CIAO1 and MMS19 deficiency: A lethal neurodegenerative phenotype caused by cytosolic Fe-S cluster protein assembly disorders

PURPOSE

The functionality of many cellular proteins depends on cofactors; yet, they have only been implicated in a minority of Mendelian diseases. Here, we describe the first 2 inherited disorders of the cytosolic iron-sulfur protein assembly system.

METHODS

Genetic testing via genome sequencing was applied to identify the underlying disease cause in 3 patients with microcephaly, congenital brain malformations, progressive developmental and neurologic impairments, recurrent infections, and a fatal outcome. Studies in patient-derived skin fibroblasts and zebrafish models were performed to investigate the biochemical and cellular consequences.

RESULTS

Metabolic analysis showed elevated uracil and thymine levels in body fluids but no pathogenic variants in DPYD, encoding dihydropyrimidine dehydrogenase. Genome sequencing identified compound heterozygosity in 2 patients for missense variants in CIAO1, encoding cytosolic iron-sulfur assembly component 1, and homozygosity for an in-frame 3-nucleotide deletion in MMS19, encoding the MMS19 homolog, cytosolic iron-sulfur assembly component, in the third patient. Profound alterations in the proteome, metabolome, and lipidome were observed in patient-derived fibroblasts. We confirmed the detrimental effect of deficiencies in CIAO1 and MMS19 in zebrafish models.

CONCLUSION

A general failure of cytosolic and nuclear iron-sulfur protein maturation caused pleiotropic effects. The critical function of the cytosolic iron-sulfur protein assembly machinery for antiviral host defense may well explain the recurrent severe infections occurring in our patients.

Loss of dlx5a/dlx6a Locus Alters Non-Canonical Wnt Signaling and Meckel's Cartilage Morphology

The dlx genes encode transcription factors that establish a proximal-distal polarity within neural crest cells to bestow a regional identity during craniofacial development. The expression regions of dlx paralogs are overlapping yet distinct within the zebrafish pharyngeal arches and may also be involved in progressive morphologic changes and organization of chondrocytes of the face. However, how each dlx paralog of dlx1a, dlx2a, dlx5a and dlx6a affects craniofacial development is still largely unknown. We report here that the average lengths of the Meckel's, palatoquadrate and ceratohyal cartilages in different dlx mutants were altered. Mutants for dlx5a-/- and dlx5i6-/-, where the entire dlx5a/dlx6a locus was deleted, have the shortest lengths for all three structures at 5 days post fertilization (dpf). This phenotype was also observed in 14 dpf larvae. Loss of dlx5i6 also resulted in increased proliferation of neural crest cells and expression of chondrogenic markers. Additionally, altered expression and function of non-canonical Wnt signaling were observed in these mutants suggesting a novel interaction between dlx5i6 locus and non-canonical Wnt pathway regulating ventral cartilage morphogenesis.

Effects of PFOS, F-53B and OBS on locomotor behaviour, the dopaminergic system and mitochondrial function in developing zebrafish (Danio rerio)

Perfluorooctanesulfonic acid (PFOS) has widely been reported to persist in the environment and to elicit neurotoxicological effects in wildlife and humans. Following the restriction of PFOS use, 6:2 chlorinated polyfluorinated ether sulfonate (F-53B) and sodium p-perfluorous nonenoxybenzene sulfonate (OBS) have emerged as novel PFOS alternatives and have been detected in the environment. However, knowledge on the toxicological effects of these alternatives remains scarce. Using developing transgenic Tg(dat:eGFP) zebrafish, we evaluated the consequences of exposure to 0, 0.1 and 1 mg/l PFOS, F-53B and OBS on the dopaminergic system, locomotor behaviour and mitochondrial function. All compounds generally reduced locomotor activity under light conditions irrespective of exposure concentration. Exposure to OBS (at all concentrations), as well as PFOS and F-53B (at 1 mg/l), significantly reduced subpallial dopaminergic neuron abundance. PFOS also significantly reduced dat and pink1 expression irrespective of exposure concentration, while F-53B and OBS tended to reduce mitochondrial pink1 and fis1 expression across concentrations without reaching statistical significance. Mitochondrial function, in the form of reduced oxygen consumption rate and marginally inhibited ATP-linked oxygen consumption rate, was affected only in response to 1 mg/l PFOS. Together, PFOS and the emerging contaminants F-53B and OBS inhibit locomotion at similar concentrations, a finding correlated with decreased dopaminergic neuron numbers in the subpallium and decreased expression of pink1. These findings are relevant to wildlife and human health, as they suggest that PFOS as well as replacement compounds affect locomotion likely in part by negatively impacting the dopamine system.

Animal Models of Neurological Disorders: Where Are We Now?

The Special Issue "Animal Models of Neurological Disorders: Where Are We Now [...].

Dopamine in Health and Disease

The neurotransmitter dopamine (DA) is generally associated with Parkinson's disease (PD) [...].

Increased Sociability in Mice Lacking Intergenic Dlx Enhancers

The Dlx homeodomain transcription factors play important roles in the differentiation and migration of GABAergic interneuron precursors. The mouse and human genomes each have six Dlx genes organized into three convergently transcribed bigene clusters (Dlx1/2, Dlx3/4, and Dlx5/6) with cis-regulatory elements (CREs) located in the intergenic region of each cluster. Amongst these, the I56i and I12b enhancers from the Dlx1/2 and Dlx5/6 locus, respectively, are active in the developing forebrain. I56i is also a binding site for GTF2I, a transcription factor whose function is associated with increased sociability and Williams-Beuren syndrome. In determining the regulatory roles of these CREs on forebrain development, we have generated mutant mouse-lines where Dlx forebrain intergenic enhancers have been deleted (I56i^(-/-), I12b^(-/-)). Loss of Dlx intergenic enhancers impairs expression of Dlx genes as well as some of their downstream targets or associated genes including Gad2 and Evf2. The loss of the I56i enhancer resulted in a transient decrease in GABA⁺ cells in the developing forebrain. The intergenic enhancer mutants also demonstrate increased sociability and learning deficits in a fear conditioning test. Characterizing mice with mutated Dlx intergenic enhancers will help us to further enhance our understanding of the role of these Dlx genes in forebrain development.

