An instructive function for Notch in promoting gliogenesis in the zebrafish retina

Nodal signaling range is regulated by proprotein convertase-mediated maturation

Tissue patterning is established by extracellular growth factors or morphogens. Although different theoretical models explaining specific patterns have been proposed, our understanding of tissue pattern establishment in vivo remains limited. In many animal species, left-right patterning is governed by a reaction-diffusion system relying on the different diffusivity of an activator, Nodal, and an inhibitor, Lefty. In a genetic screen, we identified a zebrafish loss-of-function mutant for the proprotein convertase FurinA. Embryological and biochemical experiments demonstrate that cleavage of the Nodal-related Spaw proprotein into a mature form by FurinA is required for Spaw gradient formation and activation of Nodal signaling. We demonstrate that FurinA is required cell-autonomously for the long-range signaling activity of Spaw and no other Nodal-related factors. Combined in silico and in vivo approaches support a model in which FurinA controls the signaling range of Spaw by cleaving its proprotein into a mature, extracellular form, consequently regulating left-right patterning.

miR-216a regulates snx5, a novel notch signaling pathway component, during zebrafish retinal development

Precise regulation of Notch signaling is essential for normal vertebrate development. Mind bomb (Mib) is a ubiquitin ligase that is required for activation of Notch by Notch׳s ligand, Delta. Sorting Nexin 5 (SNX5) co-localizes with Mib and Delta complexes and has been shown to directly bind to Mib. We show that microRNA-216a (miR-216a) is expressed in the retina during early development and regulates snx5 to precisely regulate Notch signaling. miR-216a and snx5 have complementary expression patterns. Knocking down miR-216a and/or overexpression of snx5 resulted in increased Notch activation. Conversely, knocking down snx5 and/or miR-216a overexpression caused a decrease in Notch activation. We propose a model in which SNX5, precisely controlled by miR-216a, is a vital partner of Mib in promoting endocytosis of Delta and subsequent activation of Notch signaling.

Notch signaling and ageing

Notch signaling is a master controller of the neural stem cell and neural development maintaining a significant role in the normal brain function. Notch genes are involved in embryogenesis, nervous system, and cardiovascular and endocrine function. On the other side, there are studies representing the involvement of Notch mutations in sporadic Alzheimer disease, other neurodegenerative diseases such as Down syndrome, Pick's and Prion's disease, and CADASIL. This manuscript attempts to present a holistic view of the positive or negative contribution of Notch signaling in the adult brain, and at the same time to present and promote the promising research fields of study.

Modulation of notch signaling pathway to prevent H2O2/menadione-induced SK-N-MC cells death by EUK134

The brain in Alzheimer's disease is under increased oxidative stress, and this may have a role in the pathogenesis and neural death in this disorder. It has been verified that numerous signaling pathways involved in neurodegenerative disorders are activated in response to reactive oxygen species (ROS). EUK134, a synthetic salen-manganese antioxidant complex, has been found to possess many interesting pharmacological activities awaiting exploration. The present study is to characterize the role of Notch signaling in apoptotic cell death of SK-N-MC cells. The cells were treated with hydrogen peroxide (H2O2) or menadione to induce oxidative stress. The free-radical scavenging capabilities of EUK134 were studied through the MTT assay, glutathione peroxidase (GPx) enzyme activity assay, and glutathione (GSH) Levels. The extents of lipid peroxidation, protein carbonyl formation, and intracellular ROS levels, as markers of oxidative stress, were also studied. Our results showed that H2O2/menadione reduced GSH levels and GPx activity. However, EUK134 protected cells against ROS-induced cell death by down-regulation of lipid peroxidation and protein carbonyl formation as well as restoration of antioxidant enzymes activity. ROS induced apoptosis and increased NICD and HES1 expression. Inhibition of NICD production proved that Notch signaling is involved in apoptosis through p53 activation. Moreover, H2O2/menadione led to Numb protein down-regulation which upon EUK134 pretreatment, its level increased and subsequently prevented Notch pathway activation. We indicated that EUK134 can be a promising candidate in designing natural-based drugs for ROS-induced neurodegenerative diseases. Collectively, ROS activated Notch signaling in SK-N-MC cells leading to cell apoptosis.

Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling during embryonic lymphangiogenesis

The VEGFC/VEGFR3 signaling pathway is essential for lymphangiogenesis (the formation of lymphatic vessels from pre-existing vasculature) during embryonic development, tissue regeneration and tumor progression. The recently identified secreted protein CCBE1 is indispensible for lymphangiogenesis during development. The role of CCBE1 orthologs is highly conserved in zebrafish, mice and humans with mutations in CCBE1 causing generalized lymphatic dysplasia and lymphedema (Hennekam syndrome). To date, the mechanism by which CCBE1 acts remains unknown. Here, we find that ccbe1 genetically interacts with both vegfc and vegfr3 in zebrafish. In the embryo, phenotypes driven by increased Vegfc are suppressed in the absence of Ccbe1, and Vegfc-driven sprouting is enhanced by local Ccbe1 overexpression. Moreover, Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of Ccbe1. Finally, CCBE1 is capable of upregulating the levels of fully processed, mature VEGFC in vitro and the overexpression of mature VEGFC rescues ccbe1 loss-of-function phenotypes in zebrafish. Taken together, these data identify Ccbe1 as a crucial component of the Vegfc/Vegfr3 pathway in the embryo.

