Nr4a2 is essential for the differentiation of dopaminergic neurons during zebrafish embryogenesis

Differential roles of NR4A2 (NURR1) paralogs in the brain and behavior of zebrafish

Dopaminergic (DAnergic) dysfunction and imbalanced dopamine (DA) levels are known contributors to the pathogenesis of numerous psychiatric and neurodegenerative disorders. Of the many identified risk factors for DA-associated disorders, nuclear receptor subfamily 4 group A2 (NR4A2; or nuclear receptor related-1 protein (NURR1)), a transcription factor involved in DAnergic differentiation, has been associated with Parkinson's disease and attention deficit hyperactive disorder (ADHD). In zebrafish, transient loss of nr4a2 was previously shown to decrease tyrosine hydroxylase (TH) expression and impair locomotion. To further characterize the roles of the two zebrafish nr4a2 paralogs, nr4a2a, and nr4a2b, we produced targeted loss-of-function mutants and examined DAnergic neuron regeneration, oxidative respiration, and behavioral traits. The loss of nr4a2a function more closely recapitulated Parkinsonian phenotypes and affected neurotrophic factor gene expression. Conversely, nr4a2b mutants displayed behavioral symptoms reminiscent of mice deficient in Nr4a2 with increased neurotrophic output. In contrast, nr4a2b mutants also displayed increased metabolic input from non-mitochondrial sources indicative of high cytosolic reactive oxygen species and perturbed mitochondrial function. The nr4a2a mutants also showed increased maximal respiration, which may suggest a compensatory mechanism to meet the metabolic requirements of DAnergic neuron health. Overall, the zebrafish mutants generated in this study helped uncover molecular mechanisms involved in DA-related disease pathologies, and in the regeneration of DAnergic neurons.

Neurotoxicity of Combined Exposure to the Heavy Metals (Pb and As) in Zebrafish (Danio rerio)

Lead (Pb) and arsenic (As) are commonly occurring heavy metals in the environment and produce detrimental impacts on the central nervous system. Although they have both been indicated to exhibit neurotoxic properties, it is not known if they have joint effects, and their mechanisms of action are likewise unknown. In this study, zebrafish were exposed to different concentrations of Pb (40 μg/L, 4 mg/L), As (32 μg/L, 3.2 mg/L) and their combinations (40 μg/L + 32 μg/L, 4 mg/L + 3.2 mg/L) for 30 days. The histopathological analyses showed significant brain damage characterized by glial scar formation and ventricular enlargement in all exposed groups. In addition, either Pb or As staining inhibited the swimming speed of zebrafish, which was enhanced by their high concentrations in a mixture. To elucidate the underlying mechanisms, we examined changes in acetylcholinesterase (AChE) activity, neurotransmitter (dopamine, 5-hydroxytryptamine) levels, HPI axis-related hormone (cortisol and epinephrine) contents and neurodevelopment-related gene expression in zebrafish brain. The observations suggest that combined exposure to Pb and As can cause abnormalities in swimming behavior and ultimately exacerbate neurotoxicity in zebrafish by interfering with the cholinergic system, dopamine and 5-hydroxytryptamine signaling, HPI axis function as well as neuronal development. This study provides an important theoretical basis for the mixed exposure of heavy metals and their toxicity to aquatic organisms.

KDM6B Negatively Regulates the Neurogenesis Potential of Apical Papilla Stem Cells via HES1

Stem cells from the apical papilla (SCAPs) are used to regulate the microenvironment of nerve defects. KDM6B, which functions as an H3K27me3 demethylase, is known to play a crucial role in neurogenesis. However, the mechanism by which KDM6B influences the neurogenesis potential of SCAPs remains unclear. We evaluated the expression of neural markers in SCAPs by using real-time RT-PCR and immunofluorescence staining. To assess the effectiveness of SCAP transplantation in the SCI model, we used the BBB scale to evaluate motor function. Additionally, toluidine blue staining and Immunofluorescence staining of NCAM, NEFM, β-III-tubulin, and Nestin were used to assess nerve tissue remodeling. Further analysis was conducted through Microarray analysis and ChIP assay to study the molecular mechanisms. Our results show that KDM6B inhibits the expression of NeuroD, TH, β-III tubulin, and Nestin. In vivo studies indicate that the SCAP-KDM6Bsh group is highly effective in restoring spinal cord structure and motor function in rats suffering from SCI. Our findings suggest that KDM6B directly binds to the HES1 promoter via regulating H3K27me3 and HES1 expression. In conclusion, our study can help understand the regulatory role of KDM6B in neurogenesis and provide more effective treatments for nerve injury.

