Identification of a dopaminergic enhancer indicates complexity in vertebrate dopamine neuron phenotype specification

CRISPR/Cas9-based QF2 knock-in at the tyrosine hydroxylase (th) locus reveals novel th-expressing neuron populations in the zebrafish mid- and hindbrain

Catecholaminergic neuron clusters are among the most conserved neuromodulatory systems in vertebrates, yet some clusters show significant evolutionary dynamics. Because of their disease relevance, special attention has been paid to mammalian midbrain dopaminergic systems, which have important functions in motor control, reward, motivation, and cognitive function. In contrast, midbrain dopaminergic neurons in teleosts were thought to be lost secondarily. Here, we generated a CRISPR/Cas9-based knock-in transgene at the th locus, which allows the expression of the Q-system transcription factor QF2 linked to the Tyrosine hydroxylase open reading frame by an E2A peptide. The QF2 knock-in allele still expresses Tyrosine hydroxylase in catecholaminergic neurons. Coexpression analysis of QF2 driven expression of QUAS fluorescent reporter transgenes and of th mRNA and Th protein revealed that essentially all reporter expressing cells also express Th/th. We also observed a small group of previously unidentified cells expressing the reporter gene in the midbrain and a larger group close to the midbrain-hindbrain boundary. However, we detected no expression of the catecholaminergic markers ddc, slc6a3, or dbh in these neurons, suggesting that they are not actively transmitting catecholamines. The identified neurons in the midbrain are located in a GABAergic territory. A coexpression analysis with anatomical markers revealed that Th-expressing neurons in the midbrain are located in the tegmentum and those close to the midbrain-hindbrain boundary are located in the hindbrain. Our data suggest that zebrafish may still have some evolutionary remnants of midbrain dopaminergic neurons.

Transcriptomic networks of gba3 governing specification of the dopaminergic neurons in zebrafish embryos

BACKGROUND

Molecular networks associated with dopaminergic (DA) neurogenesis remain undefined within mammalian models. To address this issue, the transient zebrafish model lmx1al: EGFP was generated, which expresses GFP in the DA precursor cells as well as in the DA neurons of the ventral diencephalon (VD). We found that the novel pseudogene gba3 has not been well studied in zebrafish neurogenesis.

OBJECTIVE

Crucial networks associated with gba3 transcripts were investigated because the biological functions of these networks have not been reported in DA neurogenesis in zebrafish.

METHODS

RNA isolation and sequencing were employed with GFP-expressing cells from 20-, 22-, and 24 h post-fertilization (hpf), while subsequent transcriptomic analysis generated differentially expressed genes with DA neurogenesis (DEG-DA) list. Hierarchical cluster analysis provided absolute guidance for the collection of gba3, an essential transcript that is strictly spatiotemporally expressed during DA neurogenesis, which was proven with whole-mount in situ hybridization (WISH) and knockdown and rescue of the gba3 transcripts in zebrafish embryos.

RESULTS

The gba3 transcripts were restricted to the midbrain at 24 hpf and the midbrain and pectoral fins at 30 hpf in zebrafish embryos. Functional studies with knockdown of gba3 found a diminished state in the midbrain and midbrain-hindbrain boundary (MHB) and an underdeveloped condition in the anteroposterior and dorsolateral axis relative to the wild type (WT) at 24 hpf. However, it was recovered after forced expression of gba3 transcripts at 24 hpf. Molecular markers for the DA precursors and mature neurons lmx1al, nurr1, th, and pitx3 were analyzed in the gba3 MOs. The levels of transcripts lmx1al, nurr1, and th were significantly reduced in the midbrain ventral diencephalon (VD) and hindbrain of gba3 morphants compared to the WT at 24 hpf, while expression patterns of pitx3 transcripts showed no differences in the identical regions between gba3 MOs and the controls.

CONCLUSIONS

We discuss transcriptional networks in which transcripts of gba3 plausibly govern the specification of dopaminergic neurogenesis in zebrafish embryos.

