Characterization of GFP-tagged GnRH-containing terminalis neurons in transgenic zebrafish

Application of Transgenic Zebrafish Models for Studying the Effects of Estrogenic Endocrine Disrupting Chemicals on Embryonic Brain Development

Endocrine disrupting chemicals (EDCs) are environmental pollutants that mimic hormones and/or disrupt their function. Estrogenic EDCs (eEDCs) interfere with endogenous estrogen signalling pathway(s) and laboratory animal and human epidemiological studies have provided evidence for a causal link between exposure to them during embryonic/early life and neurological impairments. However, our understanding of the molecular and cellular mechanism(s) underlying eEDCs exposure effects on brain development, tissue architecture and function and behaviour are limited. Transgenic (TG) zebrafish models offer new approach methodologies (NAMs) to help identify the modes of action (MoAs) of EDCs and their associated impacts on tissue development and function. Estrogen biosensor TG zebrafish models have been applied to study eEDC interactions and resulting transcriptional activation (via a fluorescent reporter expression) across the entire body of the developing zebrafish embryo, including in real time. These estrogen biosensor TG zebrafish models are starting to deepen our understanding of the spatiotemporal actions of eEDCs and their resulting impacts on neurological development, brain function and behaviour. In this review, we first investigate the links between early life exposure to eEDCs and neurodevelopmental alterations in model organisms (rodents and zebrafish) and humans. We then present examples of the application of estrogen biosensor and other TG zebrafish models for elucidating the mechanism(s) underlying neurodevelopmental toxicities of eEDCs. In particular we illustrate the utility of combining estrogen biosensor zebrafish models with other TG zebrafish models for understanding the effects of eEDCs on the brain, spanning cellular processes, brain circuitry, neurophysiology and behaviour. Finally, we discuss the future prospects of TG zebrafish models as experimental models for studying more complex scenarios for exposure to contaminant mixtures on neurological development and function.

Heterogeneity in GnRH and kisspeptin neurons and their significance in vertebrate reproductive biology

Vertebrate reproduction is essentially controlled by the hypothalamus-pituitary-gonadal (HPG) axis, which is a central dogma of reproductive biology. Two major hypothalamic neuroendocrine cell groups containing gonadotropin-releasing hormone (GnRH) and kisspeptin are crucial for control of the HPG axis in vertebrates. GnRH and kisspeptin neurons exhibit high levels of heterogeneity including their cellular morphology, biochemistry, neurophysiology and functions. However, the molecular foundation underlying heterogeneities in GnRH and kisspeptin neurons remains unknown. More importantly, the biological and physiological significance of their heterogeneity in reproductive biology is poorly understood. In this review, we first describe the recent advances in the neuroendocrine functions of kisspeptin-GnRH pathways. We then view the recent emerging progress in the heterogeneity of GnRH and kisspeptin neurons using morphological and single-cell transcriptomic analyses. Finally, we discuss our views on the significance of functional heterogeneity of reproductive endocrine cells and their potential relevance to reproductive health.

Circadian Vision in Zebrafish: From Molecule to Cell and from Neural Network to Behavior

Most visual system functions, such as opsin gene expression, retinal neural transmission, light perception, and visual sensitivity, display robust day-night rhythms. The rhythms persist in constant lighting conditions, suggesting the involvement of endogenous circadian clocks. While the circadian pacemakers that control the rhythms of animal behaviors are mostly found in the forebrain and midbrain, self-sustained circadian oscillators are also present in the neural retina, where they play important roles in the regulation of circadian vision. This review highlights some of the correlative studies of the circadian control of visual system functions in zebrafish. Because zebrafish maintain a high evolutionary proximity to mammals, the findings from zebrafish research may provide insights for a better understanding of the mechanisms of circadian vision in other vertebrate species including humans.

Sensory Integration: Cross-Modal Communication Between the Olfactory and Visual Systems in Zebrafish

Cross-modal sensory communication is an innate biological process that refers to the combination and/or interpretation of different types of sensory input in the brain. Often, this process conjugates with neural modulation, by which the neural signals that convey sensory information are adjusted, such as intensity, frequency, complexity, and/or novelty. Although the anatomic pathways involved in cross-modal sensory integration have been previously described, the course of development and the physiological roles of multisensory signaling integration in brain functions remain to be elucidated. In this article, I review some of the recent findings in sensory integration from research using the zebrafish models. In zebrafish, cross-modal sensory integration occurs between the olfactory and visual systems. It is mediated by the olfacto-retinal centrifugal (ORC) pathway, which originates from the terminalis nerve (TN) in the olfactory bulb and terminates in the neural retina. In the retina, the TNs synapse with the inner nuclear layer dopaminergic interplexiform cells (DA-IPCs). Through the ORC pathway, stimulation of the olfactory neurons alters the cellular activity of TNs and DA-IPCs, which in turn modulates retinal neural function and increases behavioral visual sensitivity.