Developmental Neurotoxicity of Difenoconazole in Zebrafish Embryos

Difenoconazole is a type of triazole fungicide that is widely used in the treatment of plant diseases. Triazole fungicides have been shown in several studies to impair the development of the nervous system in zebrafish embryos. There is still little known about difenoconazole-induced neurotoxicity in fish. In this study, zebrafish embryos were exposed to 0.25, 0.5, and 1 mg/L of difenoconazole solution until 120 h post-fertilization (hpf). The difenoconazole-exposed groups showed concentration-dependent inhibitory tendencies in heart rate and body length. Malformation rate and spontaneous movement of zebrafish embryos increased, and the locomotor activity decreased in the highest exposure group. The content of dopamine and acetylcholine was reduced significantly in difenoconazole treatment groups. The activity of acetylcholinesterase (AChE) was also increased after treatment with difenoconazole. Furthermore, the expression of genes involved in neurodevelopment was remarkably altered, which corresponded with the alterations of neurotransmitter content and AChE activity. These results indicated that difenoconazole might affect the development of the nervous system through influencing neurotransmitter levels, enzyme activity, and the expression of neural-related genes, ultimately leading to abnormal locomotor activity in the early stages of zebrafish.

Evaluation of the acute toxicity and neurodevelopmental inhibition of perfluorohexanoic acid (PFHxA) in zebrafish embryos

Perfluorohexanoic acid (PFHxA), a widely used emerging alternative for 8-carbon PFAAs, has been detected at a high level in the water environment. While its toxicity and environmental health risk are still largely unknown in aquatic life. The present study aimed to evaluated the possible developmental neurotoxicity induced by PFHxA exposure (0, 0.48, 2.4, and 12 mg/L for 120 h) in the zebrafish embryo. Here, both developmental endpoints, neurotransmitters concentrations, locomotor behavior were analyzed. No significant effects on mortality, malformation rate, and growth delay were detected in the low dose treatment groups except for in the high dose group (12 mg/L). A significant increase in swimming speed were noted in the 0.48 mg/L group. Other changes including neurotransmitters concentrations and green fluorescent protein (GFP) expression in Tg (HuC-GFP) zebrafish larvae were significantly increased in 12 mg/L group. Beyond that, genes related to neurodevelopment were significantly decreased in larvae. Moreover, downregulations of protein expression levels of α1-tubulin, elavl3, and gap43 were identified. These results demonstrate that the PFAAs alternative PFHxA have no significant neurodevelopmental effects on zebrafish larvae under acute low-dose exposure, while, it is important to note that PFHxA perform inhibiting effects on neurotransmitter and central nervous system under a relatively high dose. This in vivo study could provide reliable toxicity information for risk assessments of PFHxA on aquatic ecosystems. CAPSULE: PFHxA have no significant neurodevelopmental effects on zebrafish larvae under acute low-dose exposure, while exposed with relatively high-dose, could induced the alternations of neurotransmitter concentrations as well as the genes involved in the early developmental stages of zebrafish, leading to the impairment of the nervous system in zebrafish larvae.

Behavioral effects of early-life exposure to perfluorooctanoic acid might synthetically link to multiple aspects of dopaminergic neuron development and dopamine functions in zebrafish larvae

Perfluorooctanoic acid (PFOA) is known as an environmental endocrine disruptor and has developmental neurotoxicity that could be associated with behavior changes in human and animal studies. Previous studies have shown that PFOA might affect the dopaminergic nervous system. However, the mode of action underlying the effects of PFOA remains poorly understood. Our study used zebrafish as an animal model to investigate the effects of early-life PFOA exposure on dopaminergic neuron development and dopamine functions in zebrafish larvae. Zebrafish fertilized eggs were exposed to different concentrations of PFOA (0, 10, 100, 1000 μg/L). After exposure to PFOA for 7 days, the locomotor activity of zebrafish was decreased; the mRNA levels of nuclear receptor subfamily 4 group a member 2b (nr4a2b), paired box 2 and 5 (pax2, pax5), tyrosine hydroxylase 1/2 (th1/th2) and dopamine transporter (dat) were increased; mRNA and protein level of mesencephalic astrocyte-derived neurotrophic factor (manf) were decreased. Neural cell proliferation in the preoptic area of hypothalamus was increased. In conclusion, dopaminergic neuron development might be one of the targets of early-life PFOA exposure. The neurobehavior changes induced by PFOA exposure might link to multiple aspects of dopaminergic neuron development and dopamine functions in zebrafish larvae.

Single-cell analysis of the ventricular-subventricular zone reveals signatures of dorsal & ventral adult neurogenesis

The ventricular-subventricular zone (V-SVZ), on the walls of the lateral ventricles, harbors the largest neurogenic niche in the adult mouse brain. Previous work has shown that neural stem/progenitor cells (NSPCs) in different locations within the V-SVZ produce different subtypes of new neurons for the olfactory bulb. The molecular signatures that underlie this regional heterogeneity remain largely unknown. Here, we present a single-cell RNA-sequencing dataset of the adult mouse V-SVZ revealing two populations of NSPCs that reside in largely non-overlapping domains in either the dorsal or ventral V-SVZ. These regional differences in gene expression were further validated using a single-nucleus RNA-sequencing reference dataset of regionally microdissected domains of the V-SVZ and by immunocytochemistry and RNAscope localization. We also identify two subpopulations of young neurons that have gene expression profiles consistent with a dorsal or ventral origin. Interestingly, a subset of genes are dynamically expressed, but maintained, in the ventral or dorsal lineages. The study provides novel markers and territories to understand the region-specific regulation of adult neurogenesis.