Effects of low-dose methylcyclopentadienyl manganese tricarbonyl-derived manganese on the development of diencephalic dopaminergic neurons in zebrafish

Fuel additive methylcyclopentadienyl manganese tricarbonyl (MMT) is counted as an organic manganese (Mn)-derived compound. The toxic effects of Mn (alone and complexed) on dopaminergic (DA) neurotransmission have been investigated in both cellular and animal models. However, the impact of environmentally relevant Mn exposure on DA neurodevelopment is rather poorly understood. In the present study, the MMT dose of 100 μM (about 5 mg Mn/L) caused up-regulation of DA-related genes in association with cell body swelling and increase in the number of DA neurons of the ventral diencephalon subpopulation DC2. Furthermore, our analysis identified significant brain Mn bioaccumulation and enhancement of total dopamine levels in association with locomotor hyperactivity. Although DA levels were restored at adulthood, we observed a deficit in the acquisition and consolidation of memory. Collectively, these findings suggest that developmental exposure to low-level MMT-derived Mn is responsible for the selective alteration of diencephalic DA neurons and with long-lasting effects on fish explorative behaviour in adulthood.

Cerebroventricular Microinjections of MPTP on Adult Zebrafish Induces Dopaminergic Neuronal Death, Mitochondrial Fragmentation, and Sensorimotor Impairments

Mitochondria are dynamic organelles that mediate the energetic supply to cells and mitigate oxidative stress through the intricate balance of fission and fusion. Mitochondrial dysfunction is a prominent feature within Parkinson disease (PD) etiologies. To date, there have been conflicting studies of neurotoxin impact on dopaminergic cell death, mitochondrial function and behavioral impairment using adult zebrafish. Here, we performed cerebroventricular microinjections (CVMIs) of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on adult transgenic zebrafish that resulted in significant reductions in dopaminergic neurons within the telencephalon and olfactory bulbs (OB) of Tg(dat:eGFP) fish. Visualization of mCherry and mitochondrial gene expression analysis in Tg(dat:tom20 MLS:mCherry) fish reveal that MPTP induces mitochondrial fragmentation in dopaminergic neurons and the activation of the pink1/parkin pathway involved mitophagy. Moreover, the loss of dopaminergic neurons translated into a transient locomotor and olfactory phenotype. Taken together, these data can contribute to a better understanding of the mitochondrial impact on dopaminergic survivability.

Novel cross-regulation interactions between dlx genes in larval zebrafish

The homeodomain-containing transcription factors dlx1a, dlx2a, dlx5a and dlx6a are expressed in the zebrafish brain in overlapping patterns and are important in vertebrate development. Previous work in mice have suggested the overlapping expression pattern is in part due to cross-regulatory interactions among the aforementioned dlx genes. However, the extent of these interactions and whether they are conserved among vertebrates remains to be determined. Through whole-mount in situ hybridization in zebrafish dlx mutants produced by CRISPR-Cas9 mutagenesis, cross-regulatory interactions between dlx1a, dlx2a, dlx5a and dlx6a were examined from 24 to 72 h post fertilization (hpf). Notably, and different from previous work done in mouse, zebrafish dlx2a^-/- mutants continue to express dlx5a until 72hpf, whereas deletion of both enhancers within the dlx5a/dlx6a locus resulted in delayed dlx5a/dlx6a expression and relative increased dlx2a expression. These results suggest alternative regulatory elements and pathways exist to mediate dlx expression in zebrafish and may highlight evolutionary differences in gene interactions between vertebrates.

Loss of parla Function Results in Inactivity, Olfactory Impairment, and Dopamine Neuron Loss in Zebrafish

The presenilin-associated rhomboid-like (PARL) gene was found to contribute to mitochondrial morphology and function and was linked to familial Parkinson's disease (PD). The PARL gene product is a mitochondrial intramembrane cleaving protease that acts on a number of mitochondrial proteins involved in mitochondrial morphology, apoptosis, and mitophagy. To date, functional and genetic studies of PARL have been mainly performed in mammals. However, little is known about PARL function and its role in dopaminergic (DA) neuron development in vertebrates. The zebrafish genome comprises two PARL paralogs: parla and parlb. Here, we established a loss-of-function mutation in parla via CRISPR/Cas9-mediated mutagenesis. We examined DA neuron numbers in the adult brain and expression of genes associated with DA neuron function in larvae and adults. We show that loss of parla function results in loss of DA neurons, mainly in the olfactory bulb. Changes in the levels of tyrosine hydroxylase transcripts supported this neuronal loss. Expression of fis1, a gene involved in mitochondrial fission, was increased in parla mutants. Finally, we showed that loss of parla function translates into impaired olfaction and altered locomotion parameters. These results suggest a role for parla in the development and/or maintenance of DA neuron function in zebrafish.

Visualizing traumatic brain injuries

Zebrafish larvae models can be used to study the link between seizures and the neurodegeneration that follows brain trauma.

A triple-drug nanotherapy to target breast cancer cells, cancer stem cells, and tumor vasculature

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, accounting for the majority of breast cancer-related death. Due to the lack of specific therapeutic targets, chemotherapeutic agents (e.g., paclitaxel) remain the mainstay of systemic treatment, but enrich a subpopulation of cells with tumor-initiating capacity and stem-like characteristics called cancer stem cells (CSCs); thus development of a new and effective strategy for TNBC treatment is an unmet medical need. Cancer nanomedicine has transformed the landscape of cancer drug development, allowing for a high therapeutic index. In this study, we developed a new therapy by co-encapsulating clinically approved drugs, such as paclitaxel, verteporfin, and combretastatin (CA4) in polymer-lipid hybrid nanoparticles (NPs) made of FDA-approved biomaterials. Verteporfin is a drug used in the treatment of macular degeneration and has recently been found to inhibit the Hippo/YAP (Yes-associated protein) pathway, which is known to promote the progression of breast cancer and the development of CSCs. CA4 is a vascular disrupting agent and has been tested in phase II/III of clinical trials. We found that our new three drug-NP not only effectively inhibited TNBC cell viability and cell migration, but also significantly diminished paclitaxel-induced and/or CA4-induced CSC enrichment in TNBC cells, partially through inhibiting the upregulated Hippo/YAP signaling. Combination of verteporfin and CA4 was also more effective in suppressing angiogenesis in an in vivo zebrafish model than single drug alone. The efficacy and application potential of our triple drug-NPs were further assessed by using clinically relevant patient-derived xenograft (PDX) models. Triple drug-NP effectively inhibited the viability of PDX organotypic slide cultures ex vivo and stopped the growth of PDX tumors in vivo. This study developed an approach capable of simultaneously inhibiting bulk cancer cells, CSCs, and angiogenesis.

