Phototoxicity avoidance is a potential therapeutic approach for retinal dystrophy caused by EYS dysfunction

Inherited retinal dystrophies (IRDs) are progressive diseases leading to vision loss. Mutation in the eyes shut homolog (EYS) gene is one of the most frequent causes of IRD. However, the mechanism of photoreceptor cell degeneration by mutant EYS has not been fully elucidated. Here, we generated retinal organoids from induced pluripotent stem cells (iPSCs) derived from patients with EYS-associated retinal dystrophy (EYS-RD). In photoreceptor cells of RD organoids, both EYS and G protein-coupled receptor kinase 7 (GRK7), one of the proteins handling phototoxicity, were not in the outer segment, where they are physiologically present. Furthermore, photoreceptor cells in RD organoids were vulnerable to light stimuli, and especially to blue light. Mislocalization of GRK7, which was also observed in eys-knockout zebrafish, was reversed by delivering control EYS into photoreceptor cells of RD organoids. These findings suggest that avoiding phototoxicity would be a potential therapeutic approach for EYS-RD.

In Vivo Optogenetic Phase Transition of an Intrinsically Disordered Protein

Proteins containing intrinsically disordered regions (IDRs) control a wide variety of cellular processes by assembly of membrane-less organelles via IDR-mediated liquid-liquid phase separation. Dysregulated IDR-mediated phase transition has been implicated in the pathogenesis of diseases characterized by deposition of abnormal protein aggregates. Here, we describe a method to enhance interactions between the IDRs of the RNA/DNA-binding protein and TAR DNA-binding protein 43 (TDP-43) by light to drive its phase transition in the motor neurons of zebrafish. The optically controlled TDP-43 phase transition in motor neurons, in vivo, provides a unique opportunity to evaluate the impact of dysregulated TDP-43 phase transition on the physiology of motor neurons. This will help to address the etiology of neurodegenerative diseases associated with abnormal TDP-43 phase transition and aggregation, including amyotrophic lateral sclerosis (ALS).

Dysregulated TDP-43 proteostasis perturbs excitability of spinal motor neurons during brainstem-mediated fictive locomotion in zebrafish

Spinal motor neurons (SMNs) are the primary target of degeneration in amyotrophic lateral sclerosis (ALS). Degenerating motor neurons accumulate cytoplasmic TAR DNA-binding protein 43 (TDP-43) aggregates in most ALS cases. This SMN pathology can occur without mutation in the coding sequence of the TDP-43-encoding gene, TARDBP. Whether and how wild-type TDP-43 drives pathological changes in SMNs in vivo remains largely unexplored. In this study, we develop a two-photon calcium imaging setup in which tactile-evoked neural responses of motor neurons in the brainstem and spinal cord can be monitored using the calcium indicator GCaMP. We devise a piezo-assisted tactile stimulator that reproducibly evokes a brainstem descending neuron upon tactile stimulation of the head. A direct comparison between caudal primary motor neurons (CaPs) with or without TDP-43 overexpression in contiguous spinal segments demonstrates that CaPs overexpressing TDP-43 display attenuated Ca2+ transients during fictive escape locomotion evoked by the tactile stimulation. These results show that excessive amounts of TDP-43 protein reduce the neuronal excitability of SMNs and potentially contribute to asymptomatic pathological lesions of SMNs and movement disorders in patients with ALS.

[Optogenetic interrogation of TDP-43 cytotoxicity in a zebrafish ALS model]

TAR DNA-binding protein 43 (TDP-43) is an evolutionarily conserved RNA/DNA-binding protein that is nuclear-enriched in healthy cells, but deposited in the cytoplasm as aggregates in affected neurons in certain neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). We have previously developed an optogenetic TDP-43 variant (opTDP-43h) whose oligomerization status can be modulated via the CRY2olig tag, which self-assembles upon absorption of blue light. Illumination of zebrafish spinal motor neurons expressing opTDP-43h with a blue light triggers its cytoplasmic mislocalization, eventually leading to cytoplasmic deposition of opTDP-43h aggregates. Intriguingly, a light illumination-dependent transient opTDP-43 mislocalization can halt motor axon outgrowth, even in the absence of cytoplasmic deposition of opTDP-43 aggregates. These observations point toward an oligomerization-dependent, but aggregation-independent, cytotoxic effect of TDP-43 that might contribute to pathogenesis of ALS. In the present review, we would like to overview the zebrafish ALS model based on the optogenetic TDP-43, and then discuss about the potential mechanisms of TDP-43 cytotoxicity that trigger and/or promote motor neuron degeneration in ALS.

Optogenetic Phase Transition of TDP-43 in Spinal Motor Neurons of Zebrafish Larvae

Abnormal protein aggregation and selective neuronal vulnerability are two major hallmarks of neurodegenerative diseases. Causal relationships between these features may be interrogated by controlling the phase transition of a disease-associated protein in a vulnerable cell type, although this experimental approach has been limited so far. Here, we describe a protocol to induce phase transition of the RNA/DNA-binding protein TDP-43 in spinal motor neurons of zebrafish larvae for modeling cytoplasmic aggregation of TDP-43 occurring in degenerating motor neurons in amyotrophic lateral sclerosis (ALS). We describe a bacterial artificial chromosome (BAC)-based genetic method to deliver an optogenetic TDP-43 variant selectively to spinal motor neurons of zebrafish. The high translucency of zebrafish larvae allows for the phase transition of the optogenetic TDP-43 in the spinal motor neurons by a simple external illumination using a light-emitting diode (LED) against unrestrained fish. We also present a basic workflow of live imaging of the zebrafish spinal motor neurons and image analysis with freely available Fiji/ImageJ software to characterize responses of the optogenetic TDP-43 to the light illumination. This protocol enables the characterization of TDP-43 phase transition and aggregate formation in an ALS-vulnerable cellular environment, which should facilitate an investigation of its cellular and behavioral consequences.