GnRH-mediated olfactory and visual inputs promote mating-like behaviors in male zebrafish

The engagement of sexual behaviors is regulated by a number of factors which include gene expression, hormone circulation, and multi-sensory information integration. In zebrafish, when a male and a female are placed in the same container, they show mating-like behaviors regardless of whether they are kept together or separated by a net. No mating-like behaviors are observed when same-sex animals are put together. Through the olfacto-visual centrifugal pathway, activation of the terminalis nerve in the olfactory bulb increases GnRH signaling in the brain and triggers mating-like behaviors between males. In zebrafish mutants or wild-type fish in which the olfacto-visual centrifugal pathway is impaired or chemically ablated, in response to odor stimulation the mating-like behaviors between males are no longer evident. Together, the data suggest that the combination of olfactory and visual signals alter male zebrafish's mating-like behaviors via GnRH signaling.

Zebrafish Olfacto-Retinal Centrifugal Axon Projection and Distribution: Effects of Gonadotropin-Releasing Hormone and Dopaminergic Signaling

The terminalis neurons (TNs) have been described in teleost species. In zebrafish, the TNs are located in the olfactory bulb. The TNs synthesize and release gonadotropin-releasing hormone (GnRH) as one of the major neurotransmitters. The TNs project axons to many brain areas, which include the neural retina. In the retina, the TN axons synapse with dopaminergic interplexiform cells (DA-IPCs) and retinal ganglion cells (RGCs). In this research, we examine the role of GnRH and dopaminergic signaling in TN axon projection to the retina using the transgenic zebrafish Tg(GnRH-3::GFP). While the TNs developed at 34 h postfertilization (hpf), the first TN axons were not detected in the retina until 48-50 hpf, when the first DA-IPCs were differentiated. In developing embryos, inhibition of retinal GnRH signaling pathways severely interrupted the projection of TN axons to the retina. However, inhibition of retinal dopaminergic signaling produced little effect on TN axon projection. In adult retinas, inactivation of GnRH receptors disrupted the patterns of TN axon distribution, and depletion of DA-IPCs abolished the TN axons. When DA-IPCs regenerated, the TN axons reappeared. Together, the data suggest that in developing zebrafish retinas GnRH signaling is required for TN axon projection, whereas in adult retinas activation of GnRH and dopaminergic signaling transduction is required for normal distribution of the TN axons.

The Dlx5 and Foxg1 transcription factors, linked via miRNA-9 and -200, are required for the development of the olfactory and GnRH system

During neuronal development and maturation, microRNAs (miRs) play diverse functions ranging from early patterning, proliferation and commitment to differentiation, survival, homeostasis, activity and plasticity of more mature and adult neurons. The role of miRs in the differentiation of olfactory receptor neurons (ORNs) is emerging from the conditional inactivation of Dicer in immature ORN, and the depletion of all mature miRs in this system. Here, we identify specific miRs involved in olfactory development, by focusing on mice null for Dlx5, a homeogene essential for both ORN differentiation and axon guidance and connectivity. Analysis of miR expression in Dlx5^−/− olfactory epithelium pointed to reduced levels of miR-9, miR-376a and four miRs of the -200 class in the absence of Dlx5. To functionally examine the role of these miRs, we depleted miR-9 and miR-200 class in reporter zebrafish embryos and observed delayed ORN differentiation, altered axonal trajectory/targeting, and altered genesis and position of olfactory-associated GnRH neurons, i.e. a phenotype known as Kallmann syndrome in humans. miR-9 and miR-200-class negatively control Foxg1 mRNA, a fork-head transcription factor essential for development of the olfactory epithelium and of the forebrain, known to maintain progenitors in a stem state. Increased levels of z-foxg1 mRNA resulted in delayed ORN differentiation and altered axon trajectory, in zebrafish embryos. This work describes for the first time the role of specific miR (-9 and -200) in olfactory/GnRH development, and uncovers a Dlx5–Foxg1 regulation whose alteration affects receptor neuron differentiation, axonal targeting, GnRH neuron development, the hallmarks of the Kallmann syndrome.