Dopaminergic neurons regenerate following chemogenetic ablation in the olfactory bulb of adult Zebrafish (Danio rerio)

Adult zebrafish have the ability to regenerate cells of the central nervous system. However, few neuronal regeneration studies in adult zebrafish addressed their ability to regenerate specific types of neurons following cell specific ablation. We show here that treatment of transgenic Tg(dat:CFP-NTR) adult zebrafish with the prodrug metronidazole (Mtz) according to our administration regimen predominantly ablates dopamine (DA) neurons within the olfactory bulb (OB) of adult fish. Loss of DA neurons was accompanied by an impaired olfaction phenotype, as early as 1-week post-treatment, in which fish were unable to sense the presence of the repulsive stimulus cadaverine. The olfactory impairment was reversed within 45 days and coincided with the recovery of DA neuron counts in the OB. A multi-label pulse-chase analysis with BrdU and EdU over the first seventeen days-post Mtz exposure showed that newly formed DA neurons were recruited within the first nine days following exposure and led to functional and morphological recovery of the OB.

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.

Expression of dlx genes in the normal and regenerating brain of adult zebrafish

Dysfunctions in the GABAergic system lead to various pathological conditions and impaired inhibitory function is one of the causes behind neuropathies characterized by neuronal hyper excitability. The Dlx homeobox genes are involved in the development of nervous system, neural crest, branchial arches and developing appendages. Dlx genes also take part in neuronal migration and differentiation during development, more precisely, in the migration and differentiation of GABAergic neurons. Functional analysis of dlx genes has mainly been carried out in developing zebrafish embryos and larvae, however information regarding the expression and roles of these genes in the adult zebrafish brain is still lacking. The extensive neurogenesis that takes place in the adult zebrafish brain, makes them a good model for the visualization of mechanisms involving dlx genes during adulthood in physiological conditions and during regeneration of the nervous system. We have identified the adult brain regions where transcripts of dlx1a, dlx2a, dlx5a and dlx6a genes are normally found and have confirmed that within telencephalic domains, there is high overlapping expression of the four dlx paralogs with a marker for GABAergic neurons. Co-localization analyses carried with the Tg(dlx6a-1.4kbdlx5a/dlx6a:GFP) reporter line have also shown that in some areas of the diencephalon, cells expressing the dlx5a/6a bigene may have a neural stem cell identity. Furthermore, investigations in a response to stab wound lesions, have demonstrated a possible participation of the dlx5a/6a bigene, most likely of dlx5a, during regeneration of the adult zebrafish brain. These observations suggest a possible participation of dlx-expressing cells during brain regeneration in adult zebrafish and also provide information on the role of dlx genes under normal physiological conditions in adults.

PLPHP deficiency: clinical, genetic, biochemical, and mechanistic insights

Biallelic pathogenic variants in PLPBP (formerly called PROSC) have recently been shown to cause a novel form of vitamin B6-dependent epilepsy, the pathophysiological basis of which is poorly understood. When left untreated, the disease can progress to status epilepticus and death in infancy. Here we present 12 previously undescribed patients and six novel pathogenic variants in PLPBP. Suspected clinical diagnoses prior to identification of PLPBP variants included mitochondrial encephalopathy (two patients), folinic acid-responsive epilepsy (one patient) and a movement disorder compatible with AADC deficiency (one patient). The encoded protein, PLPHP is believed to be crucial for B6 homeostasis. We modelled the pathogenicity of the variants and developed a clinical severity scoring system. The most severe phenotypes were associated with variants leading to loss of function of PLPBP or significantly affecting protein stability/PLP-binding. To explore the pathophysiology of this disease further, we developed the first zebrafish model of PLPHP deficiency using CRISPR/Cas9. Our model recapitulates the disease, with plpbp-/- larvae showing behavioural, biochemical, and electrophysiological signs of seizure activity by 10 days post-fertilization and early death by 16 days post-fertilization. Treatment with pyridoxine significantly improved the epileptic phenotype and extended lifespan in plpbp-/- animals. Larvae had disruptions in amino acid metabolism as well as GABA and catecholamine biosynthesis, indicating impairment of PLP-dependent enzymatic activities. Using mass spectrometry, we observed significant B6 vitamer level changes in plpbp-/- zebrafish, patient fibroblasts and PLPHP-deficient HEK293 cells. Additional studies in human cells and yeast provide the first empirical evidence that PLPHP is localized in mitochondria and may play a role in mitochondrial metabolism. These models provide new insights into disease mechanisms and can serve as a platform for drug discovery.

Posterior axis formation requires Dlx5/Dlx6 expression at the neural plate border

Neural tube defects (NTDs), one of the most common birth defects in human, present a multifactorial etiology with a poorly defined genetic component. The Dlx5 and Dlx6 bigenic cluster encodes two evolutionary conserved homeodomain transcription factors, which are necessary for proper vertebrate development. It has been shown that Dlx5/6 genes are essential for anterior neural tube closure, however their role in the formation of the posterior structures has never been described. Here, we show that Dlx5/6 expression is required during vertebrate posterior axis formation. Dlx5 presents a similar expression pattern in neural plate border cells during posterior neurulation of zebrafish and mouse. Dlx5/6-inactivation in the mouse results in a phenotype reminiscent of NTDs characterized by open thoracic and lumbar vertebral arches and failure of epaxial muscle formation at the dorsal midline. The dlx5a/6a zebrafish morphants present posterior NTDs associated with abnormal delamination of neural crest cells showing altered expression of cell adhesion molecules and defects of motoneuronal development. Our findings provide new molecular leads to decipher the mechanisms of vertebrate posterior neurulation and might help to gather a better understanding of human congenital NTDs etiology.

High-throughput DNA Extraction and Genotyping of 3dpf Zebrafish Larvae by Fin Clipping

Zebrafish (Danio rerio) possess orthologues for 84% of the genes known to be associated with human diseases. In addition, these animals have a short generation time, are easy to handle, display a high reproductive rate, low cost, and are easily amenable to genetic manipulations by microinjection of DNA in embryos. Recent advances in gene editing tools are enabling precise introduction of mutations and transgenes in zebrafish. Disease modeling in zebrafish often leads to larval phenotypes and early death which can be challenging to interpret if genotypes are unknown. This early identification of genotypes is also needed in experiments requiring sample pooling, such as in gene expression or mass spectrometry studies. However, extensive genotypic screening is limited by traditional methods, which in most labs are performed only on adult zebrafish or in postmortem larvae. We addressed this problem by adapting a method for the isolation of PCR-ready genomic DNA from live zebrafish larvae that can be achieved as early as 72 h post-fertilization (hpf). This time and cost-effective technique, improved from a previously published genotyping protocol, allows the identification of genotypes from microscopic fin biopsies. The fins quickly regenerate as the larvae develop. Researchers are then able to select and raise the desired genotypes to adulthood by utilizing this high-throughput PCR-based genotyping procedure.