Multi-phaseted problems of TDP-43 in selective neuronal vulnerability in ALS

Transactive response DNA-binding protein 43 kDa (TDP-43) encoded by the TARDBP gene is an evolutionarily conserved heterogeneous nuclear ribonucleoprotein (hnRNP) that regulates multiple steps of RNA metabolism, and its cytoplasmic aggregation characterizes degenerating motor neurons in amyotrophic lateral sclerosis (ALS). In most ALS cases, cytoplasmic TDP-43 aggregation occurs in the absence of mutations in the coding sequence of TARDBP. Thus, a major challenge in ALS research is to understand the nature of pathological changes occurring in wild-type TDP-43 and to explore upstream events in intracellular and extracellular milieu that promote the pathological transition of TDP-43. Despite the inherent obstacles to analyzing TDP-43 dynamics in in vivo motor neurons due to their anatomical complexity and inaccessibility, recent studies using cellular and animal models have provided important mechanistic insights into potential links between TDP-43 and motor neuron vulnerability in ALS. This review is intended to provide an overview of the current literature on the function and regulation of TDP-43-containing RNP granules or membraneless organelles, as revealed by various models, and to discuss the potential mechanisms by which TDP-43 can cause selective vulnerability of motor neurons in ALS.

Illuminating ALS Motor Neurons With Optogenetics in Zebrafish

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by progressive degeneration of motor neurons in the brain and spinal cord. Spinal motor neurons align along the spinal cord length within the vertebral column, and extend long axons to connect with skeletal muscles covering the body surface. Due to this anatomy, spinal motor neurons are among the most difficult cells to observe in vivo. Larval zebrafish have transparent bodies that allow non-invasive visualization of whole cells of single spinal motor neurons, from somas to the neuromuscular synapses. This unique feature, combined with its amenability to genome editing, pharmacology, and optogenetics, enables functional analyses of ALS-associated proteins in the spinal motor neurons in vivo with subcellular resolution. Here, we review the zebrafish skeletal neuromuscular system and the optical methods used to study it. We then introduce a recently developed optogenetic zebrafish ALS model that uses light illumination to control oligomerization, phase transition and aggregation of the ALS-associated DNA/RNA-binding protein called TDP-43. Finally, we will discuss how this disease-in-a-fish ALS model can help solve key questions about ALS pathogenesis and lead to new ALS therapeutics.

Gsx2 is required for specification of neurons in the inferior olivary nuclei from Ptf1a-expressing neural progenitors in zebrafish

Neurons in the inferior olivary nuclei (IO neurons) send climbing fibers to Purkinje cells to elicit functions of the cerebellum. IO neurons and Purkinje cells are derived from neural progenitors expressing the proneural gene ptf1a In this study, we found that the homeobox gene gsx2 was co-expressed with ptf1a in IO progenitors in zebrafish. Both gsx2 and ptf1a zebrafish mutants showed a strong reduction or loss of IO neurons. The expression of ptf1a was not affected in gsx2 mutants, and vice versa. In IO progenitors, the ptf1a mutation increased apoptosis whereas the gsx2 mutation did not, suggesting that ptf1a and gsx2 are regulated independently of each other and have distinct roles. The fibroblast growth factors (Fgf) 3 and 8a, and retinoic acid signals negatively and positively, respectively, regulated gsx2 expression and thereby the development of IO neurons. mafba and Hox genes are at least partly involved in the Fgf- and retinoic acid-dependent regulation of IO neuronal development. Our results indicate that gsx2 mediates the rostro-caudal positional signals to specify the identity of IO neurons from ptf1a-expressing neural progenitors.

Optogenetic modulation of TDP-43 oligomerization accelerates ALS-related pathologies in the spinal motor neurons

Cytoplasmic aggregation of TDP-43 characterizes degenerating neurons in most cases of amyotrophic lateral sclerosis (ALS). Here, we develop an optogenetic TDP-43 variant (opTDP-43), whose multimerization status can be modulated in vivo through external light illumination. Using the translucent zebrafish neuromuscular system, we demonstrate that short-term light stimulation reversibly induces cytoplasmic opTDP-43 mislocalization, but not aggregation, in the spinal motor neuron, leading to an axon outgrowth defect associated with myofiber denervation. In contrast, opTDP-43 forms pathological aggregates in the cytoplasm after longer-term illumination and seeds non-optogenetic TDP-43 aggregation. Furthermore, we find that an ALS-linked mutation in the intrinsically disordered region (IDR) exacerbates the light-dependent opTDP-43 toxicity on locomotor behavior. Together, our results propose that IDR-mediated TDP-43 oligomerization triggers both acute and long-term pathologies of motor neurons, which may be relevant to the pathogenesis and progression of ALS.

Optogenetic approaches for inducing TDP-43 aggregation have been described previously in cellular models. Here the authors develop an approach to optogenetically induce TDP-43 aggregation in vivo using zebrafish to model ALS pathologies.

Evaluating Clinical and Cost Impacts of Achieving 90% HPV Vaccination Rate Against Cervical Cancer in Canada Using the OncoSim Cancer Simulation Model