Brain-wide perception of the emotional valence of light is regulated by distinct hypothalamic neurons

Salient sensory stimuli are perceived by the brain, which guides both the timing and outcome of behaviors in a context-dependent manner. Light is such a stimulus, which is used in treating mood disorders often associated with a dysregulated hypothalamic-pituitary-adrenal stress axis. Relationships between the emotional valence of light and the hypothalamus, and how they interact to exert brain-wide impacts remain unclear. Employing larval zebrafish with analogous hypothalamic systems to mammals, we show in free-swimming animals that hypothalamic corticotropin releasing factor (CRFHy) neurons promote dark avoidance, and such role is not shared by other hypothalamic peptidergic neurons. Single-neuron projection analyses uncover processes extended by individual CRFHy neurons to multiple targets including sensorimotor and decision-making areas. In vivo calcium imaging uncovers a complex and heterogeneous response of individual CRFHy neurons to the light or dark stimulus, with a reduced overall sum of CRF neuronal activity in the presence of light. Brain-wide calcium imaging under alternating light/dark stimuli further identifies distinct and distributed photic response neuronal types. CRFHy neuronal ablation increases an overall representation of light in the brain and broadly enhances the functional connectivity associated with an exploratory brain state. These findings delineate brain-wide photic perception, uncover a previously unknown role of CRFHy neurons in regulating the perception and emotional valence of light, and suggest that light therapy may alleviate mood disorders through reducing an overall sum of CRF neuronal activity.

α-Synuclein amplifies cytoplasmic peroxide flux and oxidative stress provoked by mitochondrial inhibitors in CNS dopaminergic neurons in vivo

Convergent evidence implicates impaired mitochondrial function and α-Synuclein accumulation as critical upstream events in the pathogenesis of Parkinson's disease, but comparatively little is known about how these factors interact to provoke neurodegeneration. We previously showed that α-Synuclein knockdown protected rat substantia nigra dopaminergic neurons from systemic exposure to the mitochondrial complex I inhibitor rotenone. Here we show that motor abnormalities prior to neuronal loss in this model are associated with extensive α-Synuclein-dependent cellular thiol oxidation. In order to elucidate the underlying events in vivo we constructed novel transgenic zebrafish that co-express, in dopaminergic neurons: (i) human α-Synuclein at levels insufficient to provoke neurodegeneration or neurobehavioral abnormalities; and (ii) genetically-encoded ratiometric fluorescent biosensors to detect cytoplasmic peroxide flux and glutathione oxidation. Live intravital imaging of the intact zebrafish CNS at cellular resolution showed unequivocally that α-Synuclein amplified dynamic cytoplasmic peroxide flux in dopaminergic neurons following exposure to the mitochondrial complex I inhibitors MPP⁺ or rotenone. This effect was robust and clearly evident following either acute or prolonged exposure to each inhibitor. In addition, disturbance of the resting glutathione redox potential following exogenous hydrogen peroxide challenge was augmented by α-Synuclein. Together these data show that α-Synuclein is a critical determinant of the redox consequences of mitochondrial dysfunction in dopaminergic neurons. The findings are important because the mechanisms underlying α-Synuclein-dependent reactive oxygen species fluxes and antioxidant suppression might provide a pharmacological target in Parkinson's disease to prevent progression from mitochondrial dysfunction and oxidative stress to cell death.

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Extensive neuronal thiol oxidation in a rat PD model is α-Synuclein-dependent.

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Peroxide flux and glutathione oxidation can be imaged in live transgenic zebrafish.

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α-Synuclein amplifies cytosolic peroxide flux in dopaminergic neurons.

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α-Synuclein exacerbates dynamic disturbances of the glutathione redox potential.

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The underlying molecular mechanisms may provide therapeutic targets in PD.

Multiple convergent hypothalamus-brainstem circuits drive defensive behavior

The hypothalamus is composed of many neuropeptidergic cell populations and directs multiple survival behaviors, including defensive responses to threats. However, the relationship between the peptidergic identity of neurons and their roles in behavior remains unclear. Here, we address this issue by studying the function of multiple neuronal populations in the zebrafish hypothalamus during defensive responses to a variety of homeostatic threats. Cellular registration of large-scale neural activity imaging to multiplexed in situ gene expression revealed that neuronal populations encoding behavioral features encompass multiple overlapping sets of neuropeptidergic cell classes. Manipulations of different cell populations showed that multiple sets of peptidergic neurons play similar behavioral roles in this fast-timescale behavior through glutamate co-release and convergent output to spinal-projecting premotor neurons in the brainstem. Our findings demonstrate that homeostatic threats recruit neurons across multiple hypothalamic cell populations, which cooperatively drive robust defensive behaviors.

gdnf affects early diencephalic dopaminergic neuron development through regulation of differentiation-associated transcription factors in zebrafish

