Transgenic zebrafish that express tyrosine hydroxylase promoter in inner retinal cells

Trace amine signaling in zebrafish models: CNS pharmacology, behavioral regulation and translational relevance

Tyramine, β-phenylethylamine, octopamine and other trace amines are endogenous substances recently recognized as important novel neurotransmitters in the brain. Trace amines act via multiple selective trace amine-associated receptors (TAARs) of the G protein-coupled receptor family. TAARs are expressed in various brain regions and modulate neurotransmission, neuronal excitability, adult neurogenesis, cognition, mood, locomotor activity and olfaction. Disrupted trace amine circuits have been implicated in various clinical neuropsychiatric disorders, including schizophrenia, Parkinson's disease, addiction, depression and anxiety. Dysregulated TAAR signaling has been linked in rodents to altered dopamine and serotonin neurotransmission, known to be associated with these psychiatric conditions. Complementing rodent genetic and pharmacological evidence, zebrafish (Danio rerio) are rapidly becoming a novel powerful model system in translational neuropharmacology research. Here, we review trace amine/TAAR neurobiology in zebrafish and discuss their developing translational utility as pharmacological and genetic models for unraveling the role of trace amines in CNS processes and brain disorders.

Caenorhabditis elegans Models to Investigate the Mechanisms Underlying Tau Toxicity in Tauopathies

The understanding of the genetic, biochemical, and structural determinants underlying tau aggregation is pivotal in the elucidation of the pathogenic process driving tauopathies and the design of effective therapies. Relevant information on the molecular basis of human neurodegeneration in vivo can be obtained using the nematode Caenorhabditis elegans (C. elegans). To this end, two main approaches can be applied: the overexpression of genes/proteins leading to neuronal dysfunction and death, and studies in which proteins prone to misfolding are exogenously administered to induce a neurotoxic phenotype. Thanks to the easy generation of transgenic strains expressing human disease genes, C. elegans allows the identification of genes and/or proteins specifically associated with pathology and the specific disruptions of cellular processes involved in disease. Several transgenic strains expressing human wild-type or mutated tau have been developed and offer significant information concerning whether transgene expression regulates protein production and aggregation in soluble or insoluble form, onset of the disease, and the degenerative process. C. elegans is able to specifically react to the toxic assemblies of tau, thus developing a neurodegenerative phenotype that, even when exogenously administered, opens up the use of this assay to investigate in vivo the relationship between the tau sequence, its folding, and its proteotoxicity. These approaches can be employed to screen drugs and small molecules that can interact with the biogenesis and dynamics of formation of tau aggregates and to analyze their interactions with other cellular proteins.

The toxic effect of triclosan and methyl-triclosan on biological pathways revealed by metabolomics and gene expression in zebrafish embryos

The omnipresence of antimicrobial triclosan (TCS) and by-products in aquatic environments is a threat to aquatic organisms. Traditionally, the adverse effects of TCS and its by-products have been evaluated by examining the phenotypic output relevant to predicting acute toxicity rather than studying the perturbation of biological pathways. Identifying alterations in the key pathways and molecular mechanisms caused by toxic chemicals helps researchers assess the ecological risks of TCS and its by-products to aquatic environments. In this study, we used metabolomics and reverse transcription qPCR to investigate the adverse effects of a wide range of concentrations of triclosan and its derivative methyl-triclosan (MTCS), ranging from relatively low environmentally relevant levels (ng/L) to high-dose concentrations (sublethal concentration), on zebrafish (Danio rerio) embryos. The metabolism and transcriptome analysis revealed changes in the metabolite and transcripts expression of zebrafish embryos after 96 h exposure at 30 μg/L and 300 μg/L of TCS, 400 μg/L of MTCS and the TCS/MTCS mixture (30 μg/L TCS + 3 μg/L MTCS and 300 μg/L TCS + 30 μg/L MTCS). Significant dysregulations in the expression of the urea transporter (UT), glucose-6-phosphate dehydrogenase (G6PD), alanine transaminase (ALT), glutamate dehydrogenase (GDH), phosphoglucomutase (PGM), and fatty acid synthase (FASN), together with changes in alanine, urea, glucose, 6-phosphogluconalactone, and palmitic acid were observed in the TCS, MTCS, and TCS/MTCS treatments. Particularly, the MTCS treatment group showed fold changes in the mRNA expression of nitrogen metabolism, energy metabolism, and fatty acid synthesis, indicating a disruption of the zebrafish embryos' biological pathways. The changes in the metabolites and gene expressions induced by the TCS, MTCS and the TCS/MTCS mixture treatment demonstrate the pathway changes in starch and sucrose metabolism, nitrogen metabolism, fatty acid synthesis, and phenylalanine, tyrosine and tryptophan biosynthesis. Therefore, our study provides better insights into the risks of the parental compound (TCS) and its by-product (MTCS), as well as the perturbation in biological pathways induced by these two compounds in aquatic environments.