Background: Cervical cancer can be largely prevented through vaccination against the human papillomavirus (HPV). In Canada, HPV vaccination of school-aged girls started in 2008 and has reached an average rate of 67% across the country. However, this rate is below a current national target of 90%. Aim: We project the difference in lifetime clinical outcomes and health system costs of achieving a 90% HPV vaccination rate versus a 67% rate for girls vaccinated in 2015 using the OncoSim-Cervical model. Methods: The OncoSim-Cervical model (version 2.5) is a microsimulation model led by the Canadian Partnership Against Cancer, with model development by Statistics Canada, to evaluate the impacts of cervical cancer interventions in Canada. It has two parts, the first, the HPV Microsimulation Model, simulates the transmission of HPV between males and females including possible modulation by vaccination and herd immunity, and provides projections of HPV infection and prevalence for input to the cervical cancer natural history component in the second part. We simulated two cohorts of 5-10 year old girls in 2015; one receiving HPV vaccination at the rate of 67% and another at 90%. Their relative lifetime cervical cancer outcomes and costs were compared. Assumptions included: 100% efficacy of the HPV vaccine; triennial cytology screening between ages 21 and 65; 90% screening recruitment of age-eligible women with 80% rescreening; and a $500 cost for a 3-dose quadrivalent HPV vaccine regimen. Projected costs were undiscounted and are in 2016 CAD. Results: Compared with the 67%-vaccinated cohort, in the 90%-vaccinated cohort there was a lifetime reduction of 23% in cervical cancer incident cases and 21% in cervical cancer deaths. Lifetime cancer treatment costs decreased by $26 million (23%), and wart treatment costs and precervical cancer costs decreased by $3.2 million (15%) and $45 million (16%) respectively. Lifetime screening costs decreased by $47 million (2%). Conclusion: Achieving a nationally-set target of 90% HPV vaccination in Canadian girls would not only save more lives but would free up funds that could be redirected to other health system needs. Planners could aim for relatively high nation-wide rates of HPV vaccination coverage to enhance their cervical cancer control strategy.

The OncoSim Cancer Simulation Platform: A Tool to Project the Population Effects of Cancer Control Interventions in Canada

Background: Cancer control interventions exert their effects over multiple decades. To evaluate diverse and competing opportunities to reduce future cancer burden it is desirable to understand long-term effects prior to any new program implementation or significant change. Internationally, modeling is becoming an accepted source of planning information for decision-makers. Aim: We will describe the construction and use of the OncoSim microsimulation model, which was developed to evaluate cancer control strategies in Canada. Methods: OncoSim is a suite of models (cancers of the lung, colorectum, cervix and breast, plus a composite 32-cancer model) used to address key policy questions and support decision-making. It is led by the Canadian Partnership Against Cancer with model development by Statistics Canada. OncoSim incorporates risk factors, cancer natural history, screening, treatment, survival and end-of-life care. Wherever possible it is informed by Canadian data sources. Models are calibrated to reproduce a range of cancer-specific statistics, e.g., current and historical Canadian cancer-specific incidence and mortality, smoking patterns, and results of screening. The site-specific models have undergone further validation by replicating reported short-term effects of cancer prevention and screening interventions. Users may customize interventions through modifying input parameters. Outputs include incidence, mortality, costs, cost-effectiveness, and resource utilization. Users from the public sector have access at no cost to OncoSim and receive extensive support from a multidisciplinary technical team. The model is continually updated to incorporate emerging knowledge. Results: OncoSim has been used to support cancer control decision-making at the national and provincial/territorial levels. Applications include: national guidelines recommendations for colorectal and lung cancer screening; comparison of cytology vs. HPV based cervical cancer screening; and integration of smoking cessation into low-dose CT lung cancer screening. Conclusion: Validated simulation models such as OncoSim can be a versatile and efficient tool for cancer control planners to evaluate and prioritize cancer control strategies.

The OncoSim model: development and use for better decision-making in Canadian cancer control

The Canadian Partnership Against Cancer was created in 2007 by the federal government to accelerate cancer control across Canada. Its OncoSim microsimulation model platform, which consists of a suite of specific cancer models, was conceived as a tool to augment conventional resources for population-level policy- and decision-making. The Canadian Partnership Against Cancer manages the OncoSim program, with funding from Health Canada and model development by Statistics Canada. Microsimulation modelling allows for the detailed capture of population heterogeneity and health and demographic history over time. Extensive data from multiple Canadian sources were used as inputs or to validate the model. OncoSim has been validated through expert consultation; assessments of face validity, internal validity, and external validity; and model fit against observed data. The platform comprises three in-depth cancer models (lung, colorectal, cervical), with another in-depth model (breast) and a generalized model (25 cancers) being in development. Unique among models of its class, OncoSim is available online for public sector use free of charge. Users can customize input values and output display, and extensive user support is provided. OncoSim has been used to support decision-making at the national and jurisdictional levels. Although simulation studies are generally not included in hierarchies of evidence, they are integral to informing cancer control policy when clinical studies are not feasible. OncoSim can evaluate complex intervention scenarios for multiple cancers. Canadian decision-makers thus have a powerful tool to assess the costs, benefits, cost-effectiveness, and budgetary effects of cancer control interventions when faced with difficult choices for improvements in population health and resource allocation.

Glycine-alanine dipeptide repeat protein contributes to toxicity in a zebrafish model of C9orf72 associated neurodegeneration

Background

The most frequent genetic cause of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) is the expansion of a GGGGCC hexanucleotide repeat in a non-coding region of the chromosome 9 open reading frame 72 (C9orf72) locus. The pathological hallmarks observed in C9orf72 repeat expansion carriers are the formation of RNA foci and deposition of dipeptide repeat (DPR) proteins derived from repeat associated non-ATG (RAN) translation. Currently, it is unclear whether formation of RNA foci, DPR translation products, or partial loss of C9orf72 predominantly drive neurotoxicity in vivo. By using a transgenic approach in zebrafish we address if the most frequently found DPR in human ALS/FTLD brain, the poly-Gly-Ala (poly-GA) protein, is toxic in vivo.

Method

We generated several transgenic UAS responder lines that express either 80 repeats of GGGGCC alone, or together with a translation initiation ATG codon forcing the translation of GA80-GFP protein upon crossing to a Gal4 driver. The GGGGCC repeat and GA80 were fused to green fluorescent protein (GFP) lacking a start codon to monitor protein translation by GFP fluorescence.

Results

Zebrafish transgenic for the GGGGCC repeat lacking an ATG codon showed very mild toxicity in the absence of poly-GA. However, strong toxicity was induced upon ATG initiated expression of poly-GA, which was rescued by injection of an antisense morpholino interfering with start codon dependent poly-GA translation. This morpholino only interferes with GA80-GFP translation without affecting repeat transcription, indicating that the toxicity is derived from GA80-GFP.