Glial cell line-derived neurotrophic factor (GDNF) has been reported to enhance dopaminergic neuron survival and differentiation in vitro and in vivo, although those results are still being debated. Glial cell line-derived neurotrophic factor (gdnf) is highly conserved in zebrafish and plays a role in enteric nervous system function. However, little is known about gdnf function in the teleost brain. Here, we employed clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 to impede gdnf function in the maintenance of dopaminergic neuron development. Genotyping of gdnf crispants revealed successful deletions of the coding region with various mutant band sizes and down-regulation of gdnf transcripts at 1, 3 and 7 day(s) post fertilization. Notably, ~20% reduction in ventral diencephalic dopaminergic neuron numbers in clusters 8 and 13 was observed in the gdnf-deficient crispants. In addition, gdnf depletion caused a modest reduction in dopaminergic neurogenesis as determined by 5-ethynyl-2'-deoxyuridine pulse chase assay. These deleterious effects could be partly attributed to deregulation of dopaminergic neuron fate specification-related transcription factors (otp,lmx1b,shha,and ngn1) in both crispants and established homozygous mutants with whole mount in-situ hybridization (WISH) on gdnf mutants showing reduced otpb and lmx1b.1 expression in the ventral diencephalon. Interestingly, locomotor function of crispants was only impacted at 7 dpf, but not earlier. Lastly, as expected, gdnf deficiency heightened crispants vulnerability to 1-methyl-4-phenylpyridinium toxic insult. Our results suggest conservation of teleost gdnf brain function with mammals and revealed the interactions between gdnf and transcription factors in dopaminergic neuron differentiation.

Chemoptogenetic ablation of neuronal mitochondria in vivo with spatiotemporal precision and controllable severity

Mitochondrial dysfunction is implicated in the pathogenesis of multiple neurological diseases, but elucidation of underlying mechanisms is limited experimentally by the inability to damage specific mitochondria in defined neuronal groups. We developed a precision chemoptogenetic approach to target neuronal mitochondria in the intact nervous system in vivo. MG2I, a chemical fluorogen, produces singlet oxygen when bound to the fluorogen-activating protein dL5** and exposed to far-red light. Transgenic zebrafish expressing dL5** within neuronal mitochondria showed dramatic MG2I- and light-dependent neurobehavioral deficits, caused by neuronal bioenergetic crisis and acute neuronal depolarization. These abnormalities resulted from loss of neuronal respiration, associated with mitochondrial fragmentation, swelling and elimination of cristae. Remaining cellular ultrastructure was preserved initially, but cellular pathology downstream of mitochondrial damage eventually culminated in neuronal death. Our work provides powerful new chemoptogenetic tools for investigating mitochondrial homeostasis and pathophysiology and shows a direct relationship between mitochondrial function, neuronal biogenetics and whole-animal behavior.

Molecular and cellular determinants of motor asymmetry in zebrafish

Asymmetries in motor behavior, such as human hand preference, are observed throughout bilateria. However, neural substrates and developmental signaling pathways that impose underlying functional lateralization on a broadly symmetric nervous system are unknown. Here we report that in the absence of over-riding visual information, zebrafish larvae show intrinsic lateralized motor behavior that is mediated by a cluster of 60 posterior tuberculum (PT) neurons in the forebrain. PT neurons impose motor bias via a projection through the habenular commissure. Acquisition of left/right identity is disrupted by heterozygous mutations in mosaic eyes and mindbomb, genes that regulate Notch signaling. These results define the neuronal substrate for motor asymmetry in a vertebrate and support the idea that haploinsufficiency for genes in a core developmental pathway destabilizes left/right identity.

Many animals show individual left/right biases in motor behaviour, but underlying neural substrates have proven elusive. Here the authors describe neurons that maintain individual, context-dependent lateralisation of swimming behaviour in zebrafish.

Dopaminergic Co-Regulation of Locomotor Development and Motor Neuron Synaptogenesis is Uncoupled by Hypoxia in Zebrafish

Hypoxic injury to the developing human brain is a complication of premature birth and is associated with long-term impairments of motor function. Disruptions of axon and synaptic connectivity have been linked to developmental hypoxia, but the fundamental mechanisms impacting motor function from altered connectivity are poorly understood. We investigated the effects of hypoxia on locomotor development in zebrafish. We found that developmental hypoxia resulted in decreased spontaneous swimming behavior in larva, and that this motor impairment persisted into adulthood. In evaluation of the diencephalic dopaminergic neurons, which regulate early development of locomotion and constitute an evolutionarily conserved component of the vertebrate dopaminergic system, hypoxia caused a decrease in the number of synapses from the descending dopaminergic diencephalospinal tract (DDT) to spinal cord motor neurons. Moreover, dopamine signaling from the DDT was coupled jointly to motor neuron synaptogenesis and to locomotor development. Together, these results demonstrate the developmental processes regulating early locomotor development and a requirement for dopaminergic projections and motor neuron synaptogenesis. Our findings suggest new insights for understanding the mechanisms leading to motor disability from hypoxic injury of prematurity.