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.

Features of the structure, development, and activity of the zebrafish noradrenergic system explored in new CRISPR transgenic lines

The noradrenergic (NA) system of vertebrates is implicated in learning, memory, arousal, and neuroinflammatory responses, but is difficult to access experimentally. Small and optically transparent, larval zebrafish offer the prospect of exploration of NA structure and function in an intact animal. We made multiple transgenic zebrafish lines using the CRISPR/Cas9 system to insert fluorescent reporters upstream of slc6a2, the norepinephrine transporter gene. These lines faithfully express reporters in NA cell populations, including the locus coeruleus (LC), which contains only about 14 total neurons. We used the lines in combination with two-photon microscopy to explore the structure and projections of the NA system in the context of the columnar organization of cell types in the zebrafish hindbrain. We found robust alignment of NA projections with glutamatergic neurotransmitter stripes in some hindbrain segments, suggesting orderly relations to neuronal cell types early in life. We also quantified neurite density in the rostral spinal cord in individual larvae with as much as 100% difference in the number of LC neurons, and found no correlation between neuronal number in the LC and projection density in the rostral spinal cord. Finally, using light sheet microscopy, we performed bilateral calcium imaging of the entire LC. We found that large-amplitude calcium responses were evident in all LC neurons and showed bilateral synchrony, whereas small-amplitude events were more likely to show interhemispheric asynchrony, supporting the potential for targeted LC neuromodulation. Our observations and new transgenic lines set the stage for a deeper understanding of the NA system.

In vivo expression of Nurr1/Nr4a2a in developing retinal amacrine subtypes in zebrafish Tg(nr4a2a:eGFP) transgenics

The Nuclear receptor subfamily 4 group A member 2 (Nr4a2) is crucial for the formation or maintenance of dopaminergic neurons in the central nervous system including the retina, where dopaminergic amacrine cells contribute to visual function. Little is known about which cells express Nr4a2 at which developmental stage. Furthermore, whether Nr4a2 functions in combination with other genes is poorly understood. Thus, we generated a novel transgenic to visualize Nr4a2 expression in vivo during zebrafish retinogenesis. A 4.1 kb fragment of the nr4a2a promoter was used to drive green fluorescent protein expression in this Tg(nr4a2a:eGFP) line. In situ hybridization showed that transgene expression follows endogenous RNA expression at a cellular level. Temporal expression and lineages were quantified using in vivo time-lapse imaging in embryos. Nr4a2 expressing retinal subtypes were characterized immunohistochemically. Nr4a2a:eGFP labeled multiple neuron subtypes including 24.5% of all amacrine interneurons. Nr4a2a:eGFP labels all tyrosine hydroxylase labeled dopaminergic amacrine cells, and other nondopaminergic GABAergic amacrine populations. Nr4a2a:eGFP is confined to a specific progenitor lineage identified by sequential expression of the bhlh transcription factor Atonal7 (Atoh7) and Pancreas transcription factor 1a (Ptf1a), and labels postmitotic postmigratory amacrine cells. Thus, developmental Nr4a2a expression indicates a role during late differentiation of specific amacrine interneurons. Tg(nr4a2a:eGFP) is an early marker of distinct neurons including dopaminergic amacrine cells. It can be utilized to assess consequences of gene manipulations and understand whether Nr4a2 only carries out its role in the presence of specific coexpressed genes. This will allow Nr4a2 use to be refined for regenerative approaches.