Conclusion

These novel transgenic C9orf72 associated repeat zebrafish models demonstrate poly-GA toxicity in zebrafish. Reduction of poly-GA protein rescues toxicity validating this therapeutic approach to treat C9orf72 repeat expansion carriers. These novel animal models provide a valuable tool for drug discovery to reduce DPR associated toxicity in ALS/FTLD patients with C9orf72 repeat expansions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0146-8) contains supplementary material, which is available to authorized users.

Visualization of Neuregulin 1 ectodomain shedding reveals its local processing in vitro and in vivo

Neuregulin1 (NRG1) plays diverse developmental roles and is likely involved in several neurological disorders including schizophrenia. The transmembrane NRG1 protein is proteolytically cleaved and released as a soluble ligand for ErbB receptors. Such post-translational processing, referred to as 'ectodomain shedding', is thought to be crucial for NRG1 function. However, little is known regarding the regulatory mechanism of NRG1 cleavage in vivo. Here, we developed a fluorescent probe, NRG1 Cleavage Indicating SenSOR (N-CISSOR), by fusing mCherry and GFP to the extracellular and intracellular domains of NRG1, respectively. N-CISSOR mimicked the subcellular localization and biochemical properties of NRG1 including cleavage dynamics and ErbB phosphorylation in cultured cells. mCherry/GFP ratio imaging of phorbol-12-myristate-13-acetate-stimulated N-CISSOR-expressing HEK293T cells enabled to monitor rapid ectodomain shedding of NRG1 at the subcellular level. Utilizing N-CISSOR in zebrafish embryos revealed preferential axonal NRG1 ectodomain shedding in developing motor neurons, demonstrating that NRG1 ectodomain shedding is spatially regulated at the subcellular level. Thus, N-CISSOR will be a valuable tool for elucidating the spatiotemporal regulation of NRG1 ectodomain shedding, both in vitro and in vivo.

Using the Cancer Risk Management Model to evaluate the health and economic impacts of cytology compared with human papillomavirus DNA testing for primary cervical cancer screening in Canada

BACKGROUND

In Canada, discussion about changing from cytology to human papillomavirus (hpv) dna testing for primary screening in cervical cancer is ongoing. However, the Canadian Task Force on Preventive Health Care has not yet made a recommendation, concluding that the evidence is insufficient.

METHODS

We used the cervical cancer and hpv transmission models of the Cancer Risk Management Model to study the health and economic outcomes of primary cytology compared with hpv dna testing in 14 screening scenarios with varying screening modalities and intervals. Projected cervical cancer cases, deaths, colposcopies, screens, costs, and incremental cost-effectiveness were evaluated. We performed sensitivity analyses for hpv dna test costs.

RESULTS

Compared with triennial cytology from age 25, 5-yearly hpv dna screening alone from age 30 resulted in equivalent incident cases and deaths, but 55% (82,000) fewer colposcopies and 43% (1,195,000) fewer screens. At hpv dna screening intervals of 3 years, whether alone or in an age-based sequence with cytology, screening costs are greater, but at intervals of more than 5 years, they are lower. Scenarios on the cost-effectiveness frontier were hpv dna testing alone every 10, 7.5, 5, or 3 years, and triennial cytology starting at age 21 or 25 when combined with hpv dna testing every 3 years.

CONCLUSIONS

Changing from cytology to hpv dna testing as the primary screening test for cervical cancer would be an acceptable strategy in Canada with respect to incidence, mortality, screening and diagnostic test volumes.

Micron scale directional coupler as a transducer for biochemical sensing

A microfabricated directional coupler (DC) was used for the detection of DNA conjugated with quantum dots. Output optical signals from DCs of a wide range of device lengths correspond well to theoretical and simulation results. Even 20 µm-long DC devices could detect changes in the output optical intensity by monitoring the near-field pattern using a CCD camera. The signal was enhanced 60 × using a 1500 µm-long DC device. For large cladding refractive-index changes between air and water, the normalized signal changed cyclically several times between 0 and 1. The results suggest that the DC can be the basis for miniaturized two-dimensionally integrated biochemical sensors.

Establishment of Gal4 transgenic zebrafish lines for analysis of development of cerebellar neural circuitry

The cerebellum is involved in some forms of motor coordination and motor learning. Here we isolated transgenic (Tg) zebrafish lines that express a modified version of Gal4-VP16 (GFF) in the cerebellar neural circuits: granule, Purkinje, or eurydendroid cells, Bergmann glia, or the neurons in the inferior olive nuclei (IO) which send climbing fibers to Purkinje cells, with the transposon Tol2 system. By combining GFF lines with Tg lines carrying a reporter gene located downstream of Gal4 binding sequences (upstream activating sequence: UAS), we investigated the anatomy and developmental processes of the cerebellar neural circuitry. Combining an IO-specific Gal4 line with a UAS reporter line expressing the photoconvertible fluorescent protein Kaede demonstrated the contralateral projections of climbing fibers. Combining a granule cell-specific Gal4 line with a UAS reporter line expressing wheat germ agglutinin (WGA) confirmed direct and/or indirect connections of granule cells with Purkinje cells, eurydendroid cells, and IO neurons in zebrafish. Time-lapse analysis of a granule cell-specific Gal4 line revealed initial random movements and ventral migration of granule cell nuclei. Transgenesis of a reporter gene with another transposon Tol1 system visualized neuronal structure at a single cell resolution. Our findings indicate the usefulness of these zebrafish Gal4 Tg lines for studying the development and function of cerebellar neural circuits.