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.

Spatial and Temporal Distribution of Dopaminergic Neurons during Development in Zebrafish

As one of the model organisms of Parkinson’s disease (PD) research, the zebrafish has its advantages, such as the 87% homology with human genome and transparent embryos which make it possible to observe the development of dopaminergic neurons in real time. However, there is no midbrain dopaminergic system in zebrafish when compared with mammals, and the location and projection of the dopaminergic neurons are seldom reported. In this study, Vmat2:GFP transgenic zebrafish was used to observe the development and distribution of dopaminergic neurons in real time. We found that diencephalons (DC) 2 and DC4 neuronal populations were detected at 24 h post fertilization (hpf). All DC neuronal populations as well as those in locus coeruleus (LC), raphe nuclei (Ra) and telencephalon were detected at 48 hpf. Axons were detected at 72 hpf. At 96 hpf, all the neuronal populations were detected. For the first time we reported axons from the posterior tubercle (PT) of ventral DC projected to subpallium in vivo. However, when compared with results from whole mount tyrosine hydroxylase (TH) immunofluorescence staining in wild type (WT) zebrafish, we found that DC2 and DC4 neuronal populations were mainly dopaminergic, while DC1, DC3, DC5 and DC6 might not. Neurons in pretectum (Pr) and telencephalon were mainly dopaminergic, while neurons in LC and Ra might be noradrenergic. Our study makes some corrections and modifications on the development, localization and distribution of zebrafish dopaminergic neurons, and provides some experimental evidences for the construction of the zebrafish PD model.

A Novel Developmental Role for Dopaminergic Signaling to Specify Hypothalamic Neurotransmitter Identity

Hypothalamic neurons expressing histamine and orexin/hypocretin (hcrt) are necessary for normal regulation of wakefulness. In Parkinson's disease, the loss of dopaminergic neurons is associated with elevated histamine levels and disrupted sleep/wake cycles, but the mechanism is not understood. To characterize the role of dopamine in the development of histamine neurons, we inhibited the translation of the two non-allelic forms of tyrosine hydroxylase (th1 and th2) in zebrafish larvae. We found that dopamine levels were reduced in both th1 and th2 knockdown, but the serotonin level and number of serotonin neurons remained unchanged. Further, we demonstrated that th2 knockdown increased histamine neuron number and histamine levels, whereas increased dopaminergic signaling using the dopamine precursor l-DOPA (l-3,4-dihydroxyphenylalanine) or dopamine receptor agonists reduced the number of histaminergic neurons. Increases in the number of histaminergic neurons were paralleled by matching increases in the numbers of hcrt neurons, supporting observations that histamine regulates hcrt neuron development. Finally, we show that histaminergic neurons surround th2-expressing neurons in the hypothalamus, and we suggest that dopamine regulates the terminal differentiation of histamine neurons via paracrine actions or direct synaptic neurotransmission. These results reveal a role for dopaminergic signaling in the regulation of neurotransmitter identity and a potential mechanism contributing to sleep disturbances in Parkinson's disease.

Live imaging of mitochondrial dynamics in CNS dopaminergic neurons in vivo demonstrates early reversal of mitochondrial transport following MPP(+) exposure

Extensive convergent evidence collectively suggests that mitochondrial dysfunction is central to the pathogenesis of Parkinson's disease (PD). Recently, changes in the dynamic properties of mitochondria have been increasingly implicated as a key proximate mechanism underlying neurodegeneration. However, studies have been limited by the lack of a model in which mitochondria can be imaged directly and dynamically in dopaminergic neurons of the intact vertebrate CNS. We generated transgenic zebrafish in which mitochondria of dopaminergic neurons are labeled with a fluorescent reporter, and optimized methods allowing direct intravital imaging of CNS dopaminergic axons and measurement of mitochondrial transport in vivo. The proportion of mitochondria undergoing axonal transport in dopaminergic neurons decreased overall during development between 2days post-fertilization (dpf) and 5dpf, at which point the major period of growth and synaptogenesis of the relevant axonal projections is complete. Exposure to 0.5-1.0mM MPP(+) between 4 and 5dpf did not compromise zebrafish viability or cause detectable changes in the number or morphology of dopaminergic neurons, motor function or monoaminergic neurochemistry. However, 0.5mM MPP(+) caused a 300% increase in retrograde mitochondrial transport and a 30% decrease in anterograde transport. In contrast, exposure to higher concentrations of MPP(+) caused an overall reduction in mitochondrial transport. This is the first time mitochondrial transport has been observed directly in CNS dopaminergic neurons of a living vertebrate and quantified in a PD model in vivo. Our findings are compatible with a model in which damage at presynaptic dopaminergic terminals causes an early compensatory increase in retrograde transport of compromised mitochondria for degradation in the cell body. These data are important because manipulation of early pathogenic mechanisms might be a valid therapeutic approach to PD. The novel transgenic lines and methods we developed will be useful for future studies on mitochondrial dynamics in health and disease.