Nerve cells, or neurons, are the central building blocks of brain circuits. Their damage, death or loss of function leads to cognitive decline. Neural stem/progenitor cells (NSPCs) first appear during embryo development, generating most of the neurons found in the nervous system. However, the adult brain retains a small subpopulation of NSPCs, which in some species are an important source of new neurons throughout life.

In the adult mouse brain the largest population of NSPCs, known as B cells, is found in an area called the ventricular-subventricular zone (V-SVZ). These V-SVZ B cells have properties of specialized support cells known as astrocytes, but they can also divide and generate intermediate ‘progenitor cells’ called C cells. These, in turn, divide to generate large numbers of young ‘A cells’ neurons that undertake a long and complex migration from V-SVZ to the olfactory bulb, the first relay in the central nervous system for the processing of smells.

Depending on their location in the V-SVZ, B cells can generate different kinds of neurons, leading to at least ten subtypes of neurons. Why this is the case is still poorly understood.

To examine this question, Cebrián-Silla, Nascimento, Redmond, Mansky et al. determined which genes were expressed in B, C and A cells from different parts of the V-SVZ. While cells within each of these populations had different expression patterns, those that originated in the same V-SVZ locations shared a set of genes, many of which associated with regional specification in the developing brain. Some, however, were intriguingly linked to hormonal regulation.

Salient differences between B cells depended on whether the cells originated closer to the top (‘dorsal’ position) or to the bottom of the brain (‘ventral’ position). This information was used to stain slices of mouse brains for the RNA and proteins produced by these genes in different regions. These experiments revealed dorsal and ventral territories containing B cells with distinct ‘gene expression’.

This study highlights the heterogeneity of NSPCs, revealing key molecular differences among B cells in dorsal and ventral areas of the V-SVZ and reinforcing the concept that the location of NSPCs determines the types of neuron they generate. Furthermore, the birth of specific types of neurons from B cells that are so strictly localized highlights the importance of neuronal migration to ensure that young neurons with specific properties reach their appropriate destination in the olfactory bulb.

The work by Cebrián-Silla, Nascimento, Redmond, Mansky et al. has identified sets of genes that are differentially expressed in dorsal and ventral regions which may contribute to regional regulation. Furthering the understanding of how adult NSPCs differ according to their location will help determine how various neuron types emerge in the adult brain.

DNA methylation and exposure to violence among African American young adult males

Exposure to violence (ETV) has been linked to epigenomics mechanisms such as DNA methylation (DNAm). We used epigenetic profiling of blood collected from 32 African American young adult males who lived in Washington DC to determine if changes in DNAm at CpG sites affiliated with nervous and immune system were associated with exposure to violence. Pathway analysis of differentially methylated regions comparing high and low ETV groups revealed an enrichment of gene sets annotated to nervous system and immune ontologies. Many of these genes are known to interact with each other which suggests DNAm alters gene function in the nervous and immune system in response to ETV. Using data from a unique age group, young African American adult males, we provide evidence that lifetime ETV could impact DNA methylation in genes impacted at Central Nervous System and Immune Function sites.

METHOD

Methylation analysis was performed on DNA collected from the blood of participants classified with either high or low lifetime ETV. Illumina®MethylationEPIC Beadchips (~850k CpG sites) were processed on the iScan System to examine whole-genome methylation differences. Differentially methylated CpG-sites between high (n ​= ​19) and low (n ​= ​13) groups were identified using linear regression with violence and substance abuse as model covariates. Gene ontology analysis was used to identify enrichment categories from probes annotated to the nearest gene.

RESULTS

A total of 595 probes (279 hypermethylated; 316 hypomethylated) annotated to 383 genes were considered differentially methylated in association with ETV. Males with high ETV showed elevated methylation in several signaling pathways but were most impacted at Central Nervous System and Immune Function affiliated sites. Eight candidate genes were identified that play important biological roles in stress response to violence with HDAC4 (10%), NR4A3 (11%), NR4A2 (12%), DSCAML1(12%), and ELAVL3 (13%) exhibiting higher levels in the low ETV group and DLGAP1 (10%), SHANK2 (10%), and NRG1(11%) having increased methylation in the high ETV group. These findings suggest that individuals subjected to high ETV may be at risk for poor health outcomes that have not been reported previously.