Single continuous lumen formation in the zebrafish gut is mediated by smoothened-dependent tissue remodeling

The formation of a single lumen during tubulogenesis is crucial for the development and function of many organs. Although 3D cell culture models have identified molecular mechanisms controlling lumen formation in vitro, their function during vertebrate organogenesis is poorly understood. Using light sheet microscopy and genetic approaches we have investigated single lumen formation in the zebrafish gut. Here we show that during gut development multiple lumens open and enlarge to generate a distinct intermediate, which consists of two adjacent unfused lumens separated by basolateral contacts. We observed that these lumens arise independently from each other along the length of the gut and do not share a continuous apical surface. Resolution of this intermediate into a single, continuous lumen requires the remodeling of contacts between adjacent lumens and subsequent lumen fusion. We show that lumen resolution, but not lumen opening, is impaired in smoothened (smo) mutants, indicating that fluid-driven lumen enlargement and resolution are two distinct processes. Furthermore, we show that smo mutants exhibit perturbations in the Rab11 trafficking pathway and demonstrate that Rab11-mediated trafficking is necessary for single lumen formation. Thus, lumen resolution is a distinct genetically controlled process crucial for single, continuous lumen formation in the zebrafish gut.

Pleiotropic effects of Sox2 during the development of the zebrafish epithalamus

The zebrafish epithalamus is part of the diencephalon and encompasses three major components: the pineal, the parapineal and the habenular nuclei. Using sox2 knockdown, we show here that this key transcriptional regulator has pleiotropic effects during the development of these structures. Sox2 negatively regulates pineal neurogenesis. Also, Sox2 is identified as the unknown factor responsible for pineal photoreceptor prepatterning and performs this function independently of the BMP signaling. The correct levels of sox2 are critical for the functionally important asymmetrical positioning of the parapineal organ and for the migration of parapineal cells as a coherent structure. Deviations from this strict control result in defects associated with abnormal habenular laterality, which we have documented and quantified in sox2 morphants.

Notch signaling regulates cardiomyocyte proliferation during zebrafish heart regeneration

The human heart's failure to replace ischemia-damaged myocardium with regenerated muscle contributes significantly to the worldwide morbidity and mortality associated with coronary artery disease. Remarkably, certain vertebrate species, including the zebrafish, achieve complete regeneration of amputated or injured myocardium through the proliferation of spared cardiomyocytes. Nonetheless, the genetic and cellular determinants of natural cardiac regeneration remain incompletely characterized. Here, we report that cardiac regeneration in zebrafish relies on Notch signaling. Following amputation of the zebrafish ventricular apex, Notch receptor expression becomes activated specifically in the endocardium and epicardium, but not the myocardium. Using a dominant negative approach, we discovered that suppression of Notch signaling profoundly impairs cardiac regeneration and induces scar formation at the amputation site. We ruled out defects in endocardial activation, epicardial activation, and dedifferentiation of compact myocardial cells as causative for the regenerative failure. Furthermore, coronary endothelial tubes, which we lineage traced from preexisting endothelium in wild-type hearts, formed in the wound despite the myocardial regenerative failure. Quantification of myocardial proliferation in Notch-suppressed hearts revealed a significant decrease in cycling cardiomyocytes, an observation consistent with a noncell autonomous requirement for Notch signaling in cardiomyocyte proliferation. Unexpectedly, hyperactivation of Notch signaling also suppressed cardiomyocyte proliferation and heart regeneration. Taken together, our data uncover the exquisite sensitivity of regenerative cardiomyocyte proliferation to perturbations in Notch signaling.

Müller glia: Stem cells for generation and regeneration of retinal neurons in teleost fish

Adult zebrafish generate new neurons in the brain and retina throughout life. Growth-related neurogenesis allows a vigorous regenerative response to damage, and fish can regenerate retinal neurons, including photoreceptors, and restore functional vision following photic, chemical, or mechanical destruction of the retina. Müller glial cells in fish function as radial-glial-like neural stem cells. During adult growth, Müller glial nuclei undergo sporadic, asymmetric, self-renewing mitotic divisions in the inner nuclear layer to generate a rod progenitor that migrates along the radial fiber of the Müller glia into the outer nuclear layer, proliferates, and differentiates exclusively into rod photoreceptors. When retinal neurons are destroyed, Müller glia in the immediate vicinity of the damage partially and transiently dedifferentiate, re-express retinal progenitor and stem cell markers, re-enter the cell cycle, undergo interkinetic nuclear migration (characteristic of neuroepithelial cells), and divide once in an asymmetric, self-renewing division to generate a retinal progenitor. This daughter cell proliferates rapidly to form a compact neurogenic cluster surrounding the Müller glia; these multipotent retinal progenitors then migrate along the radial fiber to the appropriate lamina to replace missing retinal neurons. Some aspects of the injury-response in fish Müller glia resemble gliosis as observed in mammals, and mammalian Müller glia exhibit some neurogenic properties, indicative of a latent ability to regenerate retinal neurons. Understanding the specific properties of fish Müller glia that facilitate their robust capacity to generate retinal neurons will inform and inspire new clinical approaches for treating blindness and visual loss with regenerative medicine.

Making of a retinal cell: insights into retinal cell-fate determination

Understanding the process by which an uncommitted dividing cell produces particular specialized cells within a tissue remains a fundamental question in developmental biology. Many tissues are well suited for cell-fate studies, but perhaps none more so than the developing retina. Traditionally, experiments using the retina have been designed to elucidate the influence that individual environmental signals or transcription factors can have on cell-fate decisions. Despite a substantial amount of information gained through these studies, there is still much that we do not yet understand about how cell fate is controlled on a systems level. In addition, new factors such as noncoding RNAs and regulators of chromatin have been shown to play roles in cell-fate determination and with the advent of "omics" technology more factors will most likely be identified. In this chapter we summarize both the traditional view of retinal cell-fate determination and introduce some new ideas that are providing a challenge to the older way of thinking about the acquisition of cell fates.