The parallel growth of motoneuron axons with the dorsal aorta depends on Vegfc/Vegfr3 signaling in zebrafish

Blood vessels and neurons grow often side by side. However, the molecular and cellular mechanisms underlying their parallel development remain unclear. Here, we report that a subpopulation of secondary motoneurons extends axons ventrally outside of the neural tubes and rostrocaudally as a fascicle beneath the dorsal aorta (DA) in zebrafish. We tried to clarify the mechanism by which these motoneuron axons grow beneath the DA and found that Vegfc in the DA and Vegfr3 in the motoneurons were essential for the axon growth. Forced expression of either Vegfc in arteries or Vegfr3 in motoneurons resulted in enhanced axon growth of motoneurons over the DA. Both vegfr3 morphants and vegfc morphants lost the alignment of motoneuron axons with DA. In addition, forced expression of two mutant forms of Vegfr3 in motoneurons, potentially trapping endogenous Vegfc, resulted in failure of growth of motoneuron axons beneath the DA. Finally, a vegfr3 mutant fish lacked the motoneuron axons beneath the DA. Collectively, Vegfc from the preformed DA guides the axon growth of secondary motoneurons.

Wnt/Dkk negative feedback regulates sensory organ size in zebrafish

Correct organ size must involve a balance between promotion and inhibition of cell proliferation. A mathematical model has been proposed in which an organ is assumed to produce its own growth activator as well as a growth inhibitor [1], but there is as yet no molecular evidence to support this model [2]. The mechanosensory organs of the fish lateral line system (neuromasts) are composed of a core of sensory hair cells surrounded by nonsensory support cells. Sensory cells are constantly replaced and are regenerated from surrounding nonsensory cells [3], while each organ retains the same size throughout life. Moreover, neuromasts also bud off new neuromasts, which stop growing when they reach the same size [4, 5]. Here, we show that the size of neuromasts is controlled by a balance between growth-promoting Wnt signaling activity in proliferation-competent cells and Wnt-inhibiting Dkk activity produced by differentiated sensory cells. This negative feedback loop from Dkk (secreted by differentiated cells) on Wnt-dependent cell proliferation (in surrounding cells) also acts during regeneration to achieve size constancy. This study establishes Wnt/Dkk as a novel mechanism to determine the final size of an organ.

Cellular dissection of the spinal cord motor column by BAC transgenesis and gene trapping in zebrafish

Bacterial artificial chromosome (BAC) transgenesis and gene/enhancer trapping are effective approaches for identification of genetically defined neuronal populations in the central nervous system (CNS). Here, we applied these techniques to zebrafish (Danio rerio) in order to obtain insights into the cellular architecture of the axial motor column in vertebrates. First, by using the BAC for the Mnx class homeodomain protein gene mnr2b/mnx2b, we established the mnGFF7 transgenic line expressing the Gal4FF transcriptional activator in a large part of the motor column. Single cell labeling of Gal4FF-expressing cells in the mnGFF7 line enabled a detailed investigation of the morphological characteristics of individual spinal motoneurons, as well as the overall organization of the motor column in a spinal segment. Secondly, from a large-scale gene trap screen, we identified transgenic lines that marked discrete subpopulations of spinal motoneurons with Gal4FF. Molecular characterization of these lines led to the identification of the ADAMTS3 gene, which encodes an evolutionarily conserved ADAMTS family of peptidases and is dynamically expressed in the ventral spinal cord. The transgenic fish established here, along with the identified gene, should facilitate an understanding of the cellular and molecular architecture of the spinal cord motor column and its connection to muscles in vertebrates.

Interhemispheric asymmetry of olfactory input-dependent neuronal specification in the adult brain

The vertebrate brain is anatomically and functionally asymmetric. The left and right cerebral hemispheres harbor neural stem cell niches at the ventricular-subventricular zone (V-SVZ) of the ventricular walls, where new neurons are continuously generated throughout life. However, any interhemispheric asymmetry of neural stem cell niches remains unclear. We performed gene-trap screens in adult zebrafish to identify genes that are differentially expressed in the two hemispheres and found that adult-born neurons expressing the neural zinc-finger protein Myt1 exist predominantly in the left V-SVZ. This lateralization could be reversed by left olfactory sensory deprivation-induced inactivation of Notch signaling. The olfactory behavioral preference for attractive amino acids was also impaired by sensory deprivation of the left olfactory system, but not of the right olfactory system. Our findings suggest that olfactory input generates interhemispheric differences in the fate of adult-born neurons in the zebrafish brain.

Migration of neuronal precursors from the telencephalic ventricular zone into the olfactory bulb in adult zebrafish

In the brain of adult mammals, neuronal precursors are generated in the subventricular zone in the lateral wall of the lateral ventricles and migrate into the olfactory bulbs (OBs) through a well-studied route called the rostral migratory stream (RMS). Recent studies have revealed that a comparable neural stem cell niche is widely conserved at the ventricular wall of adult vertebrates. However, little is known about the migration route of neuronal precursors in nonmammalian adult brains. Here, we show that, in the adult zebrafish, a cluster of neuronal precursors generated in the telencephalic ventricular zone migrates into the OB via a route equivalent to the mammalian RMS. Unlike the mammalian RMS, these neuronal precursors are not surrounded by glial tubes, although radial glial cells with a single cilium lined the telencephalic ventricular wall, much as in embryonic and neonatal mammals. To observe the migrating neuronal precursors in living brain tissue, we established a brain hemisphere culture using a zebrafish line carrying a GFP transgene driven by the neurogenin1 (ngn1) promoter. In these fish, GFP was observed in the neuronal precursors migrating in the RMS, some of which were aligned with blood vessels. Numerous ngn1:gfp-positive cells were observed migrating tangentially in the RMS-like route medial to the OB. Taken together, our results suggest that the RMS in the adult zebrafish telencephalon is a functional migratory pathway. This is the first evidence for the tangential migration of neuronal precursors in a nonmammalian adult telencephalon.

An mnr2b/hlxb9lb enhancer trap line that labels spinal and abducens motor neurons in zebrafish

BACKGROUND

The developing nervous system consists of a variety of cell types. Animal models that allow the visualization of specific classes of neurons are crucial for the study of neuronal networks.

RESULTS

We performed an enhancer trap screening in zebrafish and generated a collection of transgenic lines that expressed GFP in a spatially and temporally restricted manner. Among the fish generated, we identified an insertion of the enhancer trap construct in the vicinity of the mnr2b/hlxb9lb gene encoding the mnx class of homeodomain transcription factor. The insertion gave rise to GFP expression predominantly in spinal motor neurons and abducens motor neurons. During embryogenesis, GFP expression was also detected in endodermal and mesodermal tissues, where mnr2b is known to be expressed.