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Dlx5 controls the expressions of miR9 and miR-200, which target the Foxg1 mRNA

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miR-9 and -200 are needed for olfactory neurons differentiation and axon extension

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miR-9 and -200 are required for the genesis and position of GnRH neurons.

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Altered expression of miR-9 and -200 might contribute to the Kallmann disease.

Olfactory stimulation selectively modulates the OFF pathway in the retina of zebrafish

Cross-modal regulation of visual performance by olfactory stimuli begins in the retina, where dopaminergic interneurons receive projections from the olfactory bulb. However, we do not understand how olfactory stimuli alter the processing of visual signals within the retina. We investigated this question by in vivo imaging activity in transgenic zebrafish expressing SyGCaMP2 in bipolar cell terminals and GCaMP3.5 in ganglion cells. The food-related amino acid methionine reduced the gain and increased sensitivity of responses to luminance and contrast transmitted through OFF bipolar cells but not ON. The effects of olfactory stimulus were blocked by inhibiting dopamine uptake and release. Activation of dopamine receptors increased the gain of synaptic transmission in vivo and potentiated synaptic calcium currents in isolated bipolar cells. These results indicate that olfactory stimuli alter the sensitivity of the retina through the dopaminergic regulation of presynaptic calcium channels that control the gain of synaptic transmission through OFF bipolar cells.

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Olfactory stimuli regulate transmission of signals through retinal bipolar cells

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Modulation of synaptic gain and sensitivity occur in OFF bipolar cells but not ON

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An inhibitor of dopamine uptake blocks odor-induced changes in synaptic gain

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Dopamine potentiates presynaptic calcium channels in isolated bipolar cells

Early development of the gonadotropin-releasing hormone neuronal network in transgenic zebrafish

Understanding development of gonadotropin-releasing hormone (GnRH) neuronal circuits is fundamental to our understanding of reproduction, but not yet well understood. Most studies have been focused on GnRH neurons located in the hypothalamus and preoptic area (POA), which directly regulate the pituitary-gonadal axis. In zebrafish (Danio rerio), two forms of GnRH have been identified: GnRH2 and GnRH3. GnRH3 neurons in this species plays two roles: hypophysiotropic and neuromodulatory, depending on their location. GnRH3 neurons in the ventral telencephalon, POA, and hypothalamus control pituitary-gonadal function; in other areas (e.g., terminal nerve), they are neuromodulatory and without direct action on reproduction. To investigate the biology of GnRH neurons, a stable line of transgenic zebrafish was generated in which the GnRH3 promoter drives expression of a bright variant of green fluorescent protein (Emerald GFP, or EMD). This provides unprecedented sensitivity in detecting and imaging GnRH3 neurons during early embryogenesis in the transparent embryo. Using timelapse confocal imaging to monitor the time course of GnRH3:EMD expression in the live embryo, we describe the emergence and development of GnRH3 neurons in the olfactory region, hypothalamus, POA, and trigeminal ganglion. By 50 h post fertilization, these diverse groups of GnRH3 neurons project broadly in the central and peripheral nervous systems and make anatomical connections with each other. Immunohistochemistry of synaptic vesicle protein 2 (a marker of synaptic transmission) in this transgenic model suggests synaptic formation is occurring during early development of the GnRH3 neural network. Electrophysiology reveals early emergence of responsiveness to the stimulatory effects of kisspeptin in terminal nerve GnRH3 neurons. Overall, our findings reveal that the GnRH3 neuronal system is comprised of multiple populations of neurons as a complicated network.

Profiling, Bioinformatic, and Functional Data on the Developing Olfactory/GnRH System Reveal Cellular and Molecular Pathways Essential for This Process and Potentially Relevant for the Kallmann Syndrome