Impaired endolysosomal function disrupts Notch signalling in optic nerve astrocytes

Astrocytes migrate from the optic nerve into the inner retina, forming a template upon which retinal vessels develop. In the Nuc1 rat, mutation in the gene encoding βA3/A1-crystallin disrupts both Notch signalling in astrocytes and formation of the astrocyte template. Here we show that loss of βA3/A1-crystallin in astrocytes does not impede Notch ligand binding or extracellular cleavages. However, it affects vacuolar-type proton ATPase (V-ATPase) activity, thereby compromising acidification of the endolysosomal compartments, leading to reduced γ-secretase-mediated processing and release of the Notch intracellular domain (NICD). Lysosomal-mediated degradation of Notch is also impaired. These defects decrease the level of NICD in the nucleus, inhibiting the expression of Notch target genes. Overexpression of βA3/A1-crystallin in those same astrocytes restored V-ATPase activity and normal endolysosomal acidification, thereby increasing the levels of γ-secretase to facilitate optimal Notch signalling. We postulate that βA3/A1-crystallin is essential for normal endolysosomal acidification, and thereby, normal activation of Notch signalling in astrocytes.

Efficient disruption of Zebrafish genes using a Gal4-containing gene trap

Background

External development and optical transparency of embryos make zebrafish exceptionally suitable for in vivo insertional mutagenesis using fluorescent proteins to visualize expression patterns of mutated genes. Recently developed Gene Breaking Transposon (GBT) vectors greatly improve the fidelity and mutagenicity of transposon-based gene trap vectors.

Results

We constructed and tested a bipartite GBT vector with Gal4-VP16 as the primary gene trap reporter. Our vector also contains a UAS:eGFP cassette for direct detection of gene trap events by fluorescence. To confirm gene trap events, we generated a UAS:mRFP tester line. We screened 270 potential founders and established 41 gene trap lines. Three of our gene trap alleles display homozygous lethal phenotypes ranging from embryonic to late larval: nsf ^tpl6, atp1a3a^tpl10 and flr^tpl19. Our gene trap cassette is flanked by direct loxP sites, which enabled us to successfully revert nsf ^tpl6, atp1a3a^tpl10 and flr^tpl19 gene trap alleles by injection of Cre mRNA. The UAS:eGFP cassette is flanked by direct FRT sites. It can be readily removed by injection of Flp mRNA for use of our gene trap alleles with other tissue-specific GFP-marked lines. The Gal4-VP16 component of our vector provides two important advantages over other GBT vectors. The first is increased sensitivity, which enabled us to detect previously unnoticed expression of nsf in the pancreas. The second advantage is that all our gene trap lines, including integrations into non-essential genes, can be used as highly specific Gal4 drivers for expression of other transgenes under the control of Gal4 UAS.

Conclusions

The Gal4-containing bipartite Gene Breaking Transposon vector presented here retains high specificity for integrations into genes, high mutagenicity and revertibility by Cre. These features, together with utility as highly specific Gal4 drivers, make gene trap mutants presented here especially useful to the research community.

Notch receptor expression in neurogenic regions of the adult zebrafish brain

The adult zebrash brain has a remarkable constitutive neurogenic capacity. The regulation and maintenance of its adult neurogenic niches are poorly understood. In mammals, Notch signaling is involved in stem cell maintenance both in embryonic and adult CNS. To better understand how Notch signaling is involved in stem cell maintenance during adult neurogenesis in zebrafish we analysed Notch receptor expression in five neurogenic zones of the adult zebrafish brain. Combining proliferation and glial markers we identified several subsets of Notch receptor expressing cells. We found that 90 of proliferating radial glia express notch1a, notch1b and notch3. In contrast, the proliferating non-glial populations of the dorsal telencephalon and hypothalamus rarely express notch3 and about half express notch1a/1b. In the non-proliferating radial glia notch3 is the predominant receptor throughout the brain. In the ventral telencephalon and in the mitotic area of the optic tectum, where cells have neuroepithelial properties, notch1a/1b/3 are expressed in most proliferating cells. However, in the cerebellar niche, although progenitors also have neuroepithelial properties, only notch1a/1b are expressed in a high number of PCNA cells. In this region notch3 expression is mostly in Bergmann glia and at low levels in few PCNA cells. Additionally, we found that in the proliferation zone of the ventral telencephalon, Notch receptors display an apical high to basal low gradient of expression. Notch receptors are also expressed in subpopulations of oligodendrocytes, neurons and endothelial cells. We suggest that the partial regional heterogeneity observed for Notch expression in progenitor cells might be related to the cellular diversity present in each of these neurogenic niches.

Analysis of gene expression in wild-type and Notch1 mutant retinal cells by single cell profiling

BACKGROUND

The vertebrate retina comprises sensory neurons, the photoreceptors, as well as many other types of neurons and one type of glial cell. These cells are generated by multipotent and restricted retinal progenitor cells (RPCs), which express Notch1. Loss of Notch1 in RPCs late during retinal development results in the overproduction of rod photoreceptors at the expense of interneurons and glia.

RESULTS

To examine the molecular underpinnings of this observation, microarray analysis of single retinal cells from wild-type or Notch1 conditional knockout retinas was performed. In situ hybridization was carried out to validate some of the findings.