Expression of Thiaminase in Zebrafish (Danio rerio) is Lethal and Has Implications for Use as a Biocontainment Strategy in Aquaculture and Invasive Species

As the world increasingly relies on aquaculture operations to meet rising seafood demands, reliable biocontainment measures for farmed fish stocks are desired to minimize ecological impacts arising from interactions of cultured fish with wild populations. One possible biocontainment strategy is to induce a dietary dependence on a vitamin, such as thiamine (vitamin B1), required for survival. Fish expressing thiaminase (an enzyme that degrades thiamine) within a confined aquaculture facility could receive supplemental thiamine to allow survival and normal growth, whereas escapees lacking this dietary rescue would die from thiamine deficiency. To test the concept and efficacy of such a dietary dependency system (for potential future use in larger aquaculture species), we expressed thiaminase in zebrafish as a test model. We drove the expression of thiaminase under the strong ubiquitous and constitutive control of the CMV promoter which resulted in non-viable fish, indicating that the thiaminase sequence kills fish. However, the CMV promoter is too strong to allow conditional survival since the lethality could not be rescued by exogenous thiamine provided as a supplement to typical food. In addition, microinjection of 0.5 pg of thiaminase mRNA in zebrafish embryos at the one-cell stage resulted in 50% larval mortality at 5 days post-fertilization (dpf), which was partially rescued by thiamine supplementation. Evaluating the efficacy of biocontainment strategies helps assess which methods can reliably prevent ecological impacts arising from breaches in physical containment systems that release engineered organisms to nature, and consequently provides critical information for use in regulatory risk assessment processes.

Pyridoxine-Dependent Epilepsy in Zebrafish Caused by Aldh7a1 Deficiency

Pyridoxine-dependent epilepsy (PDE) is a rare disease characterized by mutations in the lysine degradation gene ALDH7A1 leading to recurrent neonatal seizures, which are uniquely alleviated by high doses of pyridoxine or pyridoxal 5'-phosphate (vitamin B6 vitamers). Despite treatment, neurodevelopmental disabilities are still observed in most PDE patients underlining the need for adjunct therapies. Over 60 years after the initial description of PDE, we report the first animal model for this disease: an aldh7a1-null zebrafish (Danio rerio) displaying deficient lysine metabolism and spontaneous and recurrent seizures in the larval stage (10 days postfertilization). Epileptiform electrographic activity was observed uniquely in mutants as a series of population bursts in tectal recordings. Remarkably, as is the case in human PDE, the seizures show an almost immediate sensitivity to pyridoxine and pyridoxal 5'-phosphate, with a resulting extension of the life span. Lysine supplementation aggravates the phenotype, inducing earlier seizure onset and death. By using mass spectrometry techniques, we further explored the metabolic effect of aldh7a1 knockout. Impaired lysine degradation with accumulation of PDE biomarkers, B6 deficiency, and low γ-aminobutyric acid levels were observed in the aldh7a1-/- larvae, which may play a significant role in the seizure phenotype and PDE pathogenesis. This novel model provides valuable insights into PDE pathophysiology; further research may offer new opportunities for drug discovery to control seizure activity and improve neurodevelopmental outcomes for PDE.

Lineage tracing of dlx1a/2a and dlx5a/6a expressing cells in the developing zebrafish brain

Lineage tracing of specific populations of progenitor cells provides crucial information about developmental programs. Four members of the Dlx homeobox gene family, Dlx1,2, 5 and 6, are involved in the specification of γ-aminobutyric acid (GABA)ergic neurons in the vertebrate forebrain. Orthologous genes in mammals and teleost show similarities in expression patterns and transcriptional regulation mechanisms. We have used lineage tracing to permanently label dlx-expressing cells in the zebrafish and have characterized the progeny of these cells in the larva and in the juvenile and adult brain. We have found that dlx1a/2a and dlx5a/6a expressing progenitors give rise, for the most part, to small populations of cells which constitute only a small proportion of GABAergic cells in the adult brain tissue. Moreover, some of the cells do not acquire a neuronal phenotype suggesting that, regardless of the time a cell expresses dlx genes in the brain, it can potentially give rise to cells other than neurons. In some instances, labeling larval dlx5a/6a-expressing cells, but not dlx1a/2a-expressing cells, results in massively expanding, widespread clonal expansion throughout the adult brain. Our data provide a detailed lineage analysis of the dlx1a/2a and dlx5a/6a expressing progenitors in the zebrafish brain and lays the foundation for further characterization of the role of these transcription factors beyond the specification of GABAergic neurons.

Functional consequences of I56ii Dlx enhancer deletion in the developing mouse forebrain

Dlx homeobox genes encode a group of transcription factors that play an essential role during developmental processes including maintaining the differentiation, proliferation and migration of GABAergic interneurons. The Dlx1/2 and Dlx5/6 genes are expressed in the forebrain and are arranged in convergently transcribed bigene clusters, with I12a/I12b and I56i/I56ii cis-regulatory elements (CREs) located in the intergenic region of each cluster respectively. We have characterized the phenotypic consequences of deleting I56ii on forebrain development and spatial patterning of corridor cells that are involved in guiding thalamocortical projections. Here we report that deletion of I56ii impairs expression of Dlx genes and that of potential targets including Gad2 as well as striatal markers Islet1, Meis2, and Ebf1. In addition, I56ii deletion reduces both the binding of DLX2 in the Dlx5/Dlx6 intergenic region and the presence of H3K9Ac at the Dlx5/Dlx6 locus, consistent with the reduced expression of these genes. Deletion of I56ii reduces the expression of the ISLET1 and CTIP2 in the striatum and disrupts the number of parvalbumin and calretinin expressing cells in the adult somatosensory cortex of the ΔI56ii mice. These data suggest an important regulatory role for I56ii in the developing forebrain by means of a potential regulatory mechanism which may regulate the expression of Dlx genes, notably Dlx6 as well as the spatial patterning of the ventral telencephalon, including possibly corridor cells.