During embryonic development, immature neurons in the olfactory epithelium (OE) extend axons through the nasal mesenchyme, to contact projection neurons in the olfactory bulb. Axon navigation is accompanied by migration of the GnRH+ neurons, which enter the anterior forebrain and home in the septo-hypothalamic area. This process can be interrupted at various points and lead to the onset of the Kallmann syndrome (KS), a disorder characterized by anosmia and central hypogonadotropic hypogonadism. Several genes has been identified in human and mice that cause KS or a KS-like phenotype. In mice a set of transcription factors appears to be required for olfactory connectivity and GnRH neuron migration; thus we explored the transcriptional network underlying this developmental process by profiling the OE and the adjacent mesenchyme at three embryonic ages. We also profiled the OE from embryos null for Dlx5, a homeogene that causes a KS-like phenotype when deleted. We identified 20 interesting genes belonging to the following categories: (1) transmembrane adhesion/receptor, (2) axon-glia interaction, (3) scaffold/adapter for signaling, (4) synaptic proteins. We tested some of them in zebrafish embryos: the depletion of five (of six) Dlx5 targets affected axonal extension and targeting, while three (of three) affected GnRH neuron position and neurite organization. Thus, we confirmed the importance of cell-cell and cell-matrix interactions and identified new molecules needed for olfactory connection and GnRH neuron migration. Using available and newly generated data, we predicted/prioritized putative KS-disease genes, by building conserved co-expression networks with all known disease genes in human and mouse. The results show the overall validity of approaches based on high-throughput data and predictive bioinformatics to identify genes potentially relevant for the molecular pathogenesis of KS. A number of candidate will be discussed, that should be tested in future mutation screens.

Functional expression of SCL/TAL1 interrupting locus (Stil) protects retinal dopaminergic cells from neurotoxin-induced degeneration

We previously isolated a dominant mutation, night blindness b (nbb), which causes a late onset of retinal dopaminergic cell degeneration in zebrafish. In this study, we cloned the zebrafish nbb locus. Sequencing results revealed that nbb is a homolog of the vertebrate SCL/TAL1 interrupting locus (Stil). The Stil gene has been shown to play important roles in the regulation of vertebrate embryonic neural development and human cancer cell proliferation. In this study, we demonstrate that functional expression of Stil is also required for neural survival. In zebrafish, decreased expression of Stil resulted in increased toxic susceptibility of retinal dopaminergic cells to 6-hydroxydopamine. Increases in Stil-mediated Shh signaling transduction (i.e. by knocking down the Shh repressor Sufu) prevented dopaminergic cell death induced by neurotoxic insult. The data suggest that the oncogene Stil also plays important roles in neural protection.

Connections of the terminal nerve and the olfactory system in two galeomorph sharks: an experimental study using a carbocyanine dye

In elasmobranchs the terminal nerve courses separately from the olfactory nerve. This characteristic makes elasmobranchs excellent models to study the anatomy and function of these two systems. Here we study the neural connections of the terminal nerve and olfactory system in two sharks by experimental tracing methods using carbocyanine dyes. The main projections from the terminal nerve system (consisting of three ganglia in Scyliorhinus canicula) course ipsilaterally to the medial septal nucleus and bilaterally to the ventromedial telencephalic pallial region. Minor terminal nerve projections were also traced ipsilaterally to diencephalic and mesencephalic levels. With regard to the olfactory connections, our results show that in sharks, unlike ray-finned fishes, the primary olfactory projections are mainly restricted to the olfactory bulb. We also performed tracer application to the olfactory bulb in order to analyze the possible central neuroanatomical relationship between the projections of the terminal nerve and the olfactory bulb. In these experiments labeled neurons and fibers were observed from telencephalic to caudal mesencephalic regions. However, we observe almost no overlap between the two systems at central levels. The afferent and the putatively efferent connections of the dogfish olfactory bulb are compared with those previously reported in other elasmobranchs. The significance of the extratelencephalic secondary olfactory projections is also discussed in a comparative context.

Characterization of voltage-activated ionic currents in the GnRH-containing terminalis nerve in transgenic zebrafish

The terminalis nerve (TN) is in a class of cranial nerves that plays important roles in animal development, physiology and behavior. Here, we report a study on the characterization of voltage-activated ionic currents in GnRH-containing TN cells in zebrafish. The experiments were performed using acutely dissociated TN cells from the transgenic zebrafish Tg (GnRH-3::GFP). In the transgenic zebrafish, the TN cells express GFP under the transcriptional control of the zebrafish GnRH-3 promoter. In all of the GnRH-containing TN cells examined, we recorded both low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium current (I(Ca)). The characteristics of the I(Ca) were similar to those described in other zebrafish cell types. However, the distribution patterns of the currents in the GnRH-containing TN cells were different in comparison to the distribution of the currents in other cell types. In addition, we characterized TTX-sensitive sodium current (I(Na)) and 4AP-sensitive and TEA-resistant potassium current (I(K)). The characteristics of voltage-activated I(Na) and I(K) in the GnRH-containing TN cells were similar to those described in other zebrafish cell types. Together, the data from this study revealed the electrophysiological properties of the GnRH-containing TN cells, thereby providing insight on the regulatory mechanisms of TN-signaling in animal physiology.