CONCLUSION

These results show that the enhancer trap construct recapitulated the expression pattern of the mnr2b gene and this transgenic line should be useful for the visualization of the spinal and abducens motor neurons in the developing nervous system.

Connexin 39.9 protein is necessary for coordinated activation of slow-twitch muscle and normal behavior in zebrafish

In many tissues and organs, connexin proteins assemble between neighboring cells to form gap junctions. These gap junctions facilitate direct intercellular communication between adjoining cells, allowing for the transmission of both chemical and electrical signals. In rodents, gap junctions are found in differentiating myoblasts and are important for myogenesis. Although gap junctions were once believed to be absent from differentiated skeletal muscle in mammals, recent studies in teleosts revealed that differentiated muscle does express connexins and is electrically coupled, at least at the larval stage. These findings raised questions regarding the functional significance of gap junctions in differentiated muscle. Our analysis of gap junctions in muscle began with the isolation of a zebrafish motor mutant that displayed weak coiling at day 1 of development, a behavior known to be driven by slow-twitch muscle (slow muscle). We identified a missense mutation in the gene encoding Connexin 39.9. In situ hybridization found connexin 39.9 to be expressed by slow muscle. Paired muscle recordings uncovered that wild-type slow muscles are electrically coupled, whereas mutant slow muscles are not. The further examination of cellular activity revealed aberrant, arrhythmic touch-evoked Ca(2+) transients in mutant slow muscle and a reduction in the number of muscle fibers contracting in response to touch in mutants. These results indicate that Connexin 39.9 facilitates the spreading of neuronal inputs, which is irregular during motor development, beyond the muscle cells and that gap junctions play an essential role in the efficient recruitment of slow muscle fibers.

A transgenic zebrafish for monitoring in vivo microtubule structures

The microtubule (MT) cytoskeleton plays crucial roles in brain development by regulating the proliferation of neuronal progenitor cells, neuronal migration and axon guidance. Methods for monitoring MT in the intact brain, however, have been limited in vertebrates. Here, we report a transgenic zebrafish line for monitoring MT in vivo. This reporter line carries a transgene encoding the green fluorescent protein (GFP) -tagged tubulin gene linked to the upstream activating sequence (UAS), the recognition sequence of the yeast Gal4 transcriptional activator. By crossing this reporter line with appropriate transgenic Gal4 driver lines, we induced the GFP-tagged tubulin in various cell types from the embryonic stages to the adult stage. In larvae expressing the modified tubulin, individual MT filaments and other MT structures, including the mitotic spindles in proliferating neuronal progenitor cells, were clearly visualized. Therefore, the transgenic UAS reporter line should be useful for directly monitoring MTs in the intact brain.

Chemically-Enhanced GaAs Maskless Etching Using a Novel Focused Ion Beam Etching System with a Chlorine Molecular and Radical Beam

Chlorine-enhanced GaAs maskless etching using a novel focused-ion-beametching (FIBE) system has been examined for establishing high-rate and smooth FIBE. The system is composed of an air-locked ultrahigh-vacuum chamber, a 30 KeV Ga+ FIB column and two kinds of chlorine-irradiation nozzles. A fine nozzle enabled us to irradiate a high-density Cl2 flux on a desired, small area of the sample while retaining a sufficiently low surrounding-gas pressure for stable Ga+ FIB emission. Highly chemically-enhanced sputtering yields (up to 50 GaAs molecules per incident ion) were obtained. At the maximum yield, line-scanned deep-groove (6.5 um) etching with a smooth surface, capable of fabricating a laser-cavity optical mirror, was demonstrated. The chemical-enhancement effect showed high FIB-scanning-time dependence. This effect was also observed by irradiating with a plasma-dissociated Cl radicals using a novel radical beam gun. An analytical model, based on the Ga+-ion bombardment on the chlorine-adsorbed substrate surface, suggested that the maximum chemical enhancement is obtained when the Ga+-FIB scanning time is adjusted to the chlorine-coverage time, given by the Cl2-molecule or Cl-radical flux density.

zTrap: zebrafish gene trap and enhancer trap database

BACKGROUND

We have developed genetic methods in zebrafish by using the Tol2 transposable element; namely, transgenesis, gene trapping, enhancer trapping and the Gal4FF-UAS system. Gene trap constructs contain a splice acceptor and the GFP or Gal4FF (a modified version of the yeast Gal4 transcription activator) gene, and enhancer trap constructs contain the zebrafish hsp70l promoter and the GFP or Gal4FF gene. By performing genetic screens using these constructs, we have generated transgenic zebrafish that express GFP and Gal4FF in specific cells, tissues and organs. Gal4FF expression is visualized by creating double transgenic fish carrying a Gal4FF transgene and the GFP reporter gene placed downstream of the Gal4-recognition sequence (UAS). Further, the Gal4FF-expressing cells can be manipulated by mating with UAS effector fish. For instance, when fish expressing Gal4FF in specific neurons are crossed with the UAS:TeTxLC fish carrying the tetanus neurotoxin gene downstream of UAS, the neuronal activities are inhibited in the double transgenic fish. Thus, these transgenic fish are useful to study developmental biology and neurobiology.

DESCRIPTION

To increase the usefulness of the transgenic fish resource, we developed a web-based database named zTrap http://kawakami.lab.nig.ac.jp/ztrap/. The zTrap database contains images of GFP and Gal4FF expression patterns, and genomic DNA sequences surrounding the integration sites of the gene trap and enhancer trap constructs. The integration sites are mapped onto the Ensembl zebrafish genome by in-house Blat analysis and can be viewed on the zTrap and Ensembl genome browsers. Furthermore, zTrap is equipped with the functionality to search these data for expression patterns and genomic loci of interest. zTrap contains the information about transgenic fish including UAS reporter and effector fish.