CONCLUSIONS

The majority of Notch1-mutant cells lost expression of known Notch target genes. These cells also had low levels of RPC and cell cycle genes, and robustly up-regulated rod precursor genes. In addition, single wild-type cells, in which cell cycle marker genes were down-regulated, expressed markers of both rod photoreceptors and interneurons.

Insulinoma-associated 1a (Insm1a) is required for photoreceptor differentiation in the zebrafish retina

The zinc-finger transcription factor insulinoma-associated 1 (Insm1, previously IA-1) is expressed in the developing nervous and neuroendocrine systems, and is required for cell type specific differentiation. Expression of Insm1 is largely absent in the adult, although it is present in neurogenic regions of the adult brain and zebrafish retina. While expression of Insm1 has also been observed in the embryonic retina of numerous vertebrate species, its function during retinal development has remained unexplored. Here, we demonstrate that in the developing zebrafish retina, insm1a is required for photoreceptor differentiation. Insm1a-deficient embryos were microphthalmic and displayed defects in rod and cone photoreceptor differentiation. Rod photoreceptor cells were more sensitive to loss of insm1a expression than were cone photoreceptor cells. Additionally, we provide evidence that insm1a regulates cell cycle progression of retinoblasts, and functions upstream of the bHLH transcription factors ath5/atoh7 and neurod, and the photoreceptor specification genes crx and nr2e3. Finally, we show that insm1a is negatively regulated by Notch-Delta signaling. Taken together, our data demonstrate that Insm1 influences neuronal subtype differentiation during retinal development.

A comparative analysis of Müller glia-mediated regeneration in the vertebrate retina

This article reviews the current state of knowledge regarding the potential of Müller glia to become neuronal progenitor cells in the avian retina. We compare and contrast the remarkable proliferative and neurogenic capacity of Müller glia in the fish retina to the limited capacity of Müller glia in avian and rodent retinas. We summarize recent findings regarding the secreted factors, signaling pathways and cell intrinsic factors that have been implicated in the formation of Müller glia-derived progenitors. We discuss several key similarities and differences between the fish, rodent and chick model systems, highlighting several of the key transcription factors and signaling pathways that regulate the formation of Müller glia-derived progenitors.

Dynamic expression of secreted Frizzled-related protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia

Wnt proteins have been implicated in regulating a variety of developmental processes in the CNS. Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of the spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression ofsFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.

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.

Notch signaling coordinates cellular proliferation with differentiation during zebrafish fin regeneration

Zebrafish can completely regenerate amputated fins via formation of a blastema, a proliferative mass of undifferentiated precursor cells. During regenerative growth, blastema proliferation must be tightly coordinated with cellular differentiation, but little is known about how this is achieved. Here, we show that Notch signaling is essential for maintenance of blastema cells in a proliferative undifferentiated state. We found that the Notch pathway is activated in response to fin amputation in the highly proliferative region of the blastema. Chemical interference with Notch signaling resulted in a complete block of regeneration. Notch signaling was not required for the earliest known cellular processes during blastema formation, i.e. dedifferentiation and migration of osteoblasts, but specifically interfered with proliferation of blastema cells. Interestingly, overactivation of the pathway via misexpression of the intracellular domain of the Notch receptor (NICD) likewise inhibited regenerative outgrowth. In NICD-overexpressing fins, overall blastemal cell proliferation was not enhanced, but expanded into proximal regions where cellular differentiation normally occurs. Similarly, blastemal and epidermal gene expression territories invaded proximal regions upon sustained Notch activation. Concomitantly, NICD overexpression suppressed differentiation of osteoblasts and caused an expansion of the undifferentiated blastema. Together, these data suggest that Notch signaling activity maintains blastemal cells in a proliferative state and thus coordinates proliferation with differentiation during regenerative growth.

Notch regulates blastema proliferation and prevents differentiation during adult zebrafish fin regeneration

Zebrafish have the capacity to regenerate several organs, including the heart and fins. Fin regeneration is epimorphic, involving the formation at the amputation plane of a mass of undifferentiated, proliferating mesenchymal progenitor-like cells, called blastema. This tissue provides all the cell types that form the fin, so that after damage or amputation the fin pattern and structure are fully restored. How blastema cells remain in this progenitor-like state is poorly understood. Here, we show that the Notch pathway plays an essential role during fin regeneration. Notch signalling is activated during blastema formation and remains active throughout the regeneration process. Chemical inhibition or morpholino-mediated knockdown of Notch signalling impairs fin regeneration via decreased proliferation accompanied by reduced expression of Notch target genes in the blastema. Conversely, overexpression of a constitutively active form of the Notch1 receptor (N1ICD) in the regenerating fin leads to increased proliferation and to the expansion of the blastema cell markers msxe and msxb, as well as increased expression of the proliferation regulator aldh1a2. This blastema expansion prevents regenerative fin outgrowth, as indicated by the reduction in differentiating osteoblasts and the inhibition of bone regeneration. We conclude that Notch signalling maintains blastema cells in a plastic, undifferentiated and proliferative state, an essential requirement for fin regeneration.