Chemical and Genetic Zebrafish Models to Define Mechanisms of and Treatments for Dopaminergic Neurodegeneration

The zebrafish (Danio rerio) is routinely used in biological studies as a vertebrate model system that provides unique strengths allowing applications in studies of neurodevelopmental and neurodegenerative diseases. One specific advantage is that the neurotransmitter systems are highly conserved throughout vertebrate evolution, including between zebrafish and humans. Disruption of the dopaminergic signaling pathway is linked to multiple neurological disorders. One of the most common is Parkinson’s disease, a neurodegenerative disease associated with the loss of dopaminergic neurons, among other neuropathological characteristics. In this review, the development of the zebrafish’s dopaminergic system, focusing on genetic control of the dopaminergic system, is detailed. Second, neurotoxicant models used to study dopaminergic neuronal loss, including 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the pesticides paraquat and rotenone, and 6-hydroxydopamine (6-OHDA), are described. Next, zebrafish genetic knockdown models of dj1, pink1, and prkn established for investigating mechanisms of Parkinson’s disease are discussed. Chemical modulators of the dopaminergic system are also highlighted to showcase the applicability of the zebrafish to identify mechanisms and treatments for neurodegenerative diseases such as Parkinson’s disease associated with the dopaminergic system.

Developmental neurotoxicity of Microcystis aeruginosa in the early life stages of zebrafish

Accumulating evidence suggests that cyanotoxins can exert neurotoxic effects on exposed aquatic organisms but most studies have focused on purified toxins rather than on the more complex effects of cyanobacterial blooms. To evaluate this issue in an environmentally relevant model, we assessed the developmental neurotoxicity induced by Microcystis aeruginosa on newly hatched zebrafish. After four days of exposure, the locomotor activity of zebrafish larvae was significantly decreased with increasing algae concentration. The levels of both acetylcholinesterase (AChE) and dopamine (DA) were decreased, accompanied by a decline in ache, chrna7 and manf and a compensatory increase in nr4a2b transcription. Furthermore, the expression of nine marker genes for nervous system function or development, namely, elavl3, gap43, gfap, mbp, nestin, ngn1, nkx2.2a, shha and syn2a, similarly decreased after algal exposure. These results demonstrated that Microcystis aeruginosa exposure affected cholinergic and dopaminergic neurotransmitter systems, the transcription of key nervous system genes, and consequently the activity level of larval zebrafish. Importantly, discrepancies in the neurotoxic effects observed in this study and in previous reports that were based on exposure to pure cyanotoxin highlight the necessity for further investigation of cyanobacterial bloom mixtures when assessing the ecotoxicity of cyanobacteria.

Parental transfer of microcystin-LR induced transgenerational effects of developmental neurotoxicity in zebrafish offspring

Microcystin-LR (MCLR) has been reported to cause developmental neurotoxicity in zebrafish, but there are few studies on the mechanisms of MCLR-induced transgenerational effects of developmental neurotoxicity. In this study, zebrafish were exposed to 0, 1, 5, and 25 μg/L MCLR for 60 days. The F1 zebrafish embryos from the above-mentioned parents were collected and incubated in clean water for 120 h for hatching. After examining the parental zebrafish and F1 embryos, MCLR was detected in the gonad of adults and F1 embryos, indicating MCLR could potentially be transferred from parents to offspring. The larvae also showed a serious hypoactivity. The contents of dopamine, dihydroxyphenylacetic acid (DOPAC), serotonin, gamma-aminobutyric acid (GABA) and acetylcholine (ACh) were further detected, but only the first three neurotransmitters showed significant reduction in the 5 and 25 μg/L MCLR parental exposure groups. In addition, the acetylcholinesterase (AChE) activity was remarkably decreased in MCLR parental exposure groups, while the expression levels of manf, bdnf, ache, htr1ab, htr1b, htr2a, htr1aa, htr5a, DAT, TH1 and TH2 genes coincided with the decreased content of neurotransmitters (dopamine, DOPAC and serotonin) and the activity of AChE. Neuronal development related genes, α1-tubulin, syn2a, mbp, gfap, elavl3, shha and gap43 were also measured, but gap43 was the gene only up-regulated. Our results demonstrated MCLR could be transferred to offspring, and subsequently induce developmental neurotoxicity in F1 zebrafish larvae by disturbing the neurotransmitter systems and neuronal development.

Comparative DNA Methylation Profiling Reveals an Immunoepigenetic Signature of HIV-related Cognitive Impairment

Monocytes/macrophages contribute to the neuropathogenesis of HIV-related cognitive impairment (CI); however, considerable gaps in our understanding of the precise mechanisms driving this relationship remain. Furthermore, whether a distinct biological profile associated with HIV-related CI resides in immune cell populations remains unknown. Here, we profiled DNA methylomes and transcriptomes of monocytes derived from HIV-infected individuals with and without CI using genome-wide DNA methylation and gene expression profiling. We identified 1,032 CI-associated differentially methylated loci in monocytes. These loci related to gene networks linked to the central nervous system (CNS) and interactions with HIV. Most (70.6%) of these loci exhibited higher DNA methylation states in the CI group and were preferentially distributed over gene bodies and intergenic regions of the genome. CI-associated DNA methylation states at 12 CpG sites associated with neuropsychological testing performance scores. CI-associated DNA methylation also associated with gene expression differences including CNS genes CSRNP1 (P = 0.017), DISC1 (P = 0.012), and NR4A2 (P = 0.005); and a gene known to relate to HIV viremia, THBS1 (P = 0.003). This discovery cohort data unveils cell type-specific DNA methylation patterns related to HIV-associated CI and provide an immunoepigenetic DNA methylation “signature” potentially useful for corroborating clinical assessments, informing pathogenic mechanisms, and revealing new therapeutic targets against CI.