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.

Transgenic zebrafish expressing green fluorescent protein in dopaminergic neurons of the ventral diencephalon

We have generated a line of transgenic zebrafish, Tg(dat:EGFP), in which the green fluorescent protein (GFP) is expressed under the control of cis-regulatory elements of the dopamine transporter (dat) gene. In Tg(dat:EGFP) fish, dopamine (DA) neurons are labeled with GFP, including those in ventral diencephalon (vDC) clusters, amacrine cells in the retina, in the olfactory bulb, in the pretectum, and in the caudal hypothalamus. In the vDC, DA neurons of groups 2-6 are correctly labeled with GFP, based on colocalization analyses. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) treatments induced a modest but significant loss of DA neurons in groups 2-6 of the vDC. This transgenic line will be useful for the study of DA neuron development and in models of DA neuron loss.

Two types of tyrosine hydroxylase-immunoreactive neurons in the zebrafish retina

The purpose of the present study is to identify the dopaminergic amacrine (DA) cells in the inner nuclear layer (INL) of zebrafish retina through immunocytochemistry and quantitative analysis. Two types of tyrosine hydroxylase-immunoreactive (TH-IR) cells appeared on the basis of dendritic morphology and stratification patterns in the inner plexiform layer (IPL). The first (DA1) was bistratified, with branching planes in both s1 and s5 of the IPL. The second (DA2) was diffuse, with dendritic processes branched throughout the IPL. DA1 and DA2 cells corresponded morphologically to A(on)(-s1/s5) and A(diffuse)(-1) (Connaughton et al., 2004). The average number of total TH-IR cells was 1088±79cells per retina (n=5), and the mean density was 250±27cells/mm(2). Their density was highest in the mid central region of ventrotemporal retina and lowest in the periphery of dorsonasal retina. Quantitatively, 45.71% of the TH-IR cells were DA1 cells, while 54.29% were DA2 cells. No TH-IR cells expressed calbindin D28K, calretinin or parvalbumin, markers for the various INL cells present in several animals. Therefore the TH-IR cells in zebrafish are limited to very specific subpopulations of the amacrine cells.

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

The terminalis nerve (TN) has been described in all vertebrate species, in which it plays important roles in animal behavior and physiology. In teleost fish, the TN is located in the olfactory bulb and in its nerve tract. Here, we report a study on the characterization of the TN cell development, axon projection and physiology in zebrafish (Danio rerio). We have generated several lines of transgenic zebrafish [Tg (GnRH-3::GFP)] that express GFP in the TN cells. The transgenes are expressed under the transcriptional control of the zebrafish GnRH-3 promoter. During development, the first GFP-positive TN cell was identified at approximately 34 h post-fertilization (hpf). By 38 hpf, several clusters of TN cells were identified in the olfactory bulb and olfactory nerve tract. In the olfactory bulb, the TN cells projected axons caudally. In the forebrain, some of the TN axons extended caudally, but most crossed the midline of the brain at the commissural anterior. In the midbrain, some of the TN axons extended dorsally towards the tectum, whereas other axons extended caudally, or extended ventrally to the optic nerve where they entered the neural retina. We also examined the cell membrane property of the TN cells. Using patch-clamp techniques, we recorded spontaneous and evoked action potentials from isolated TN cells. We examined the expression of glutamate receptors in the TN cells. The data shed light on the mechanisms of TN function in the nervous system and in the regulation of animal physiology.