Development of hypothalamic serotoninergic neurons requires Fgf signalling via the ETS-domain transcription factor Etv5b

Serotonin is a monoamine neurotransmitter that is involved in numerous physiological functions and its dysregulation is implicated in various psychiatric diseases. In all non-placental vertebrates, serotoninergic (5-HT) neurons are present in several regions of the brain, including the hypothalamus. In placental mammals, however, 5-HT neurons are located in the raphe nuclei only. In all species, though, 5-HT neurons constitute a functionally and molecularly heterogeneous population. How the non-raphe 5-HT populations are developmentally encoded is unknown. Using the zebrafish model we show that, in contrast to the raphe populations, hypothalamic 5-HT neurons are generated independently of the ETS-domain transcription factor Pet1 (Fev). By applying a combination of pharmacological tools and gene knockdown and/or overexpression experiments, we demonstrate that Fgf signalling acts via another ETS-domain transcription factor, Etv5b (Erm), to induce hypothalamic 5-HT neurons. We provide evidence that Etv5b exerts its effects by regulating cell cycle parameters in 5-HT progenitors. Our results highlight a novel role for Etv5b in neuronal development and provide support for the existence of a developmental heterogeneity among 5-HT neurons in their requirement for ETS-domain transcription factors.

Quantitative genetic analysis of retinal degeneration in the blind cavefish Astyanax mexicanus

The retina is the light-sensitive tissue of the eye that facilitates vision. Mutations within genes affecting eye development and retinal function cause a host of degenerative visual diseases, including retinitis pigmentosa and anophthalmia/microphthalmia. The characin fish Astyanax mexicanus includes both eyed (surface fish) and eyeless (cavefish) morphs that initially develop eyes with normal retina; however, early in development, the eyes of cavefish degenerate. Since both surface and cave morphs are members of the same species, they serve as excellent evolutionary mutant models with which to identify genes causing retinal degeneration. In this study, we crossed the eyed and eyeless forms of A. mexicanus and quantified the thickness of individual retinal layers among 115 F₂ hybrid progeny. We used next generation sequencing (RAD-seq) and microsatellite mapping to construct a dense genetic map of the Astyanax genome, scan for quantitative trait loci (QTL) affecting retinal thickness, and identify candidate genes within these QTL regions. The map we constructed for Astyanax includes nearly 700 markers assembled into 25 linkage groups. Based on our scans with this map, we identified four QTL, one each associated with the thickness of the ganglion, inner nuclear, outer plexiform, and outer nuclear layers of the retina. For all but one QTL, cavefish alleles resulted in a clear reduction in the thickness of the affected layer. Comparative mapping of genetic markers within each QTL revealed that each QTL corresponds to an approximately 35 Mb region of the zebrafish genome. Within each region, we identified several candidate genes associated with the function of each affected retinal layer. Our study is the first to examine Astyanax retinal degeneration in the context of QTL mapping. The regions we identify serve as a starting point for future studies on the genetics of retinal degeneration and eye disease using the evolutionary mutant model Astyanax.

Id2a functions to limit Notch pathway activity and thereby influence the transition from proliferation to differentiation of retinoblasts during zebrafish retinogenesis

During vertebrate retinogenesis, the precise balance between retinoblast proliferation and differentiation is spatially and temporally regulated through a number of intrinsic factors and extrinsic signaling pathways. Moreover, there are complex gene regulatory network interactions between these intrinsic factors and extrinsic pathways, which ultimately function to determine when retinoblasts exit the cell cycle and terminally differentiate. We recently uncovered a cell non-autonomous role for the intrinsic HLH factor, Id2a, in regulating retinoblast proliferation and differentiation, with Id2a-deficient retinae containing an abundance of proliferative retinoblasts and an absence of terminally differentiated retinal neurons and glia. Here, we report that Id2a function is necessary and sufficient to limit Notch pathway activity during retinogenesis. Id2a-deficient retinae possess elevated levels of Notch pathway component gene expression, while retinae overexpressing id2a possess reduced expression of Notch pathway component genes. Attenuation of Notch signaling activity by DAPT or by morpholino knockdown of Notch1a is sufficient to rescue both the proliferative and differentiation defects in Id2a-deficient retinae. In addition to regulating Notch pathway activity, through a novel RNA-Seq and differential gene expression analysis of Id2a-deficient retinae, we identify a number of additional intrinsic and extrinsic regulatory pathway components whose expression is regulated by Id2a. These data highlight the integral role played by Id2a in the gene regulatory network governing the transition from retinoblast proliferation to terminal differentiation during vertebrate retinogenesis.

miR-9 controls the timing of neurogenesis through the direct inhibition of antagonistic factors

The timing of commitment and cell-cycle exit within progenitor populations during neurogenesis is a fundamental decision that impacts both the number and identity of neurons produced during development. We show here that microRNA-9 plays a key role in this process through the direct inhibition of targets with antagonistic functions. Across the ventricular zone of the developing zebrafish hindbrain, miR-9 expression occurs at a range of commitment stages. Abrogating miR-9 function transiently delays cell-cycle exit, leading to the increased generation of late-born neuronal populations. Target protection analyses in vivo identify the progenitor-promoting genes her6 and zic5 and the cell-cycle exit-promoting gene elavl3/HuC as sequential targets of miR-9 as neurogenesis proceeds. We propose that miR-9 activity generates an ambivalent progenitor state poised to respond to both progenitor maintenance and commitment cues, which may be necessary to adjust neuronal production to local extrinsic signals during late embryogenesis.