Delayed effects of methylmercury on the mitochondria of dopaminergic neurons and developmental toxicity in zebrafish larvae (Danio rerio)

Methylmercury (MeHg) is a known neurotoxicant affecting the central nervous system but effects on dopaminergic (DA) neurons are not well understood. Wild-type zebrafish (Danio rerio) and two transgenic lines: Tg(dat:eGFP) expressing enhanced green fluorescent protein (eGFP) in DA neuron clusters and Tg(dat:tom20 MLS-mCherry) expressing red fluorescence (mCherry) targeted to mitochondria of DA neurons were used to evaluate the effects of micromolar MeHg exposure on DA neuron and whole animal motor function during early development. Three-day-old larvae were exposed to micromolar concentrations of MeHg (0.03, 0.06, and 0.3μM) in system water. Exposure to 0.3μM MeHg caused mortality and significant morphological abnormalities including edema, curvature of the spine, and hemorrhages in zebrafish larvae after a 48h exposure period. At 0.06μM MeHg, the appearance of morphological abnormalities was delayed for 72h and far less severe, whereas 0.03μM MeHg did not cause any morphological defects or mortalities. A delayed but significant reduction in locomotor ability and mCherry fluorescence in specific brain regions in the 0.06μM MeHg exposed larvae suggests that DA neuron function rather than neuron numbers was compromised. Double immunolabeling with tyrosine hydroxylase and pan neural staining showed no effect of MeHg exposure. We have established Tg(dat:tom20 MLS-mCherry) zebrafish larvae as a model which can be used to assess MeHg neurotoxicity and that exposure to low dose MeHg (0.06μM) during development may predispose DA neurons to impairment caused by changes in mitochondrial dynamics.

Rotenone Neurotoxicity Causes Dopamine Neuron Loss in Zebrafish

ABSTRACT:Objectives: We sought to determine whether rotenone, a commonly used pesticide, exhibits neurotoxicity in zebrafish by causing dopamine neuron loss through rotenone-induced oxidative damage. Methods: We exposed transgenic zebrafish embryos expressing green fluorescent protein under the control of the cis-regulatory elements of dopamine transporter (dat) to rotenone to determine the neurotoxic effects of rotenone on dopamine neuron abundance and pattern distribution, as well as the presence of apoptotic markers. The oxidative stress potential of rotenone on embryos was assessed using a live MitoSOX Red assay, and behavioural testing on adult zebrafish was assessed using video recordings of midline crossing events. Results: Zebrafish embryos treated with rotenone displayed a 50% reduction in dopamine neurons in the ventral diencephalon when exposed to 30µM rotenone (n=6, p<0.001), and rotenone-exposed zebrafish raised to adulthood demonstrate an anxiety-like behaviour (n=5, p<0.01). Furthermore embryos exposed to rotenone also demonstrated a logarithmic increase in markers of oxidative damage (n=3, p<0.001) and apoptotic activity in their diencephalic neurons. Conclusions: These results show that rotenone can induce dopamine neuron loss in zebrafish, providing a useful model for studying the environmental causes of Parkinson’s disease. RÉSUMÉ:Objectif: Nous cherchons à déterminer si la roténone, un élément commun dans les pesticides, démontre de la neurotoxicité dans les poissons-zèbres en causant une perte de dopamine dans leurs neurones à travers le dommage oxydatif induit par la roténone. Méthode: Nous avons exposé des embryons de poissons-zèbres transgéniques qui expriment la protéine fluorescente verte sous le contrôle d’éléments cis-régulateurs des transporteurs sélectifs de dopamine (dat) à la roténone pour déterminer les effets neurotoxiques de ce dernier sur les niveaux dopaminergiques dans leurs neurones. De plus, nous avons évalué la présence de marqueurs apoptotiques. Le stress oxydatif potentiel de la roténone sur les embryons a été analysé par le « live MitoSOX Red assay » et les tests comportementaux sur les poissons-zèbres adultes furent analysés en utilisant des enregistrements vidéo. Résultats: Les embryons de poissons-zèbres qui ont été traités avec la roténone ont démontré une réduction de dopamine de 50% dans les neurones localisés dans le diencéphale ventral, quand exposés à 30µM de roténone (n=6, p<0.001). Ils ont également illustré une augmentation logarithmique dans les marqueurs de dommage oxydatif (n=3, p<0.001) et une activité apoptotique dans les neurones du diencéphale. Les poissons-zèbres exposés à de la roténone qui ont atteint l’âge adulte ont démontré des comportements d’anxiété (n=5, p<0.01). Conclusion: Les résultats démontrent que la roténone peut induire une perte dopaminergique dans les neurones des poissons-zèbres. Ces résultats s’avèrent utiles pour étudier davantage les causes environnementales reliées à la maladie de Parkinson.

Transgenic Zebrafish Expressing mCherry in the Mitochondria of Dopaminergic Neurons

Genetic mutations and environmental toxins are known to affect mitochondrial health and have been implicated in the progressive degeneration of dopaminergic neurons in Parkinson's disease. To visualize mitochondria in dopaminergic neurons of live zebrafish, we used the regulatory elements of the dopamine transporter (dat) gene to target a reporter, mCherry, after fusion with the mitochondrial localizing signal (MLS) of Tom20. Immunoblot analysis of mitochondrial and cytosolic fractions from Tg(dat:tom20 MLS-mCherry) larvae shows that mCherry is efficiently targeted to the mitochondria. Confocal imaging of live fish was carried out from 1 day postfertilization (dpf) to 9 dpf. We also colocalized dat mRNA expression with the mCherry protein in the olfactory bulb (OB), subpallium (SP), pretectum (Pr), diencephalic clusters 2 and 3 (DC2/3), caudal hypothalamus (Hc), locus coeruleus (LC), anterior preoptic area (POa), retinal amacrine cells (RAC), caudal hypothalamus (Hc), and preoptic area (PO). Treating Tg(dat:tom20 MLS-mCherry) larvae with the dopaminergic neurotoxin MPTP (1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine) at 2 or 3 dpf resulted in a decrease in mCherry fluorescence in the pretectum, olfactory bulb, subpallium, diencephalic clusters 2 and 3, and the caudal hypothalamus. Labeling of mitochondria in nigrostriatal dopaminergic neurons of zebrafish could allow their visualization in vivo following genetic or pharmacological manipulations.