Microfluidic-aided genotyping of zebrafish in the first 48 h with 100% viability

This paper introduces an innovative method for genotyping 1-2 days old zebrafish embryos, without sacrificing the life/health of the embryos. The method utilizes microfluidic technology to extract and collect a small amount of genetic material from the chorionic fluid or fin tissue of the embryo. Then, using conventional DNA extraction, PCR amplification, and high resolution melt analysis with fluorescent DNA detection techniques, the embryo is genotyped. The chorionic fluid approach was successful 78% of the time while the fin clipping method was successful 100% of the time. Chorionic fluid was shown to only contain DNA from the embryo and not from the mother. These results suggest a novel method to genotype zebrafish embryos that can facilitate high-throughput screening, while maintaining 100% viability of the embryo.

Transgenic FingRs for Live Mapping of Synaptic Dynamics in Genetically-Defined Neurons

Tools for genetically-determined visualization of synaptic circuits and interactions are necessary to build connectomics of the vertebrate brain and to screen synaptic properties in neurological disease models. Here we develop a transgenic FingR (fibronectin intrabodies generated by mRNA display) technology for monitoring synapses in live zebrafish. We demonstrate FingR labeling of defined excitatory and inhibitory synapses, and show FingR applicability for dissecting synapse dynamics in normal and disease states. Using our system we show that chronic hypoxia, associated with neurological defects in preterm birth, affects dopaminergic neuron synapse number depending on the developmental timing of hypoxia.

Whole-brain activity mapping onto a zebrafish brain atlas

In order to localize the neural circuits involved in generating behaviors, it is necessary to assign activity onto anatomical maps of the nervous system. Using brain registration across hundreds of larval zebrafish, we have built an expandable open-source atlas containing molecular labels and definitions of anatomical regions, the Z-Brain. Using this platform and immunohistochemical detection of phosphorylated extracellular signal–regulated kinase (ERK) as a readout of neural activity, we have developed a system to create and contextualize whole-brain maps of stimulus- and behavior-dependent neural activity. This mitogen-activated protein kinase (MAP)-mapping assay is technically simple, and data analysis is completely automated. Because MAP-mapping is performed on freely swimming fish, it is applicable to studies of nearly any stimulus or behavior. Here we demonstrate our high-throughput approach using pharmacological, visual and noxious stimuli, as well as hunting and feeding. The resultant maps outline hundreds of areas associated with behaviors.

A vertebrate-conserved cis-regulatory module for targeted expression in the main hypothalamic regulatory region for the stress response

Background

The homeodomain transcription factor orthopedia (Otp) is an evolutionarily conserved regulator of neuronal fates. In vertebrates, Otp is necessary for the proper development of different regions of the brain and is required in the diencephalon to specify several hypothalamic cell types, including the cells that control the stress response. To understand how this widely expressed transcription factor accomplishes hypothalamus-specific functions, we performed a comprehensive screening of otp cis-regulatory regions in zebrafish.

Results

Here, we report the identification of an evolutionarily conserved vertebrate enhancer module with activity in a restricted area of the forebrain, which includes the region of the hypothalamus that controls the stress response. This region includes neurosecretory cells producing Corticotropin-releasing hormone (Crh), Oxytocin (Oxt) and Arginine vasopressin (Avp), which are key components of the stress axis. Lastly, expression of the bacterial nitroreductase gene under this specific enhancer allowed pharmacological attenuation of the stress response in zebrafish larvae.