Notch-Rbpj signaling is required for the development of noradrenergic neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the main source of noradrenaline in the brain and is implicated in a broad spectrum of physiological and behavioral processes. However, genetic pathways controlling the development of noradrenergic neurons in the mammalian brain are largely unknown. We report here that Rbpj, a key nuclear effector in the Notch signaling pathway, plays an essential role in LC neuron development in the mouse. Conditional inactivation of Rbpj in the dorsal rhombomere (r) 1, where LC neurons are born, resulted in a dramatic increase in the number of Phox2a- and Phox2b-expressing early-differentiating LC neurons, and dopamine-β-hydroxylase- and tyrosine-hydroxylase-expressing late-differentiating LC neurons. In contrast, other neuronal populations derived from the dorsal r1 were either reduced or unchanged. In addition, a drastic upregulation of Ascl1, an essential factor for noradrenergic neurogenesis, was observed in dorsal r1 of conditional knockout mice. Through genomic sequence analysis and EMSA and ChIP assays, a conserved Rbpj-binding motif was identified within the Ascl1 promoter. A luciferase reporter assay revealed that Rbpj per se could induce Ascl1 transactivation but this effect was counteracted by its downstream-targeted gene Hes1. Moreover, our in vitro gene transfection and in ovo electroporation assays showed that Rbpj upregulated Ascl1 expression when Hes1 expression was knocked down, although it also exerted a repressive effect on Ascl1 expression in the presence of Hes1. Thus, our results provide the first evidence that Rbpj functions as a key modulator of LC neuron development by regulating Ascl1 expression directly, and indirectly through its target gene Hes1.

Sox9b is a key regulator of pancreaticobiliary ductal system development

The pancreaticobiliary ductal system connects the liver and pancreas to the intestine. It is composed of the hepatopancreatic ductal (HPD) system as well as the intrahepatic biliary ducts and the intrapancreatic ducts. Despite its physiological importance, the development of the pancreaticobiliary ductal system remains poorly understood. The SRY-related transcription factor SOX9 is expressed in the mammalian pancreaticobiliary ductal system, but the perinatal lethality of Sox9 heterozygous mice makes loss-of-function analyses challenging. We turned to the zebrafish to assess the role of SOX9 in pancreaticobiliary ductal system development. We first show that zebrafish sox9b recapitulates the expression pattern of mouse Sox9 in the pancreaticobiliary ductal system and use a nonsense allele of sox9b, sox9b(fh313), to dissect its function in the morphogenesis of this structure. Strikingly, sox9b(fh313) homozygous mutants survive to adulthood and exhibit cholestasis associated with hepatic and pancreatic duct proliferation, cyst formation, and fibrosis. Analysis of sox9b(fh313) mutant embryos and larvae reveals that the HPD cells appear to mis-differentiate towards hepatic and/or pancreatic fates, resulting in a dysmorphic structure. The intrahepatic biliary cells are specified but fail to assemble into a functional network. Similarly, intrapancreatic duct formation is severely impaired in sox9b(fh313) mutants, while the embryonic endocrine and acinar compartments appear unaffected. The defects in the intrahepatic and intrapancreatic ducts of sox9b(fh313) mutants worsen during larval and juvenile stages, prompting the adult phenotype. We further show that Sox9b interacts with Notch signaling to regulate intrahepatic biliary network formation: sox9b expression is positively regulated by Notch signaling, while Sox9b function is required to maintain Notch signaling in the intrahepatic biliary cells. Together, these data reveal key roles for SOX9 in the morphogenesis of the pancreaticobiliary ductal system, and they cast human Sox9 as a candidate gene for pancreaticobiliary duct malformation-related pathologies.

Different levels of Notch signaling regulate quiescence, renewal and differentiation in pancreatic endocrine progenitors

Genetic studies have implicated Notch signaling in the maintenance of pancreatic progenitors. However, how Notch signaling regulates the quiescent, proliferative or differentiation behaviors of pancreatic progenitors at the single-cell level remains unclear. Here, using single-cell genetic analyses and a new transgenic system that allows dynamic assessment of Notch signaling, we address how discrete levels of Notch signaling regulate the behavior of endocrine progenitors in the zebrafish intrapancreatic duct. We find that these progenitors experience different levels of Notch signaling, which in turn regulate distinct cellular outcomes. High levels of Notch signaling induce quiescence, whereas lower levels promote progenitor amplification. The sustained downregulation of Notch signaling triggers a multistep process that includes cell cycle entry and progenitor amplification prior to endocrine differentiation. Importantly, progenitor amplification and differentiation can be uncoupled by modulating the duration and/or extent of Notch signaling downregulation, indicating that these processes are triggered by distinct levels of Notch signaling. These data show that different levels of Notch signaling drive distinct behaviors in a progenitor population.

Fbxw7 regulates Notch to control specification of neural precursors for oligodendrocyte fate

Background

In the developing vertebrate nervous system elevated levels of Notch signaling activity can block neurogenesis and promote formation of glial cells. The mechanisms that limit Notch activity to balance formation of neurons and glia from neural precursors are poorly understood.

Results

By screening for mutations that disrupt oligodendrocyte development in zebrafish we found one allele, called vu56, that produced excess oligodendrocyte progenitor cells (OPCs). Positional cloning revealed that the vu56 allele is a mutation of fbxw7, which encodes the substrate recognition component of a ubiquitin ligase that targets Notch and other proteins for degradation. To investigate the basis of the mutant phenotype we performed in vivo, time-lapse imaging, which revealed that the increase in OPC number resulted from production of extra OPCs by ventral spinal cord precursors and not from changes in OPC proliferation or death. Notch signaling activity was elevated in spinal cord precursors of fbxw7 mutant zebrafish and inhibition of Notch signaling suppressed formation of excess OPCs.

Conclusion

Notch signaling promotes glia cell formation from neural precursors in vertebrate embryos. Our data indicate that Fbxw7 helps attenuate Notch signaling during zebrafish neural development thereby limiting the number of OPCs.