Chemogenetic ablation of dopaminergic neurons leads to transient locomotor impairments in zebrafish larvae

To determine the impact of a controlled loss of dopaminergic neurons on locomotor function, we generated transgenic zebrafish, Tg(dat:CFP-NTR), expressing a cyan fluorescent protein-nitroreductase fusion protein (CFP-NTR) under the control of dopamine transporter (dat) cis-regulatory elements. Embryonic and larval zebrafish express the transgene in several groups of dopaminergic neurons, notably in the olfactory bulb, telencephalon, diencephalon and caudal hypothalamus. Administration of the pro-drug metronidazole (Mtz) resulted in activation of caspase 3 in CFP-positive neurons and in a reduction in dat-positive cells by 5 days post-fertilization (dpf). Loss of neurons coincided with impairments in global locomotor parameters such as swimming distance, percentage of time spent moving, as well as changes in tail bend parameters such as time to maximal bend and angular velocity. Dopamine levels were transiently decreased following Mtz administration. Recovery of some of the locomotor parameters was observed by 7 dpf. However, the total numbers of dat-expressing neurons were still decreased at 7, 12, or 14 dpf, even though there was evidence for production of new dat-expressing cells. Tg(dat:CFP-NTR) zebrafish provide a model to correlate altered dopaminergic neuron numbers with locomotor function and to investigate factors influencing regeneration of dopaminergic neurons.

Relative developmental toxicities of pentachloroanisole and pentachlorophenol in a zebrafish model (Danio rerio)

Pentachloroanisole (PCA) and pentachlorophenol (PCP) are chlorinated aromatic compounds that have been found in the environment and in human populations. The objective of this study is to characterize the effects of PCA in comparison to those of PCP on development at environmental relevant levels using a fish model. Zebrafish embryos were exposed to 0.1, 1, 10, 100, 500, 1000 μg/L PCA and PCP respectively for 96 h. Malformation observation, LC50 testing for survival rate at 96 hours post fertilization (hpf) and EC50 testing for hatching rate at 72 hpf indicated that the developmental toxicity of PCP was about 15 times higher than that of PCA. PCP exposure at 10 μg/L resulted in elevated 3, 3', 5-triiodothyronine (T3) levels and decreased thyroxine (T4) levels, whereas PCA had no effects on T3 or T4 levels. PCP and PCA exposure at 1 and 10 μg/L showed possible hyperthyroid effects similar to that of T3, due to increased relative mRNA expression of synapsin I (SYN), iodothyronine deiodinase type III (Dio3), thyroid hormone receptor alpha a (THRαa) and thyroid hormone receptor beta (THRβ), and decreased expression of iodothyronine deiodinase type II (Dio2). The results indicate that both PCA and PCP exposure can cause morphological deformities, possibly affect the timing and coordination of development in the central nervous system, and alter thyroid hormone levels by disrupting thyroid hormone regulating pathways. However, the developmental toxicity of PCA is at least ten times lower than that of PCP. Our results on the relative developmental toxicities of PCA and PCP and the possible underlying mechanisms will be useful to support interpretation of envrionmental concentrations and body burden levels observed in human populations.

The dlx5a/dlx6a genes play essential roles in the early development of zebrafish median fin and pectoral structures

The Dlx5 and Dlx6 genes encode homeodomain transcription factors essential for the proper development of limbs in mammalian species. However, the role of their teleost counterparts in fin development has received little attention. Here, we show that dlx5a is an early marker of apical ectodermal cells of the pectoral fin buds and of the median fin fold, but also of cleithrum precursor cells during pectoral girdle development. We propose that early median fin fold establishment results from the medial convergence of dlx5a-expressing cells at the lateral edges of the neural keel. Expression analysis also shows involvement of dlx5a during appendage skeletogenesis. Using morpholino-mediated knock down, we demonstrate that disrupted dlx5a/6a function results in pectoral fin agenesis associated with misexpression of bmp4, fgf8a, and1 and msx genes. In contrast, the median fin fold presents defects in mesenchymal cell migration and actinotrichia formation, whereas the initial specification seems to occur normally. Our results demonstrate that the dlx5a/6a genes are essential for the induction of pectoral fin outgrowth, but are not required during median fin fold specification. The dlx5a/6a knock down also causes a failure of cleithrum formation associated with a drastic loss of runx2b and col10a1 expression. The data indicate distinct requirements for dlx5a/6a during median and pectoral fin development suggesting that initiation of unpaired and paired fin formation are not directed through the same molecular mechanisms. Our results refocus arguments on the mechanistic basis of paired appendage genesis during vertebrate evolution.

Developmental processes regulated by the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway: highlights from animal studies

The 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is the rate-limiting enzyme in the biosynthesis of cholesterol and isoprenoids, which are substrates required for post-translational modification of signalling proteins that can potentially regulate various aspects of embryonic development. The HMGCR transcripts are detectable during early embryogenesis in both invertebrates and vertebrates, which suggests a conserved developmental requirement for mevalonate derivatives. Consistently, recent animal and in vitro studies have yielded valuable insights into potential morphogenic parameters that are modulated by HMGCR activity. These developmental end-points include brain and craniofacial morphogenesis, PGC migration and survival, myocardial epithelial migration and fusion, EC migration and survival, and vascular stabilization. By providing a synthesis of these studies, we hope that this review will highlight the need to comprehensively examine the entire suite of developmental processes regulated by HMGCR.

Heterogeneous conservation of Dlx paralog co-expression in jawed vertebrates

Background

The Dlx gene family encodes transcription factors involved in the development of a wide variety of morphological innovations that first evolved at the origins of vertebrates or of the jawed vertebrates. This gene family expanded with the two rounds of genome duplications that occurred before jawed vertebrates diversified. It includes at least three bigene pairs sharing conserved regulatory sequences in tetrapods and teleost fish, but has been only partially characterized in chondrichthyans, the third major group of jawed vertebrates. Here we take advantage of developmental and molecular tools applied to the shark Scyliorhinus canicula to fill in the gap and provide an overview of the evolution of the Dlx family in the jawed vertebrates. These results are analyzed in the theoretical framework of the DDC (Duplication-Degeneration-Complementation) model.

Results

The genomic organisation of the catshark Dlx genes is similar to that previously described for tetrapods. Conserved non-coding elements identified in bony fish were also identified in catshark Dlx clusters and showed regulatory activity in transgenic zebrafish. Gene expression patterns in the catshark showed that there are some expression sites with high conservation of the expressed paralog(s) and other expression sites with events of paralog sub-functionalization during jawed vertebrate diversification, resulting in a wide variety of evolutionary scenarios within this gene family.

Conclusion

Dlx gene expression patterns in the catshark show that there has been little neo-functionalization in Dlx genes over gnathostome evolution. In most cases, one tandem duplication and two rounds of vertebrate genome duplication have led to at least six Dlx coding sequences with redundant expression patterns followed by some instances of paralog sub-functionalization. Regulatory constraints such as shared enhancers, and functional constraints including gene pleiotropy, may have contributed to the evolutionary inertia leading to high redundancy between gene expression patterns.