Conclusion

Vertebrates share many cellular and molecular components of the stress response and our work identified a striking conservation at the cis-regulatory level of a key hypothalamic developmental gene. In addition, this enhancer provides a useful tool to manipulate and visualize stress-regulatory hypothalamic cells in vivo with the long-term goal of understanding the ontogeny of the stress axis in vertebrates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-014-0041-x) contains supplementary material, which is available to authorized users.

Expression of arginine vasotocin receptors in the developing zebrafish CNS

Vasotocin/vasopressin is a neuropeptide that regulates social and reproductive behaviors in a variety of animals including fish. Arginine vasotocin (AVT) is expressed by cells in the ventral hypothalamic and preoptic areas in the diencephalon during embryogenesis in zebrafish suggesting that vasotocin might mediate other functions within the CNS prior to the development of social and reproductive behaviors. In order to examine potential early roles for vasotocin we cloned two zebrafish vasotocin receptors homologous to AVPR1a. The receptors are expressed primarily in the CNS in similar but generally non-overlapping patterns. Both receptors are expressed in the forebrain, midbrain and hindbrain by larval stage. Of note, AVTR1a-expressing neurons in the hindbrain appear to be contacted by the axons of preoptic neurons in the forebrain that include avt+ neurons and sensory axons in the lateral longitudinal fasciculus (LLF). Furthermore, AVTR1a-expressing hindbrain neurons extend axons into the medial longitudinal fasciculus (MLF) that contains axons of many neurons thought to be involved in locomotor responses to sensory stimulation. One hypothesis consistent with this anatomy is that AVT signaling mediates or gates sensory input to motor circuits in the hindbrain and spinal cord.

Toward developmental models of psychiatric disorders in zebrafish

Psychiatric disorders are a diverse set of diseases that affect all aspects of mental function including social interaction, thinking, feeling, and mood. Although psychiatric disorders place a large economic burden on society, the drugs available to treat them are often palliative with variable efficacy and intolerable side-effects. The development of novel drugs has been hindered by a lack of knowledge about the etiology of these diseases. It is thus necessary to further investigate psychiatric disorders using a combination of human molecular genetics, gene-by-environment studies, in vitro pharmacological and biochemistry experiments, animal models, and investigation of the non-biological basis of these diseases, such as environmental effects. Many psychiatric disorders, including autism spectrum disorder, attention-deficit/hyperactivity disorder, mental retardation, and schizophrenia can be triggered by alterations to neural development. The zebrafish is a popular model for developmental biology that is increasingly used to study human disease. Recent work has extended this approach to examine psychiatric disorders as well. However, since psychiatric disorders affect complex mental functions that might be human specific, it is not possible to fully model them in fish. In this review, I will propose that the suitability of zebrafish for developmental studies, and the genetic tools available to manipulate them, provide a powerful model to study the roles of genes that are linked to psychiatric disorders during neural development. The relative speed and ease of conducting experiments in zebrafish can be used to address two areas of future research: the contribution of environmental factors to disease onset, and screening for novel therapeutic compounds.

Sim1a and Arnt2 contribute to hypothalamo-spinal axon guidance by regulating Robo2 activity via a Robo3-dependent mechanism

Precise spatiotemporal control of axon guidance factor expression is a prerequisite for formation of functional neuronal connections. Although Netrin/Dcc- and Robo/Slit-mediated attractive and repulsive guidance of commissural axons have been extensively studied, little is known about mechanisms controlling mediolateral positioning of longitudinal axons in vertebrates. Here, we use a genetic approach in zebrafish embryos to study pathfinding mechanisms of dopaminergic and neuroendocrine longitudinal axons projecting from the hypothalamus into hindbrain and spinal cord. The transcription factors Sim1a and Arnt2 contribute to differentiation of a defined population of dopaminergic and neuroendocrine neurons. We show that both factors also control aspects of axon guidance: Sim1a or Arnt2 depletion results in displacement of hypothalamo-spinal longitudinal axons towards the midline. This phenotype is suppressed in robo3 guidance receptor mutant embryos. In the absence of Sim1a and Arnt2, expression of the robo3 splice isoform robo3a.1 is increased in the hypothalamus, indicating negative control of robo3a.1 transcription by these factors. We further provide evidence that increased Robo3a.1 levels interfere with Robo2-mediated repulsive axon guidance. Finally, we show that the N-terminal domain unique to Robo3a.1 mediates the block of Robo2 repulsive activity. Therefore, Sim1a and Arnt2 contribute to control of lateral positioning of longitudinal hypothalamic-spinal axons by negative regulation of robo3a.1 expression, which in turn attenuates the repulsive activity of Robo2.