HB-EGF is necessary and sufficient for Müller glia dedifferentiation and retina regeneration

Müller glia (MG) dedifferentiation into a cycling population of multipotent progenitors is crucial to zebrafish retina regeneration. The mechanisms underlying MG dedifferentiation are unknown. Here we report that heparin-binding epidermal-like growth factor (HB-EGF) is rapidly induced in MG residing at the injury site and that pro-HB-EGF ectodomain shedding is necessary for retina regeneration. Remarkably, HB-EGF stimulates the formation of multipotent MG-derived progenitors in the uninjured retina. We show that HB-EGF mediates its effects via an EGFR/MAPK signal transduction cascade that regulates the expression of regeneration-associated genes, like ascl1a and pax6(b). We also uncover an HB-EGF/Ascl1a/Notch/hb-egf(a)-signaling loop that helps define the zone of injury-responsive MG. Finally, we show that HB-EGF acts upstream of the Wnt/β-catenin-signaling cascade that controls progenitor proliferation. These data provide a link between extracellular signaling and regeneration-associated gene expression in the injured retina and suggest strategies for stimulating retina regeneration in mammals.

Building the posterior lateral line system in zebrafish

The posterior lateral line (pLL) in zebrafish has emerged as an excellent system to study how a sensory organ system develops. Here we review recent studies that illustrate how interactions between multiple signaling pathways coordinate cell fate,morphogenesis, and collective migration of cells in the posterior lateral line primordium. These studies also illustrate how the pLL system is contributing much more broadly to our understanding of mechanisms operating during the growth, regeneration, and self-organization of other organ systems during development and disease.

F3/Contactin acts as a modulator of neurogenesis during cerebral cortex development

The expression of the cell recognition molecule F3/Contactin (CNTN1) is generally associated with the functions of post-mitotic neurons. In the embryonic cortex, however, we find it expressed by proliferating ventricular zone (VZ) precursors. In contrast to previous findings in the developing cerebellum, F3/Contactin transgenic overexpression in the early cortical VZ promotes proliferation and expands the precursor pool at the expense of neurogenesis. At later stages, when F3/Contactin levels subside, however, neurogenesis resumes, suggesting that F3/Contactin expression in the VZ is inversely related to neurogenesis and plays a role in a feedback control mechanism, regulating the orderly progression of cortical development. The modified F3/Contactin profile therefore results in delayed corticogenesis, as judged by downregulation in upper and lower layer marker expression and by BrdU birth dating, indicating that, in this transgenic model, increased F3/Contactin levels counteract neuronal precursor commitment. These effects also occur in primary cultures and are reproduced by addition of an F3/Fc fusion protein to wild type cultures. Together, these data indicate a completely novel function for F3/Contactin. Parallel changes in the generation of the Notch Intracellular Domain and in the expression of the Hes-1 transcription factor indicate that activation of the Notch pathway plays a role in this phenotype, consistent with previous in vitro reports that F3/Contactin is a Notch1 ligand.

Automated reporter quantification in vivo: high-throughput screening method for reporter-based assays in zebrafish

Reporter-based assays underlie many high-throughput screening (HTS) platforms, but most are limited to in vitro applications. Here, we report a simple whole-organism HTS method for quantifying changes in reporter intensity in individual zebrafish over time termed, Automated Reporter Quantification in vivo (ARQiv). ARQiv differs from current “high-content” (e.g., confocal imaging-based) whole-organism screening technologies by providing a purely quantitative data acquisition approach that affords marked improvements in throughput. ARQiv uses a fluorescence microplate reader with specific detection functionalities necessary for robust quantification of reporter signals in vivo. This approach is: 1) Rapid; achieving true HTS capacities (i.e., >50,000 units per day), 2) Reproducible; attaining HTS-compatible assay quality (i.e., Z'-factors of ≥0.5), and 3) Flexible; amenable to nearly any reporter-based assay in zebrafish embryos, larvae, or juveniles. ARQiv is used here to quantify changes in: 1) Cell number; loss and regeneration of two different fluorescently tagged cell types (pancreatic beta cells and rod photoreceptors), 2) Cell signaling; relative activity of a transgenic Notch-signaling reporter, and 3) Cell metabolism; accumulation of reactive oxygen species. In summary, ARQiv is a versatile and readily accessible approach facilitating evaluation of genetic and/or chemical manipulations in living zebrafish that complements current “high-content” whole-organism screening methods by providing a first-tier in vivo HTS drug discovery platform.

One step forward: the use of transgenic zebrafish tumor model in drug screens

The zebrafish (Danio rerio) has been an experimental model in the developmental biology and toxicology since the 1950s. In recent years, with the aid of transgenic technology, it has also gained an increasing popularity to model human diseases, including various cancers. As a feasible vertebrate model for large-scale chemical screens, the zebrafish has also given us a new option for the search of potential anticancer drugs. It is hopeful that in the near future with automation and analytical tools, drug development processes will be significantly shortened for quick and effective identification of candidate drugs.

Fgf signaling controls pharyngeal taste bud formation through miR-200 and Delta-Notch activity

Taste buds, the taste sensory organs, are conserved in vertebrates and composed of distinct cell types, including taste receptor, basal/presynaptic and support cells. Here, we characterize zebrafish taste bud development and show that compromised Fgf signaling in the larva results in taste bud reduction and disorganization. We determine that Fgf activity is required within pharyngeal endoderm for formation of Calb2b(+) cells and reveal miR-200 and Delta-Notch signaling as key factors in this process. miR-200 knock down shows that miR-200 activity is required for taste bud formation and in particular for Calb2b(+) cell formation. Compromised delta activity in mib(-/-) dramatically reduces the number of Calb2b(+) cells and increases the number of 5HT(+) cells. Conversely, larvae with increased Notch activity and ascl1a(-/-) mutants are devoid of 5HT(+) cells, but have maintained and increased Calb2b(+) cells, respectively. These results show that Delta-Notch signaling is required for intact taste bud organ formation. Consistent with this, Notch activity restores Calb2b(+) cell formation in pharyngeal endoderm with compromised Fgf signaling, but fails to restore the formation of these cells after miR-200 knock down. Altogether, this study provides genetic evidence that supports a novel model where Fgf regulates Delta-Notch signaling, and subsequently miR-200 activity, in order to promote taste bud cell type differentiation.