The ascl1a and dlx genes have a regulatory role in the development of GABAergic interneurons in the zebrafish diencephalon

During development of the mouse forebrain interneurons, the Dlx genes play a key role in a gene regulatory network (GRN) that leads to the GABAergic phenotype. Here, we have examined the regulatory relationships between the ascl1a, dlx, and gad1b genes in the zebrafish forebrain. Expression of ascl1a overlaps with dlx1a in the telencephalon and diencephalon during early forebrain development. The loss of Ascl1a function results in a loss of dlx expression, and subsequent losses of dlx5a and gad1b expression in the diencephalic prethalamus and hypothalamus. Loss of Dlx1a and Dlx2a function, and, to a lesser extent, of Dlx5a and Dlx6a, impairs gad1b expression in the prethalamus and hypothalamus. We conclude that dlx1a/2a act downstream of ascl1a but upstream of dlx5a/dlx6a and gad1b to activate GABAergic specification. This pathway is conserved in the diencephalon, but has diverged between mammals and teleosts in the telencephalon.

The 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway regulates developmental cerebral-vascular stability via prenylation-dependent signalling pathway

Spontaneous intracranial hemorrhage is a debilitating form of stroke, often leading to death or permanent cognitive impairment. Many of the causative genes and the underlying mechanisms implicated in developmental cerebral-vascular malformations are unknown. Recent in vitro and in vivo studies in mice have shown inhibition of the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway to be effective in stabilizing cranial vessels. Using a combination of pharmacological and genetic approaches to specifically inhibit the HMGCR pathway in zebrafish (Danio rerio), we demonstrate a requirement for this metabolic pathway in developmental vascular stability. Here we report that inhibition of HMGCR function perturbs cerebral-vascular stability, resulting in progressive dilation of blood vessels, followed by vessel rupture, mimicking cerebral cavernous malformation (CCM)-like lesions in humans and murine models. The hemorrhages in the brain are rescued by prior exogenous supplementation with geranylgeranyl pyrophosphate (GGPP), a 20-carbon metabolite of the HMGCR pathway, required for the membrane localization and activation of Rho GTPases. Consistent with this observation, morpholino-induced depletion of the β-subunit of geranylgeranyltransferase I (GGTase I), an enzyme that facilitates the post-translational transfer of the GGPP moiety to the C-terminus of Rho family of GTPases, mimics the cerebral hemorrhaging induced by the pharmacological and genetic ablation of HMGCR. In embryos with cerebral hemorrhage, the endothelial-specific expression of cdc42, a Rho GTPase involved in the regulation of vascular permeability, was significantly reduced. Taken together, our data reveal a metabolic contribution to the stabilization of nascent cranial vessels, requiring protein geranylgeranylation acting downstream of the HMGCR pathway.

Functional prediction and physiological characterization of a novel short trans-membrane protein 1 as a subunit of mitochondrial respiratory complexes

Mitochondrial respiration is mediated by a set of multisubunit assemblies of proteins that are embedded in the mitochondrial inner membranes. Respiratory complexes do not only contain central catalytic subunits essential for the bioenergetic transformation, but also many short trans-membrane subunits (sTMs) that are implicated in the proper assembly of complexes. Defects in sTMs have been discovered in some human neurodegenerative diseases. Here we identify a new subunit that we named Stmp1 and have characterized its function using both computational and experimental approaches. Stmp1 is a short trans-membrane protein, and sequence/structure analysis revealed that it shares common features like the small size, presence of a single or two TM region, and a COOH-terminal charged region, as many typical sTMs of respiratory complexes. In situ hybridization and RT-PCR assays showed that the Stmp1 expression is ubiquitous throughout zebrafish embryogenesis. In adults, Stmp1 expression was highest in the brain compared with muscle and liver. In zebrafish larvae (3-5 days postfertilization), antisense morpholino oligonucleotide-mediated knockdown of the Stmp1 gene (Stmp1-MO) resulted in a series of mild morphological defects, including abnormal shape of head and jaw and cardiac edema. Larvae injected with the Stmp1-MO had negligible responses to touch stimuli. By ventilation frequency analysis we found that Stmp1-MO-injected zebrafish displayed a severe dysfunction of ventilatory activities when exposed to hypoxic conditions, suggesting a defective mitochondrial activity induced by the loss of Stmp1. Phylogenetic profiling of known respiratory sTMs compared with Stmp1 revealed that all defined sTMs from four respiratory complexes have restricted or variable phyletic distribution, indicating that they are products of evolutionary innovations to fulfill lineage-related functional requirements for respiratory complexes. Thus, being present in animals, filasterea, choanoflagellida, amoebozoa, and plants, Stmp1 may have evolved to confer a new or complementary regulation of respiratory activities.

Influences of dietary biotin and avidin on growth, survival, deficiency syndrome and hepatic gene expression of juvenile Nile tilapia Oreochromis niloticus

This study was undertaken to assess the interactive effects of dietary biotin and avidin on growth, feed conversion, survival and deficiency syndrome of tilapia and to determine the influence of dietary biotin deficiency on the expression of key genes related to biotin metabolism in tilapia. Six iso-nitrogenous and iso-energetic diets based on a common purified basal diet (vitamin-free casein as the protein source) were prepared for this study. The six dietary groups were 0 g avidin with 0 mg biotin (A0B0), 0 g avidin with 0.06 mg biotin/kg diet (A0B1), four avidin-supplemented diets incorporating at a incremental concentrations 0.25, 0.5, 1.0 and 2.0 g/kg diet with 0.06 mg biotin/kg diet (A15B1, A30B1, A60B1 and A120B1). Fish were hand-fed three times a day to apparent satiation for 12 weeks. Each diet was fed to three replicate groups of fish. Fish were kept in glass aquaria in a recirculating aquaculture system under standardized environmental conditions. Growth was significantly higher in fish that received the biotin-supplemented diet (A0B1), compared to diets lacking biotin or supplemented with avidin. Tilapia fed higher concentration of avidin-supplemented diets (A60B1 and A120B1) showed significant growth depression and displayed severe deficiency syndromes such as lethargy, anorexia, circular swimming and convulsions, which ultimately lead to death. There was a strong proportional linear relationship between the avidin content of the diet and feed conversion ratio, FCR (y = 0.43x + 0.135; r = 0.960; P < 0.001) and strong inverse relationship with protein efficiency ratio, PER (y = -0.309x + 2.195; r = 0.961; P < 0.0001). Elevated levels of biotinidase, pyruvate carboxylase, propionyl-CoA carboxylase-A and propionyl-CoA carboxylase-B transcripts were noted in fish fed all graded level of avidin-supplemented diets. A broken-line analysis indicated that feeding tilapia a diet with 44.5 times more avidin than the dietary biotin requirement can induce deficiency syndromes including retarded growth, when analyzing the data of percentage weight gain.