Unconventional neurotrophic factors CDNF and MANF: Structure, physiological functions and therapeutic potential

Cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) promote the survival of midbrain dopaminergic neurons which degenerate in Parkinson's disease (PD). However, CDNF and MANF are structurally and functionally clearly distinct from the classical, target-derived neurotrophic factors (NTFs) that are solely secreted proteins. In cells, CDNF and MANF localize in the endoplasmic reticulum (ER) and evidence suggests that MANF, and possibly CDNF, is important for the maintenance of ER homeostasis. MANF expression is particularly high in secretory tissues with extensive protein production and thus a high ER protein folding load. Deletion of MANF in mice results in a diabetic phenotype and the activation of unfolded protein response (UPR) in the pancreatic islets. However, information about the intracellular and extracellular mechanisms of MANF and CDNF action is still limited. Here we will discuss the structural motifs and physiological functions of CDNF and MANF as well as their therapeutic potential for the treatment of neurodegenerative diseases and diabetes. Currently available knockout models of MANF and CDNF in mice, zebrafish and fruit fly will increase information about the biology of these interesting proteins.

The developmental neurotoxicity of polybrominated diphenyl ethers: Effect of DE-71 on dopamine in zebrafish larvae

The potential neurotoxicity of polybrominated diphenyl ethers (PBDEs) is still a great concern. In the present study, the authors investigated whether exposure to PBDEs could affect the neurotransmitter system and cause developmental neurotoxicity in zebrafish. Zebrafish embryos (2 h postfertilization) were exposed to different concentrations of the PBDE mixture DE-71 (0-100 μg/L). The larvae were harvested at 120 h postfertilization, and the impact on dopaminergic signaling was investigated. The results revealed significant reductions in content of whole-body dopamine and its metabolite, dihydroxyphenylacetic acid, in DE-71-exposed larvae. The transcription of genes involved in the development of dopaminergic neurons (e.g., manf, bdnf, and nr4a2b) was significantly downregulated upon exposure to DE-71. Also, DE-71 resulted in a significant decrease of tyrosine hydroxylase and dopamine transporter protein levels in dopaminergic neurons. The expression level of tyrosine hydroxylase in forebrain neurons was assessed by whole-mount immunofluorescence, and the results further demonstrated that the tyrosine hydroxylase protein expression level was reduced in dopaminergic neurons. In addition to these molecular changes, the authors observed reduced locomotor activity in DE-71-exposed larvae. Taken together, the results of the present study demonstrate that acute exposure to PBDEs can affect dopaminergic signaling by disrupting the synthesis and transportation of dopamine in zebrafish, thereby disrupting normal neurodevelopment. In accord with its experimental findings, the present study extends knowledge of the mechanisms governing PBDE-induced developmental neurotoxicity.

MicroRNA profiling and the role of microRNA-132 in neurodegeneration using a rat model

MicroRNAs (miRs) are endogenous small RNAs that regulate gene expression at the post-transcriptional level by mediating mRNA degradation or transcriptional inhibition. MiRs were implicated in the pathogenesis of numerous neurodegenerative diseases, including Parkinson's disease (PD). In this study we analyzed the possible role of miRs in the neurodegenerative process in a spontaneous autosomal recessive rat model for neurodegeneration developed in our laboratory. To investigate the role of miRs in the etiology of PD, we conducted miR expression profiling using microarrays. We found 20 miRs that are deregulated in affected rats and many of these are implicated in neurodegenerative disease, including PD. In this study we were particularly interested in the expression of miR-132, a miR that has been reported to be highly expressed in neurons, and to have a potential role in neurodegenerative diseases. We found a significant increase in miR-132 in affected rats by microarray and the result was confirmed by qPCR. Next we analyzed one of the known downstream targets of miR-132, nuclear receptor related 1 protein (Nurr1), which is essential in neurogenesis of midbrain dopaminergic neurons. Western blot analysis and immunohistochemistry revealed a significant decrease in Nurr1 protein expression in the mesencephalic neurons. Finally, we found a significant decrease in both serum and mesencephalon brain tissue of brain-derived neurotrophic factor (BDNF), which is known to be a direct target of Nurr1. Taken together, our findings suggest that miR-132 can regulate Nurr1 levels and might influence the development and function of midbrain dopaminergic neurons.

