosphorylation of EGFR at a previously identified lysosome-targeting motif that subsequently recruits Cbl. This is followed by tyrosine phosphorylation of c-Cbl at a site flanking its RING finger, which enables receptor ubiquitination and degradation. Whereas all three members of the Cbl family can enhance ubiquitination, two oncogenic Cbl variants, whose RING fingers are defective and phosphorylation sites are missing, are unable to desensitize EGFR. Our study identifies Cbl proteins as components of the ubiquitin ligation machinery and implies that they similarly suppress many other signaling pathways.

The RING finger of c-Cbl mediates desensitization of the epidermal growth factor receptor

Ligand-induced activation of surface receptors, including the epidermal growth factor receptor (EGFR), is followed by a desensitization process involving endocytosis and receptor degradation. c-Cbl, a tyrosine phosphorylation substrate shared by several signaling pathways, accelerates desensitization by recruiting EGFR and increasing receptor polyubiquitination. Here we demonstrate that the RING type zinc finger of c-Cbl is essential for ubiquitination and subsequent desensitization of EGFR. Mutagenesis of a single cysteine residue impaired the ability of c-Cbl to enhance both down-regulation and ubiquitination of EGFR in living cells, although the mutant retained binding to the activated receptor. Consequently, the mutant form of c-Cbl acquired a dominant inhibitory function and lost the ability to inhibit signaling downstream to EGFR. In vitro reconstitution of EGFR ubiquitination implies that the RING finger plays an essential direct role in ubiquitin ligation. Our results attribute to the RING finger of c-Cbl a causative role in endocytic sorting of EGFR and desensitization of signal transduction.

c-Cbl/Sli-1 regulates endocytic sorting and ubiquitination of the epidermal growth factor receptor

Ligand-induced down-regulation of two growth factor receptors, EGF receptor (ErbB-1) and ErbB-3, correlates with differential ability to recruit c-Cbl, whose invertebrate orthologs are negative regulators of ErbB. We report that ligand-induced degradation of internalized ErbB-1, but not ErbB-3, is mediated by transient mobilization of a minor fraction of c-Cbl into ErbB-1-containing endosomes. This recruitment depends on the receptor's tyrosine kinase activity and an intact carboxy-terminal region. The alternative fate is recycling of internalized ErbBs to the cell surface. Cbl-mediated receptor sorting involves covalent attachment of ubiquitin molecules, and subsequent lysosomal and proteasomal degradation. The oncogenic viral form of Cbl inhibits down-regulation by shunting endocytosed receptors to the recycling pathway. These results reveal an endosomal sorting machinery capable of controlling the fate, and, hence, signaling potency, of growth factor receptors.

Pathogenic poxviruses reveal viral strategies to exploit the ErbB signaling network

Virulence of poxviruses, the causative agents of smallpox, depends on virus-encoded growth factors related to the mammalian epidermal growth factor (EGF). Here we report that the growth factors of Shope fibroma virus, Myxoma virus and vaccinia virus (SFGF, MGF and VGF) display unique patterns of specificity to ErbB receptor tyrosine kinases; whereas SFGF is a broad-specificity ligand, VGF binds primarily to ErbB-1 homodimers, and the exclusive receptor for MGF is a heterodimer comprised of ErbB-2 and ErbB-3. In spite of 10- to 1000-fold lower binding affinity to their respective receptors, the viral ligands are mitogenically equivalent or even more potent than their mammalian counterparts. This remarkable enhancement of cell growth is due to attenuation of receptor degradation and ubiquitination, which leads to sustained signal transduction. Our results imply that signal potentiation and precise targeting to specific receptor combinations contribute to cell transformation at sites of poxvirus infection, and they underscore the importance of the often ignored low-affinity ligand-receptor interactions.

Differential expression of NDF/neuregulin receptors ErbB-3 and ErbB-4 and involvement in inhibition of neuronal differentiation

Two receptor tyrosine kinases, ErB-3 and ErbB-4, mediate signaling by Neu differentiation factors (NDFs, also called neuregulins), while ErbB-1 and ErbB-2 serve as co-receptors. We show that the two NDF/neuregulin receptors differ in spatial and temporal expression patterns: The kinase-defective receptor, ErbB-3, is expressed primarily in epithelial layers of various organs, in the peripheral nervous system, and in adult brain, whereas ErbB-4 is restricted to the developing central nervous system and to the embryonic heart. An example of alternating expression of the two receptors is provided by the developing cerebellum: During postnatal cerebellar development, ErbB-4 expression slightly decreases along with a decline in NDF transcription, whereas ErbB-3 expression commences after the peak of neurogenesis. To study functional differences, we established primary brain cultures and found that ErbB-3 was expressed only in oligodendrocytes, whereas ErbB-4 expression was shared by oligodendrocytes, astrocytes and neurons. Blocking the action of endogenous NDF in vitro, by using a soluble form of ErbB-4, accelerated neurite outgrowth in both primary cultures and in neuronal-type cultures of the P19 teratocarcinoma, suggesting an inhibitory effect of NDF on neural differentiation. Apparently, ErbB-3 is associated with proliferation of P19 cells, whereas ErbB-4 correlates with a differentiated phenotype. We conclude that the two NDF receptors play distinct, rather than redundant, developmental and physiological roles.

Bivalence of EGF-like ligands drives the ErbB signaling network

Signaling by epidermal growth factor (EGF)-like ligands is mediated by an interactive network of four ErbB receptor tyrosine kinases, whose mechanism of ligand-induced dimerization is unknown. We contrasted two existing models: a conformation-driven activation of a receptor-intrinsic dimerization site and a ligand bivalence model. Analysis of a Neu differentiation factor (NDF)-induced heterodimer between ErbB-3 and ErbB-2 favors a bivalence model; the ligand simultaneously binds both ErbB-3 and ErbB-2, but, due to low-affinity of the second binding event, ligand bivalence drives dimerization only when the receptors are membrane anchored. Results obtained with a chimera and isoforms of NDF/neuregulin predict that each terminus of the ligand molecule contains a distinct binding site. The C-terminal low-affinity site has broad specificity, but it prefers interaction with ErbB-2, an oncogenic protein acting as a promiscuous low-affinity subunit of the three primary receptors. Thus, ligand bivalence enables signal diversification through selective recruitment of homo- and heterodimers of ErbB receptors, and it may explain oncogenicity of erbB-2/HER2.