Zebrafish Parla- and Parlb-deficiency affects dopaminergic neuron patterning and embryonic survival

Many genes associated with familial Parkinson's disease contribute to mitochondrial morphology and function. Some of these genes, for example, Pink1 and Parkin, are part of a common pathway. The presenilin-associated rhomboid-like (PARL) gene was recently linked to familial Parkinson's disease. The PARL gene product is found in the inner mitochondrial membrane and cleaves the optic atrophy 1 protein, involved in mitochondrial morphology and apoptosis. In Drosophila, the PARL-related rhomboid-7 gene acts upstream of pink1 and parkin. However, such a genetic relationship is still unknown in vertebrates. Here, we show that the zebrafish genome comprises two parl paralogs: parla and parlb. Morpholino-mediated loss of parla and/or parlb function resulted in mild neurodegeneration, as evidenced by a lower density of dopaminergic neurons. Patterning of dopaminergic neurons was also perturbed in the ventral diencephalon. Morphants exhibited extensive cell death throughout the entire body as well as increased larval mortality. The morphant phenotype could be rescued by injection of human PARL mRNA, but not catalytically inactive PARL, suggesting functional conservation between the human and zebrafish proteins. More importantly, the zebrafish pink1 mRNA as well as the human PINK1 mRNA, but not kinase-dead nor Parkinson's disease-linked mutant PINK1 mRNA, also rescued the morphant phenotype, providing evidence that Parl genes may function upstream of Pink1, as part of a conserved pathway in vertebrates.

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.

Modeling neurodegeneration in zebrafish

The zebrafish, Danio rerio, has been established as an excellent vertebrate model for the study of developmental biology and gene function. It also has proven to be a valuable model to study human diseases. Here, we reviewed recent publications using zebrafish to study the pathology of human neurodegenerative diseases including Parkinson's, Huntington's, and Alzheimer's. These studies indicate that zebrafish genes and their human homologues have conserved functions with respect to the etiology of neurodegenerative diseases. The characteristics of the zebrafish and the experimental approaches to which it is amenable make this species a useful complement to other animal models for the study of pathologic mechanisms of neurodegenerative diseases and for the screening of compounds with therapeutic potential.

In vivo and in vitro assessment of cardiac beta-adrenergic receptors in larval zebrafish (Danio rerio)

β-Adrenergic receptors (βARs) are crucial for maintaining the rate and force of cardiac muscle contraction in vertebrates. Zebrafish (Danio rerio) have one β1AR gene and two β2AR genes (β2aAR and β2bAR). We examined the roles of these receptors in larval zebrafish in vivo by assessing the impact of translational gene knockdown on cardiac function. Zebrafish larvae lacking β1AR expression by morpholino knockdown displayed lower heart rates than control fish, whereas larvae deficient in both β2aAR and β2bAR expression exhibited significantly higher heart rates than controls. These results suggested a potential inhibitory role for one or both β2AR genes. By using cultured HEK293 cells transfected with zebrafish βARs, we demonstrated that stimulation with adrenaline or procaterol (a β2AR agonist) resulted in an increase in intracellular cAMP levels in cells expressing any of the three zebrafish βARs. In comparison with its human βAR counterpart, zebrafish β2aAR expressed in HEK293 cells appeared to exhibit a unique binding affinity profile for adrenergic ligands. Specifically, zebrafish β2aAR had a high binding affinity for phenylephrine, a classical α-adrenergic receptor agonist. The zebrafish receptors also had distinct ligand binding affinities for adrenergic agonists when compared with human βARs in culture, with zebrafish β2aAR being distinct from human β2AR and zebrafish β2bAR. Overall, this study provides insight into the function and evolution of both fish and mammalian β-adrenergic receptors.

A key role for poly(ADP-ribose) polymerase 3 in ectodermal specification and neural crest development

Background

The PARP family member poly(ADP-ribose) polymerase 3 (PARP3) is structurally related to the well characterized PARP1 that orchestrates cellular responses to DNA strand breaks and cell death by the synthesis of poly(ADP-ribose). In contrast to PARP1 and PARP2, the functions of PARP3 are undefined. Here, we reveal critical functions for PARP3 during vertebrate development.

Principal Findings

We have used several in vitro and in vivo approaches to examine the possible functions of PARP3 as a transcriptional regulator, a function suggested from its previously reported association with several Polycomb group (PcG) proteins. We demonstrate that PARP3 gene occupancy in the human neuroblastoma cell line SK-N-SH occurs preferentially with developmental genes regulating cell fate specification, tissue patterning, craniofacial development and neurogenesis. Addressing the significance of this association during zebrafish development, we show that morpholino oligonucleotide-directed inhibition of parp3 expression in zebrafish impairs the expression of the neural crest cell specifier sox9a and of dlx3b/dlx4b, the formation of cranial sensory placodes, inner ears and pectoral fins. It delays pigmentation and severely impedes the development of the median fin fold and tail bud.

Conclusion

Our findings demonstrate that Parp3 is crucial in the early stages of zebrafish development, possibly by exerting its transcriptional regulatory functions as early as during the specification of the neural plate border.

The relationship between dlx and gad1 expression indicates highly conserved genetic pathways in the zebrafish forebrain

The Dlx genes encode a family of transcription factors important for the development of the vertebrate forebrain. These genes have very similar expression domains during the development of the telencephalon in mice and play a role in gamma-aminobutyric acid (GABAergic) interneuron differentiation. We have used triple fluorescent in situ hybridization to study the relative expression domains of the dlx and gad1 genes in the zebrafish telencephalon and diencephalon. We also generated transgenic zebrafish with regulatory elements from the zebrafish dlx1a/2a locus. The zebrafish dlx regulatory elements recapitulated dlx expression in the forebrain and mimicked the relationship between the expression of the dlx genes and gad1. Finally, we show that a putative enhancer located downstream of dlx2b can also activate reporter gene expression in a tissue-specific manner similar to endogenous dlx2b expression. Our results indicate the dlx genes are regulated by an evolutionarily conserved genetic pathway and may play a role in GABAergic interneuron differentiation in the zebrafish forebrain.