Correlation of Nr4a2 expression with the neuron progenitors in adult zebrafish brain

Our previous study showed that although Nr4a2b transcripts have little co-localization with tyrosine hydroxylase (TH) in the posterior tuberculum area, knockdown of Nr4a2 caused a decrease in the number of TH-positive (TH(+)) neurons in the posterior tuberculum area. It suggests that Nr4a2 expression in the progenitors may play an important role in regulating differentiation rather than survival of TH(+) progenitors in the posterior tuberculum area during early zebrafish embryogenesis. In this study, we determined the correlation between TH and Nr4a2 in adult zebrafish brain and found that Nr4a2b was co-localized with the spindle-shaped TH(+) cells in the posterior tuberculum area and some small round TH(+) cells in the pretectum area, but not with large pear-shaped TH(+) cells in adult zebrafish diencephalon. In the pretectum area, Nr4a2(+) cells were localized next to the dorsal side of TH(+) cells. Furthermore, we demonstrated that Nr4a2 was co-expressed with nestin in the progenitors of pretectum area and caudal periventricular hypothalamic zones with a lateral symmetry pattern beside the diencephalic ventricle. Co-expression of Nr4a2 and nestin in these areas was remarkably declined with aging. These findings indicate that Nr4a2 is expressed in the neuronal progenitors and plays a crucial role in the differentiation process of dopamine neuron from the stem cell. The change in Nr4a2 expression with aging suggests its possible association with neurodegenerative diseases.

Evolutionary development of neural systems in vertebrates and beyond

The emerging field of "neuro-evo-devo" is beginning to reveal how the molecular and neural substrates that underlie brain function are based on variations in evolutionarily ancient and conserved neurochemical and neural circuit themes. Comparative work across bilaterians is reviewed to highlight how early neural patterning specifies modularity of the embryonic brain, which lays a foundation on which manipulation of neurogenesis creates adjustments in brain size. Small variation within these developmental mechanisms contributes to the evolution of brain diversity. Comparing the specification and spatial distribution of neural phenotypes across bilaterians has also suggested some major brain evolution trends, although much more work on profiling neural connections with neurochemical specificity across a wide diversity of organisms is needed. These comparative approaches investigating the evolution of brain form and function hold great promise for facilitating a mechanistic understanding of how variation in brain morphology, neural phenotypes, and neural networks influences brain function and behavioral diversity across organisms.

Forebrain and hindbrain development in zebrafish is sensitive to ethanol exposure involving agrin, Fgf, and sonic hedgehog function

BACKGROUND

Ethanol is a teratogen that affects numerous developmental processes in the nervous system, which includes development and survival of GABAergic and glutamatergic neurons. Possible molecular mechanisms accounting for ethanol's effects on nervous system development include perturbed fibroblast growth factor (Fgf) and Sonic hedgehog (Shh) signaling. In zebrafish, forebrain GABAergic neuron development is dependent on Fgf19 and Shh signaling. The present study was conducted to test the hypothesis that ethanol affects GABAergic and glutamatergic neuron development by disrupting Fgf, Shh, and agrin function.

METHODS

Zebrafish embryos were exposed to varying concentrations of ethanol during a range of developmental stages, in the absence or presence of morpholino oligonucleotides (MOs) that disrupt agrin or Shh function. In situ hybridization was used to analyze glutamic acid decarboxylase (GAD1) gene expression, as well as markers of glutamatergic neurons.

RESULTS

Acute ethanol exposure results in marked reduction in GAD1 gene expression in forebrain and hindbrain, and reduction of glutamatergic neuronal markers in hindbrain. Subthreshold ethanol exposure, combined with agrin or Shh MO treatment, produces a similar diminution in expression of markers for GABAergic and glutamatergic neurons. Consistent with the ethanol effects on Fgf and Shh pathways, Fgf19, Fgf8, or Shh mRNA overexpression rescues ethanol-induced decreases in GAD1 and Atonal1a gene expression.

CONCLUSIONS

These studies demonstrate that GABAergic and glutamatergic neuron development in zebrafish forebrain or cerebellum is sensitive to ethanol exposure, and provides additional evidence that a signaling pathway involving agrin, Fgfs and Shh may be a critical target of ethanol exposure during zebrafish embryogenesis.

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.

MANF regulates dopaminergic neuron development in larval zebrafish

Mesencephalic astrocyte derived neurotrophic factor (MANF) is recognized as a dopaminergic neurotrophic factor, which can protect dopaminergic neurons from neurotoxic damage. However, little is known about the function of MANF during the vertebrate development. Here, we report that MANF expression is widespread during embryonic development and in adult organs analyzed by qPCR and in situ hybridization in zebrafish. Knockdown of MANF expression with antisense splice-blocking morpholino oligonucleotides resulted in no apparent abnormal phenotype. Nevertheless, the dopamine level of MANF morphants was lower than that of the wild type larvae, the expression levels of the two tyrosine hydroxylase gene transcripts were decreased and a decrease in neuron number in certain groups of th1 and th2 cells in the diencephalon region in MANF morphants was observed. These defects were rescued by injection of exogenous manf mRNA. Strikingly, manf mRNA could partly restore the decrease of th1 positive cells in Nr4a2-deficient larvae. These results suggest that MANF is involved in the regulation of the development of dopaminergic system in zebrafish.