The conserved dopaminergic diencephalospinal tract mediates vertebrate locomotor development in zebrafish larvae

The most conserved part of the vertebrate dopaminergic system is the orthopedia (otp)-expressing diencephalic neuronal population that constitutes the dopaminergic diencephalospinal tract (DDT). Although studies in the neonatal murine spinal cord in vitro suggest an early locomotor role of the DDT, the function of the DDT in developing vertebrates in vivo remains unknown. Here, we investigated the role of the DDT in the locomotor development of zebrafish larvae. To assess the development of the behavioral and neural locomotor pattern, we used high-throughput video tracking in combination with peripheral nerve recordings. We found a behavioral and neural correspondence in the developmental switch from an immature to mature locomotor pattern. Blocking endogenous dopamine receptor 4 (D(4)R) signaling in vivo either before or after the developmental switch prevented or reversed the switch, respectively. Spinal transections of post-switch larvae reestablished the immature locomotor pattern, which was rescued to a mature-like pattern via spinal D(4)R agonism. Selective chemogenetic ablation of otp b (otpb) neurons that contribute to the DDT perpetuated the immature locomotor pattern in vivo. This phenotype was recapitulated by diencephalic transections that removed the dopaminergic otpb population and was rescued to a mature-like locomotor pattern by D(4)R agonism. We conclude that the dopaminergic otpb population, via the DDT, is responsible for spinal D(4)R signaling to mediate the developmental switch to the mature locomotor pattern of zebrafish. These results, integrated with the mammalian literature, suggest that the DDT represents an evolutionarily conserved neuromodulatory system that is necessary for normal vertebrate locomotor development.

Dopaminergic and noradrenergic circuit development in zebrafish

Dopaminergic and noradrenergic neurons constitute some of the major far projecting systems in the vertebrate brain and spinal cord that modulate the activity of circuits controlling a broad range of behaviors. Degeneration or dysfunction of dopaminergic neurons has also been linked to a number of neurological and psychiatric disorders, including Parkinson's disease.Zebrafish (Danio rerio) have emerged over the past two decades into a major genetic vertebrate model system,and thus contributed to a better understanding of developmental mechanisms controlling dopaminergic neuron specification and axonogenesis. In this review, we want to focus on conserved and dynamic aspects of the different catecholaminergic systems, which may help to evaluate the zebrafish as a model for dopaminergic and noradrenergic cellular specification and circuit function as well as biomedical aspects of catecholamine systems.

Deep brain photoreceptors control light-seeking behavior in zebrafish larvae

Most vertebrates process visual information using elaborately structured photosensory tissues, including the eyes and pineal. However, there is strong evidence that other tissues can detect and respond to photic stimuli. Many reports suggest that photosensitive elements exist within the brain itself and influence physiology and behavior; however, a long-standing puzzle has been the identity of the neurons and photoreceptor molecules involved. We tested whether light cues influence behavior in zebrafish larvae through deep brain photosensors. We found that larvae lacking eyes and pineal perform a simple light-seeking behavior triggered by loss of illumination ("dark photokinesis"). Neuroanatomical considerations prompted us to test orthopedia (otpa)-deficient fish, which show a profound reduction in dark photokinesis. Using targeted genetic ablations, we narrowed the photosensitive region to neurons in the preoptic area. Neurons in this region express several photoreceptive molecules, but expression of the melanopsin opn4a is selectively lost in otpa mutants, suggesting that opn4a mediates dark photokinesis. Our findings shed light on the identity and function of deep brain photoreceptors and suggest that otpa specifies an ancient population of sensory neurons that mediate behavioral responses to light.

Zebrafish foxP2 zinc finger nuclease mutant has normal axon pathfinding

foxP2, a forkhead-domain transcription factor, is critical for speech and language development in humans, but its role in the establishment of CNS connectivity is unclear. While in vitro studies have identified axon guidance molecules as targets of foxP2 regulation, and cell culture assays suggest a role for foxP2 in neurite outgrowth, in vivo studies have been lacking regarding a role for foxP2 in axon pathfinding. We used a modified zinc finger nuclease methodology to generate mutations in the zebrafish foxP2 gene. Using PCR-based high resolution melt curve analysis (HRMA) of G0 founder animals, we screened and identified three mutants carrying nonsense mutations in the 2(nd) coding exon: a 17 base-pair (bp) deletion, an 8bp deletion, and a 4bp insertion. Sequence analysis of cDNA confirmed that these were frameshift mutations with predicted early protein truncations. Homozygous mutant fish were viable and fertile, with unchanged body morphology, and no apparent differences in CNS apoptosis, proliferation, or patterning at embryonic stages. There was a reduction in expression of the known foxP2 target gene cntnap2 that was rescued by injection of wild-type foxP2 transcript. When we examined axon pathfinding using a pan-axonal marker or transgenic lines, including a foxP2-neuron-specific enhancer, we did not observe any axon guidance errors. Our findings suggest that foxP2 is not necessary for axon pathfinding during development.