Zebrafish models of Tauopathy

Tauopathies are a group of incurable neurodegenerative diseases, in which loss of neurons is accompanied by intracellular deposition of fibrillar material composed of hyperphosphorylated forms of the microtubule-associated protein Tau. A zebrafish model of Tauopathy could complement existing murine models by providing a platform for genetic and chemical screens, in order to identify novel therapeutic targets and compounds with disease-modifying potential. In addition, Tauopathy zebrafish would be useful for hypothesis-driven experiments, especially those exploiting the potential to deploy in vivo imaging modalities. Several considerations, including conservation of specialized neuronal and other cellular populations, and biochemical pathways implicated in disease pathogenesis, suggest that the zebrafish brain is an appropriate setting in which to model these complex disorders. Novel transgenic zebrafish lines expressing wild-type and mutant forms of human Tau in CNS neurons have recently been reported. These studies show evidence that human Tau undergoes disease-relevant changes in zebrafish neurons, including somato-dendritic relocalization, hyperphosphorylation and aggregation. In addition, preliminary evidence suggests that Tau transgene expression can precipitate neuronal dysfunction and death. These initial studies are encouraging that the zebrafish holds considerable promise as a model in which to study Tauopathies. Further studies are necessary to clarify the phenotypes of transgenic lines and to develop assays and models suitable for unbiased high-throughput screening approaches. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Genetic zebrafish models of neurodegenerative diseases

As a consequence of the widespread use of zebrafish in developmental biology studies, an extensive array of experimental tools and techniques has been assembled; it has recently become apparent that these might be exploited in the analysis of human neurodegenerative diseases. A surprising degree of functional conservation has been demonstrated between human genes implicated in neurodegenerative diseases and their zebrafish orthologues. In zebrafish models of recessive parkinsonism, Parkin or Pink1 knockdown gave rise to specific loss of dopamine neurons; in a zebrafish model of recessive spinal muscular atrophy, loss of Smn1 function caused specific motor axonal defects. In addition, pathological features of several dominant diseases were replicated by transgenic over-expression of mutant human proteins, including Tau, Huntingtin, and SOD1. In some cases, conservation of relevant cellular pathways was sufficient that disease-specific posttranslational changes to the respective proteins were found in the zebrafish models. These data collectively suggest that the zebrafish can be an appropriate setting in which to model the molecular events underlying human neuropsychiatric disease. Consequently, novel findings yielded by studies in zebrafish models may be applicable to human diseases; this is an exciting prospect, in view of the many potential uses of zebrafish models, for example, screening for lead therapeutic compounds, rapid functional assessments of putative modifier genes, and live observation of pathogenic mechanisms in vivo.

Transgenic zebrafish models of neurodegenerative diseases

Since the introduction of the zebrafish as a model for the study of vertebrate developmental biology, an extensive array of techniques for its experimental manipulation and analysis has been developed. Recently it has become apparent that these powerful methodologies might be deployed in order to elucidate the pathogenesis of human neurodegenerative diseases and to identify candidate therapeutic approaches. In this article, we consider evidence that the zebrafish central nervous system provides an appropriate setting in which to model human neurological disease and we review techniques and resources available for generating transgenic models. We then examine recent publications showing that appropriate phenotypes can be provoked in the zebrafish through transgenic manipulations analogous to genetic abnormalities known to cause human tauopathies, polyglutamine diseases or motor neuron degenerations. These studies show proof of concept that findings in zebrafish models can be applicable to the pathogenic mechanisms underlying human diseases. Consequently, the prospects for providing novel insights into neurodegenerative diseases by exploiting transgenic zebrafish models and discovery-driven approaches seem favorable.

Cis-acting elements responsible for dopaminergic neuron-specific expression of zebrafish slc6a3 (dopamine transporter) in vivo are located remote from the transcriptional start site