Analysis of transcriptional codes for zebrafish dopaminergic neurons reveals essential functions of Arx and Isl1 in prethalamic dopaminergic neuron development

Distinct groups of dopaminergic neurons develop at defined anatomical sites in the brain to modulate function of a large diversity of local and far-ranging circuits. However, the molecular identity as judged from transcription factor expression has not been determined for all dopaminergic groups. Here, we analyze regional expression of transcription factors in the larval zebrafish brain to determine co-expression with the Tyrosine hydroxylase marker in dopaminergic neurons. We define sets of transcription factors that clearly identify each dopaminergic group. These data confirm postulated relations to dopaminergic groups defined for mammalian systems. We focus our functional analysis on prethalamic dopaminergic neurons, which co-express the transcription factors Arx and Isl1. Morpholino-based knockdown reveals that both Arx and Isl1 are strictly required for prethalamic dopaminergic neuron development and appear to act in parallel. We further show that Arx contributes to patterning in the prethalamic region, while Isl1 is required for differentiation of prethalamic dopaminergic neurons.

The vertebrate mesolimbic reward system and social behavior network: a comparative synthesis

All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation.

miR-132 regulates the differentiation of dopamine neurons by directly targeting Nurr1 expression

Although it is well established that embryonic stem (ES) cells have the potential to differentiate into dopamine neurons, the molecular basis of this process, particularly the role of microRNAs (miRNAs), remains largely unknown. Here we report that miR-132 plays a key role in the differentiation of dopamine neurons by directly regulating the expression of Nurr1 (also known as nuclear receptor subfamily 4 group A member 2; Nr4a2). We constructed a mouse ES cell line CGR8, which stably expresses GFP under the tyrosine hydroxylase (TH) promoter, so the TH-positive neurons could be easily sorted using fluorescence-activated cell sorting (FACS). Then, we performed a miRNA array analysis on the purified TH-positive neurons and found that 45 of 585 miRNAs had more than a fivefold change in expression level during dopamine neuron differentiation. Among the 45 miRNAs, we were particularly interested in miR-132 because this miRNA has been reported to be highly expressed in neurons and to have a potential role in neurodegenerative diseases. We found that the direct downregulation of endogenous miR-132 induced by miR-132 antisense oligonucleotide (miR-132-ASO) promoted the differentiation of TH-positive neurons, whereas ectopic expression of miR-132 in ES cells reduced the number of differentiated TH-positive neurons but did not change the total number of differentiated neurons. Furthermore, we identified that miR-132-ASO could substantially reverse the miR-132-mediated suppression of TH-positive neuron differentiation. Moreover, through a bioinformatics assay we identified the Nurr1 gene as a potential molecular target of miR-132. Using a luciferase-reporter assay and western blot analysis, we demonstrated that miR-132 could directly regulate the expression of Nurr1. Collectively, our data provide the first evidence that miR-132 is an important molecule regulating ES cell differentiation into dopamine neurons by directly targeting Nurr1 gene expression.

Nurr1 regulates Top IIβ and functions in axon genesis of mesencephalic dopaminergic neurons

Background

NURR1 (also named as NR4A2) is a member of the steroid/thyroid hormone receptor family, which can bind to DNA and modulate expression of target genes. Previous studies have shown that NURR1 is essential for the nigral dopaminergic neuron phenotype and function maintenance, and the defects of the gene are possibly associated with Parkinson's disease (PD).

Results

In this study, we used new born Nurr1 knock-out mice combined with Affymetrix genechip technology and real time polymerase chain reaction (PCR) to identify Nurr1 regulated genes, which led to the discovery of several transcripts differentially expressed in the nigro-striatal pathway of Nurr1 knock-out mice. We found that an axon genesis gene called Topoisomerase IIβ (Top IIβ) was down-regulated in Nurr1 knock-out mice and we identified two functional NURR1 binding sites in the proximal Top IIβ promoter. While in Top IIβ null mice, we saw a significant loss of dopaminergic neurons in the substantial nigra and lack of neurites along the nigro-striatal pathway. Using specific TOP II antagonist ICRF-193 or Top IIβ siRNA in the primary cultures of ventral mesencephalic (VM) neurons, we documented that suppression of TOP IIβ expression resulted in VM neurites shortening and growth cones collapsing. Furthermore, microinjection of ICRF-193 into the mouse medial forebrain bundle (MFB) led to the loss of nigro-striatal projection.