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.

Hypoxia disruption of vertebrate CNS pathfinding through ephrinB2 Is rescued by magnesium

The mechanisms of hypoxic injury to the developing human brain are poorly understood, despite being a major cause of chronic neurodevelopmental impairments. Recent work in the invertebrate Caenorhabditis elegans has shown that hypoxia causes discrete axon pathfinding errors in certain interneurons and motorneurons. However, it is unknown whether developmental hypoxia would have similar effects in a vertebrate nervous system. We have found that developmental hypoxic injury disrupts pathfinding of forebrain neurons in zebrafish (Danio rerio), leading to errors in which commissural axons fail to cross the midline. The pathfinding defects result from activation of the hypoxia-inducible transcription factor (hif1) pathway and are mimicked by chemical inducers of the hif1 pathway or by expression of constitutively active hif1α. Further, we found that blocking transcriptional activation by hif1α helped prevent the guidance defects. We identified ephrinB2a as a target of hif1 pathway activation, showed that knock-down of ephrinB2a rescued the guidance errors, and showed that the receptor ephA4a is expressed in a pattern complementary to the misrouting axons. By targeting a constitutively active form of ephrinB2a to specific neurons, we found that ephrinB2a mediates the pathfinding errors via a reverse-signaling mechanism. Finally, magnesium sulfate, used to improve neurodevelopmental outcomes in preterm births, protects against pathfinding errors by preventing upregulation of ephrinB2a. These results demonstrate that evolutionarily conserved genetic pathways regulate connectivity changes in the CNS in response to hypoxia, and they support a potential neuroprotective role for magnesium.

How hypoxia damages the developing human brain is poorly understood, despite being a major cause of life-long neurologic and psychiatric problems. Premature infants are especially at risk for these problems, with increased rates of attention-deficit disorder, autism, cerebral palsy, epilepsy, psychiatric disorders, and cognitive impairment. It is unknown whether hypoxia can cause errors in the connections of neurons in the vertebrate nervous system. We used zebrafish, a vertebrate model animal, to answer this question. We found that hypoxic injury causes errors in how neurons connect. We went on to determine that a specific genetic pathway, the hif1 pathway, is activated by hypoxia and turns on downstream genes, which cause the connection problems. One of the genes activated by hif1, ephrinB2a, is responsible for many of the connection problems. Importantly, magnesium, used as a treatment for some preterm births, is able to help protect against the neuron connection errors. Our results show that hypoxia in vertebrates does cause errors in neuron connections and that magnesium can help prevent this.

Regulation of terminal differentiation programs in the nervous system

The generation of individual neuron types in the nervous system is a multistep process whose endpoint is the expression of neuron type-specific batteries of terminal differentiation genes that determine the functional properties of a neuron. This review focuses on the regulatory mechanisms that are involved in controlling the terminally differentiated state of a neuron. I review several case studies from invertebrate and vertebrate nervous systems that reveal that many terminal differentiation features of a neuron are coregulated via terminal selector transcription factors that initiate and maintain terminal differentiation programs.

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.

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.

Gal80 intersectional regulation of cell-type specific expression in vertebrates

Characterization and functional manipulation of specific groups of neurons in the vertebrate central nervous system (CNS) remains a major hurdle for understanding complex circuitry and functions. In zebrafish, the Gal4/UAS system has permitted expression of transgenes and enhancer trap screens, but is often limited by broad expression domains. We have developed a method for cell-type specific expression using Gal80 inhibition of Gal4-dependent expression. We show that native Gal4 is able to drive strong expression, that Gal80 can inhibit this expression, and that overlapping Gal4 and Gal80 expression can achieve "intersectional" expression in spatially and genetically defined subsets of neurons. We also optimize Gal80 for expression in vertebrates, track Gal80 expression with a co-expressed fluorescent marker, and use a temperature-sensitive allele of Gal80 to temporally regulate its function. These data demonstrate that Gal80 is a powerful addition to the genetic techniques available to map and manipulate neural circuits in zebrafish.