CONCLUSION

zTrap is a useful resource to find gene trap and enhancer trap fish lines that express GFP and Gal4FF in desired patterns, and to find insertions of the gene trap and enhancer trap constructs that are located within or near genes of interest. These transgenic fish can be utilized to observe specific cell types during embryogenesis, to manipulate their functions, and to discover novel genes and cis-regulatory elements. Therefore, zTrap should facilitate studies on genomics, developmental biology and neurobiology utilizing the transgenic zebrafish resource.

Sex reversal in zebrafish fancl mutants is caused by Tp53-mediated germ cell apoptosis

The molecular genetic mechanisms of sex determination are not known for most vertebrates, including zebrafish. We identified a mutation in the zebrafish fancl gene that causes homozygous mutants to develop as fertile males due to female-to-male sex reversal. Fancl is a member of the Fanconi Anemia/BRCA DNA repair pathway. Experiments showed that zebrafish fancl was expressed in developing germ cells in bipotential gonads at the critical time of sexual fate determination. Caspase-3 immunoassays revealed increased germ cell apoptosis in fancl mutants that compromised oocyte survival. In the absence of oocytes surviving through meiosis, somatic cells of mutant gonads did not maintain expression of the ovary gene cyp19a1a and did not down-regulate expression of the early testis gene amh; consequently, gonads masculinized and became testes. Remarkably, results showed that the introduction of a tp53 (p53) mutation into fancl mutants rescued the sex-reversal phenotype by reducing germ cell apoptosis and, thus, allowed fancl mutants to become fertile females. Our results show that Fancl function is not essential for spermatogonia and oogonia to become sperm or mature oocytes, but instead suggest that Fancl function is involved in the survival of developing oocytes through meiosis. This work reveals that Tp53-mediated germ cell apoptosis induces sex reversal after the mutation of a DNA-repair pathway gene by compromising the survival of oocytes and suggests the existence of an oocyte-derived signal that biases gonad fate towards the female developmental pathway and thereby controls zebrafish sex determination.

Mechanism of development of ionocytes rich in vacuolar-type H(+)-ATPase in the skin of zebrafish larvae

Mitochondrion-rich cells (MRCs), or ionocytes, play a central role in aquatic species, maintaining body fluid ionic homeostasis by actively taking up or excreting ions. Since their first description in 1932 in eel gills, extensive morphological and physiological analyses have yielded important insights into ionocyte structure and function, but understanding the developmental pathway specifying these cells remains an ongoing challenge. We previously succeeded in identifying a key transcription factor, Foxi3a, in zebrafish larvae by database mining. In the present study, we analyzed a zebrafish mutant, quadro (quo), deficient in foxi1 gene expression and found that foxi1 is essential for development of an MRC subpopulation rich in vacuolar-type H(+)-ATPase (vH-MRC). foxi1 acts upstream of Delta-Notch signaling that determines sporadic distribution of vH-MRC and regulates foxi3a expression. Through gain- and loss-of-function assays and cell transplantation experiments, we further clarified that (1) the expression level of foxi3a is maintained by a positive feedback loop between foxi3a and its downstream gene gcm2 and (2) Foxi3a functions cell-autonomously in the specification of vH-MRC. These observations provide a better understanding of the differentiation and distribution of the vH-MRC subtype.

The Tol2-mediated Gal4-UAS method for gene and enhancer trapping in zebrafish

The Gal4-UAS system provides powerful tools to analyze the function of genes and cells in vivo and has been extensively employed in Drosophila. The usefulness of this approach relies on the P element-mediated Gal4 enhancer trapping, which can efficiently generate transgenic fly lines expressing Gal4 in specific cells. Similar approaches, however, had not been developed in vertebrate systems due to the lack of an efficient transgenesis method. We have been developing transposon techniques by using the madaka fish Tol2 element. Taking advantage of its ability to generate genome-wide insertions, we developed the Gal4 gene trap and enhancer trap methods in zebrafish that enabled us to create various transgenic fish expressing Gal4 in specific cells. The Gal4-expressing cells can be visualized and manipulated in vivo by crossing the transgenic Gal4 lines with transgenic lines carrying various reporter and effector genes downstream of UAS (upstream activating sequence). Thus, the Gal4 gene trap and enhancer trap methods together with UAS lines now make detailed analyses of genes and cells in zebrafish feasible. Here, we describe the protocols to perform Gal4 gene trap and enhancer trap screens in zebrafish and their application to the studies of vertebrate neural circuits.

A novel conserved evx1 enhancer links spinal interneuron morphology and cis-regulation from fish to mammals

Spinal interneurons are key components of locomotor circuits, driving such diverse behaviors as swimming in fish and walking in mammals. Recent work has linked the expression of evolutionarily conserved transcription factors to key features of interneurons in diverse species, raising the possibility that these interneurons are functionally related. Consequently, the determinants of interneuron subtypes are predicted to share conserved cis-regulation in vertebrates with very different spinal cords. Here, we establish a link between cis-regulation and morphology of spinal interneurons that express the Evx1 homeodomain transcription factor from fish to mammals. Using comparative genomics, and complementary transgenic approaches, we have identified a novel enhancer of evx1, that includes two non-coding elements conserved in vertebrates. We show that pufferfish evx1 transgenes containing this enhancer direct reporter expression to a subset of spinal commissural interneurons in zebrafish embryos. Pufferfish, zebrafish and mouse evx1 downstream genomic enhancers label selectively Evx1(+) V0 commissural interneurons in chick and rat embryos. By dissecting the zebrafish evx1 enhancer, we identify a role for a 25 bp conserved cis-element in V0-specific gene expression. Our findings support the notion that spinal interneurons shared between distantly related vertebrates, have been maintained in part via the preservation of highly conserved cis-regulatory modules.