The purpose of this study was to analyze the transcriptional regulation of the zebrafish solute carrier family 6 member 3 gene (slc6a3, dopamine transporter, dat), as a first step towards isolating regulatory sequences useful for driving transgene expression within dopaminergic neurons of the zebrafish CNS in vivo. We found that the 3.0 kb slc6a3 mRNA is expressed in each of the major groups of dopaminergic neurons previously identified in the zebrafish CNS. The slc6a3 gene spans >20 kb of genomic DNA and contains 15 exons. The genomic organization of slc6a3 is highly conserved with respect to its human orthologue, including the presence of an untranslated first exon. The promoter lacks a canonical TATA box and there are multiple transcriptional start sites. Functional analysis of cis-acting elements responsible for the expression pattern of slc6a3 was carried out by generating stable transgenic zebrafish lines expressing fluorescent reporters under transcriptional control of fragments of slc6a3 genomic sequence. The region between -2 kb and +5 kb with respect to the transcriptional start site contains the core slc6a3 promoter, in addition to neuronal enhancers and/or non-neuronal repressors that restrict expression to the CNS, but this region lacks cis-acting elements responsible for slc6a3 expression in dopaminergic neurons. The upstream sequence between -6 kb and -2 kb contains an enhancer element that drives slc6a3 expression in dopaminergic neurons of the pretectal region, and additional sequences that partially repress expression in non-dopaminergic neurons. However, expression of slc6a3 in dopaminergic neurons of the ventral diencephalon and telencephalon is dependent on elements that lie outside the region -6 kb to +5 kb. These data provide a detailed analysis of the slc6a3 gene and show that its expression in different populations of dopamine neurons is driven by discrete enhancers, rather than a single target sequence for a terminal factor involved in specifying neurochemical phenotype.

Cloning and spatial and temporal expression of the zebrafish dopamine D1 receptor

Dopamine plays important roles in the regulation of central nervous system (CNS) development and functions. In vertebrates, two families of dopamine receptors, collectively known as dopamine D1 and D2 receptors, have been identified. Recently, dopamine receptors have been targeted by pharmacological and therapeutic studies of neurological disorders, such as Parkinson's disease. Here, we report a study on the molecular characterization of dopamine D1 receptor in zebrafish (Danio rerio). We cloned the full-length cDNA of a zebrafish dopamine D1 receptor, designated as drd1. The sequence of drd1 shares high homology to the sequences of dopamine D1 receptors in mammalian, amphibian, and other fish species. drd1 is expressed in the CNS. The first drd1 expression was observed at approximately 30 hours postfertilization, at which time the expression was seen in the developing diencephalon and hindbrain. In developing retinas, the expression of drd1 was detected in the inner nuclear layer with the exception of the marginal zones. In adult retinas, drd1 expression was detected in most cell types in the inner and outer nuclear layers as well as ganglion cell layer. Differential expression of drd1 in developing and adult retinas may play various roles in regulating visual system functions.

Differential regulation of the zebrafish orthopedia 1 gene during fate determination of diencephalic neurons

BACKGROUND

The homeodomain transcription factor Orthopedia (Otp) is essential in restricting the fate of multiple classes of secreting neurons in the neuroendocrine hypothalamus of vertebrates. However, there is little information on the intercellular factors that regulate Otp expression during development.

RESULTS

Here, we identified two otp orthologues in zebrafish (otp1 and otp2) and explored otp1 in the context of the morphogenetic pathways that specify neuroectodermal regions. During forebrain development, otp1 is expressed in anterior groups of diencephalic cells, positioned in the preoptic area (PO) (anterior alar plate) and the posterior tuberculum (PT) (posterior basal plate). The latter structure is characterized by Tyrosine Hydroxylase (TH)-positive cells, suggesting a role for otp1 in the lineage restriction of catecholaminergic (CA) neurons. Disruptions of Hedgehog (HH) and Fibroblast Growth Factor (FGF) pathways point to the ability of SHH protein to trigger otp1 expression in PO presumptive neuroblasts, with the attenuating effect of Dzip1 and FGF8. In addition, our data disclose otp1 as a determinant of CA neurons in the PT, where otp1 activity is strictly dependent on Nodal signaling and it is not responsive to SHH and FGF.

CONCLUSION

In this study, we pinpoint the evolutionary importance of otp1 transcription factor in cell states of the diencephalon anlage and early neuronal progenitors. Furthermore, our data indicate that morphogenetic mechanisms differentially regulate otp1 expression in alar and basal plates.