Conclusion

Taken together, our findings suggest that Top IIβ might be a down-stream target of Nurr1, which might influence the processes of axon genesis in dopaminergic neurons via the regulation of TOP IIβ expression. The Nurr1-Top IIβ interaction may shed light on the pathologic role of Nurr1 defect in the nigro-striatal pathway deficiency associated with PD.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Characterization of the dopamine system in the brain of the túngara frog, Physalaemus pustulosus

Dopamine is an evolutionarily ancient neurotransmitter that plays an essential role in mediating behavior. In vertebrates, dopamine is central to the mesolimbic reward system, a neural network concerned with the valuation of stimulus salience, and to the nigrostriatal motor system and hypothalamic nuclei involved in the regulation of locomotion and social behavior. In amphibians, dopaminergic neurons have been mapped out in several species, yet the distribution of dopaminoreceptive cells is unknown. The túngara frog, Physalaemus pustulosus, is an excellent model system for the study of neural mechanisms by which valuations of stimuli salience and social decisions are made, especially in the context of mate choice. In order to better understand where dopamine acts to regulate social decisions in this species, we have determined the distribution of putative dopaminergic cells (using tyrosine hydroxylase immunohistochemistry) and cells receptive to dopaminergic signaling (using DARPP-32 immunohistochemistry) throughout the brain of P. pustulosus. The distribution of dopaminergic cells was comparable to other anurans. DARPP-32 immunoreactivity was identified in key brain regions known to modulate social behavior in other vertebrates including the proposed anuran homologues of the mammalian amygdalar complex, nucleus accumbens, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area/substantia nigra pars compacta. Due to its widespread distribution, DARPP-32 likely also plays many roles in non-limbic brain regions that mediate non-social information processing. These results significantly extend our understanding of the distribution of the dopaminergic system in the anuran brain and beyond.

Distribution of sex steroid hormone receptors in the brain of an African cichlid fish, Astatotilapia burtoni

Sex steroid hormones released from the gonads play an important role in mediating social behavior across all vertebrates. Many effects of these gonadal hormones are mediated by nuclear steroid hormone receptors, which are crucial for integration in the brain of external (e.g., social) signals with internal physiological cues to produce an appropriate behavioral output. The African cichlid fish Astatotilapia burtoni presents an attractive model system for the study of how internal cues and external social signals are integrated in the brain as males display robust plasticity in the form of two distinct, yet reversible, behavioral and physiological phenotypes depending on the social environment. In order to better understand where sex steroid hormones act to regulate social behavior in this species, we have determined the distribution of the androgen receptor, estrogen receptor alpha, estrogen receptor beta, and progesterone receptor mRNA and protein throughout the telencephalon and diencephalon and some mesencephalic structures of A. burtoni. All steroid hormone receptors were found in key brain regions known to modulate social behavior in other vertebrates including the proposed teleost homologs of the mammalian amygdalar complex, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area. Overall, there is high concordance of mRNA and protein labeling. Our results significantly extend our understanding of sex steroid pathways in the cichlid brain and support the important role of nuclear sex steroid hormone receptors in modulating social behaviors in teleosts and across vertebrates.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Nuclear receptor unfulfilled regulates axonal guidance and cell identity of Drosophila mushroom body neurons

Nuclear receptors (NRs) comprise a family of ligand-regulated transcription factors that control diverse critical biological processes including various aspects of brain development. Eighteen NR genes exist in the Drosophila genome. To explore their roles in brain development, we knocked down individual NRs through the development of the mushroom bodies (MBs) by targeted RNAi. Besides recapitulating the known MB phenotypes for three NRs, we found that unfulfilled (unf), an ortholog of human photoreceptor specific nuclear receptor (PNR), regulates axonal morphogenesis and neuronal subtype identity. The adult MBs develop through remodeling of γ neurons plus de-novo elaboration of both α′/β′ and α/β neurons. Notably, unf is largely dispensable for the initial elaboration of γ neurons, but plays an essential role in their re-extension of axons after pruning during early metamorphosis. The subsequently derived MB neuron types also require unf for extension of axons beyond the terminus of the pruned bundle. Tracing single axons revealed misrouting rather than simple truncation. Further, silencing unf in single-cell clones elicited misguidance of axons in otherwise unperturbed MBs. Such axon guidance defects may occur as MB neurons partially lose their subtype identity, as evidenced by suppression of various MB subtype markers in unf knockdown MBs. In sum, unf governs axonal morphogenesis of multiple MB neuron types, possibly through regulating neuronal subtype identity.

Orphan nuclear receptor Nur77 is required for the differentiation of C6 glioma cells induced by cholera toxin

AIM

To investigate a possible regulator gene involved in the cholera toxin-induced differentiation of rat C6 glioma cells.

METHODS

The global changes in the mRNA expression pattern induced by cholera toxin were analyzed using gene chip microarray. The selected gene was then silenced by RNA interference or overexpressed with an ORF plasmid to determine its necessity in this process.

RESULTS

Nur77, a member of the orphan nuclear receptor family (NR4A), was markedly up-regulated during the process of differentiation. Furthermore, RNAi of nur77 attenuated the induction effect of cholera toxin on C6 cells, whereas overexpression of nur77 led to similarly differentiated behavior, including morphologic and biomarker changes, as well as cell cycle arrest.

CONCLUSION

Nur77 participated actively and essentially as an important regulator in the cholera toxin-induced differentiation of C6 cells.

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

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