Targeted gene expression by the Gal4-UAS system in zebrafish

Targeted gene expression by the Gal4-UAS system is a powerful methodology for analyzing function of genes and cells in vivo and has been extensively used in genetic studies in Drosophila. On the other hand, the Gal4-UAS system had not been applied effectively to vertebrate systems for a long time mainly due to the lack of an efficient transgenesis method. Recently, a highly efficient transgenesis method using the medaka fish Tol2 transposable element was developed in zebrafish. Taking advantage of the Tol2 transposon system, we and other groups developed the Gal4 gene trap and enhancer trap methods and established various transgenic fish expressing Gal4 in specific cells. By crossing such Gal4 lines with transgenic fish lines harboring various reporter genes and effector genes downstream of UAS (upstream activating sequence), specific cells can be visualized and manipulated in vivo by targeted gene expression. Thus, the Gal4 gene trap and enhancer trap approaches together with various UAS lines should be important tools for investigating roles of genes and cells in vertebrates.

Insertional mutagenesis by the Tol2 transposon-mediated enhancer trap approach generated mutations in two developmental genes: tcf7 and synembryn-like

Gene trap and enhancer trap methods using transposon or retrovirus have been recently described in zebrafish. However, insertional mutants using these methods have not been reported. We report here development of an enhancer trap method by using the Tol2 transposable element and identification and characterization of insertional mutants. We created 73 fish lines that carried single copy insertions of an enhancer trap construct, which contained the zebrafish hsp70 promoter and the GFP gene, in their genome and expressed GFP in specific cells, tissues and organs, indicating that the hsp70 promoter is highly capable of responding to chromosomal enhancers. First, we analyzed genomic DNA surrounding these insertions. Fifty-one of them were mapped onto the current version of the genomic sequence and 43% (22/51) were located within transcribed regions, either exons or introns. Then, we crossed heterozygous fish carrying the same insertions and identified two insertions that caused recessive mutant phenotypes. One disrupted the tcf7 gene, which encodes a transcription factor of the Tcf/Lef family mediating Wnt signaling, and caused shorter and wavy median fin folds and pectoral fins. We knocked down Lef1, another member of the Tcf/Lef family also expressed in the fin bud, in the tcf7 mutant, and revealed functional redundancy of these factors and their essential role in establishment of the apical ectodermal ridge (AER). The other disrupted the synembryn-like gene (synbl), a homolog of the C. elegans synembryn gene, and caused embryonic lethality and small pigment spots. The pigment phenotype was rescued by application of forskolin, an activator of adenylyl cyclase, suggesting that the synbl gene activates the Galpha(S) pathway leading to activation of adenylyl cyclase. We thus demonstrated that the transposon-mediated enhancer trap approach can indeed create insertional mutations in developmental genes. Our present study provides a basis for the development of efficient transposon-mediated insertional mutagenesis in a vertebrate.

Gamma-tubulin complex-mediated anchoring of spindle microtubules to spindle-pole bodies requires Msd1 in fission yeast

The anchoring of microtubules to subcellular structures is critical for cell polarity and motility. Although the process of anchoring cytoplasmic microtubules to the centrosome has been studied in some detail, it is not known how spindle microtubules are anchored to the mitotic centrosome and, particularly, whether anchoring and nucleation of mitotic spindles are functionally separate. Here, we show that a fission yeast coiled-coil protein, Msd1, is required for anchoring the minus end of spindle microtubules to the centrosome equivalent, the spindle-pole body (SPB). msd1 deletion causes spindle microtubules to abnormally extend beyond SPBs, which results in chromosome missegregation. Importantly, this protruding spindle is phenocopied by the amino-terminal deletion mutant of Alp4, a component of the gamma-tubulin complex (gamma-TuC), which lacks the potential Msd1-interacting domain. We propose that Msd1 interacts with gamma-TuC, thereby specifically anchoring the minus end of microtubules to SPBs without affecting microtubule nucleation.

Targeting neural circuitry in zebrafish using GAL4 enhancer trapping

We present a pilot enhancer trap screen using GAL4 to drive expression of upstream activator sequence (UAS)-linked transgenes in expression patterns dictated by endogenous enhancers in zebrafish. The patterns presented include expression in small subsets of neurons throughout the larval brain, which in some cases persist into adult. Through targeted photoconversion of UAS-driven Kaede and variegated expression of UAS-driven GFP in single cells, we begin to characterize the cellular components of labeled circuits.

A rapid method for protein extraction from fission yeast

Researchers working with fission yeast conduct protein extraction widely and frequently, but this includes the handling of glass beads, and hence is laborious and cumbersome, especially when dealing with a large number of samples. Here we describe a rapid and reliable method for preparing protein extract from fission yeast, one which is applicable to routine western blotting.

The V260I mutation in fission yeast alpha-tubulin Atb2 affects microtubule dynamics and EB1-Mal3 localization and activates the Bub1 branch of the spindle checkpoint

We have identified a novel temperature-sensitive mutant of fission yeast alpha-tubulin Atb2 (atb2-983) that contains a single amino acid substitution (V260I). Atb2-983 is incorporated into the microtubules, and their overall structures are not altered noticeably, but microtubule dynamics is compromised during interphase. atb2-983 displays a high rate of chromosome missegregation and is synthetically lethal with deletions in a subset of spindle checkpoint genes including bub1, bub3, and mph1, but not with mad1, mad2, and mad3. During early mitosis in this mutant, Bub1, but not Mad2, remains for a prolonged period in the kinetochores that are situated in proximity to one of the two SPBs (spindle pole bodies). High dosage mal3(+), encoding EB1 homologue, rescues atb2-983, suggesting that Mal3 function is compromised. Consistently, Mal3 localization and binding between Mal3 and Atb2-983 are impaired significantly, and a mal3 single mutant, such as atb2-983, displays prolonged Bub1 kinetochore localization. Furthermore in atb2-983 back-and-forth centromere oscillation during prometaphase is abolished. Intriguingly, this oscillation still occurs in the mal3 mutant, indicating that there is another defect independent of Mal3. These results show that microtubule dynamics is important for coordinated execution of mitotic events, in which Mal3 plays a vital role.