Forward and reverse genetic approaches for the analysis of vertebrate development in the zebrafish

The development of facile forward and reverse genetic approaches has propelled the deconvolution of gene function in biology. While the origins of these techniques reside in the study of single-cell or invertebrate organisms, in many cases these approaches have been applied to vertebrate model systems to gain powerful insights into gene function during embryonic development. This perspective provides a summary of the major forward and reverse genetic approaches that have contributed to the study of vertebrate gene function in zebrafish, which has become an established model for the study of animal development.

BMP signaling orchestrates photoreceptor specification in the zebrafish pineal gland in collaboration with Notch

A variety of signaling pathways have been shown to regulate specification of neuronal subtype identity. However, the mechanisms by which future neurons simultaneously process information from multiple pathways to establish their identity remain poorly understood. The zebrafish pineal gland offers a simple system with which to address questions concerning the integration of signaling pathways during neural specification as it contains only two types of neurons - photoreceptors and projection neurons. We have previously shown that Notch signaling inhibits the projection neuron fate. Here, we show that BMP signaling is both necessary and sufficient to promote the photoreceptor fate. We also demonstrate that crosstalk between BMP and Notch signaling is required for the inhibition of a projection neuron fate in future photoreceptors. In this case, BMP signaling is required as a competence factor for the efficient activation of Notch targets. Our results indicate that both the induction of a photoreceptor fate and the interaction with Notch relies on a canonical BMP/Smad5 pathway. However, the activation of Notch-dependent transcription does not require a canonical Smad5-DNA interaction. Our results provide new insights into how multiple signaling influences are integrated during cell fate specification in the vertebrate CNS.

A somitic Wnt16/Notch pathway specifies haematopoietic stem cells

Haematopoietic stem cells (HSCs) are a self-renewing population that continuously replenish all blood and immune cells during the lifetime of an individual^{1, 2}. HSCs are used clinically to treat a wide array of diseases, including acute leukaemias and congenital blood disorders, but obtaining suitable numbers of cells and finding immune compatible donors remain serious problems. These concerns have led to an interest in the conversion of embryonic stem cells or induced pluripotent stem cells into HSCs, which is not possible using current methodologies. To accomplish this goal, it is critical to understand the native mechanisms involved in specification of HSCs during embryonic development. Here we demonstrate that Wnt16 controls a novel genetic regulatory network required for HSC specification. Non-canonical signaling by Wnt16 is required for somitic expression of the Notch ligands deltaC (dlc) and deltaD (dld), and these ligands are in turn required for establishment of definitive haematopoiesis. Notch signalling downstream of Dlc/Dld is earlier than, and distinct from known cell-autonomous requirements for Notch, strongly suggesting that novel Notch-dependent relay signal(s) induce the first HSCs in parallel to other established pathways. Our results demonstrate that somite-specific gene expression is required for the production of haemogenic endothelium.

Zebrafish models for cancer

First established as a valuable vertebrate model system for studying development, zebrafish have emerged as an attractive animal system for modeling human cancers. Major technical advances have been essential for the generation of zebrafish cancer models relevant to human diseases. These models develop tumors in various organ sites that bear striking resemblance to human malignances, both histologically and genetically. Thus, the focus of cancer research in zebrafish has transcended the need to validate zebrafish as a viable model organism to study cancer biology. With the significant advantages of in vivo imaging, the power of forward genetics, well-established high efficiency for transgenesis, and ease of transplantation, further exploration of the zebrafish cancer models not only will generate unique insights into underlying mechanisms of cancer but will also provide platforms useful for drug discovery.

Fli+ etsrp+ hemato-vascular progenitor cells proliferate at the lateral plate mesoderm during vasculogenesis in zebrafish

BACKGROUND

Vasculogenesis, the de novo formation of blood vessels from precursor cells is critical for a developing embryo. However, the signals and events that dictate the formation of primary axial vessels remain poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we use ets-related protein-1 (etsrp), which is essential for vascular development, to analyze the early stages of vasculogenesis in zebrafish. We found etsrp(+) cells of the head, trunk and tail follow distinct developmental sequences. Using a combination of genetic, molecular and chemical approaches, we demonstrate that fli(+)etsrp(+) hemato-vascular progenitors (FEVPs) are proliferating at the lateral plate mesoderm (LPM). The Shh-VEGF-Notch-Hey2 signaling pathway controls the proliferation process, and experimental modulation of single components of this pathway alters etsrp(+) cell numbers at the LPM.

CONCLUSIONS/SIGNIFICANCE

This study for the first time defines factors controlling proliferation, and cell numbers of pre-migratory FEVPs in zebrafish.

Neuron-specific expression of atp6v0c2 in zebrafish CNS

Vacuolar ATPase (V-ATPase) is a multi-subunit enzyme that plays an important role in the acidification of a variety of intracellular compartments. ATP6V0C is subunit c of the V(0) domain that forms the proteolipid pore of the enzyme. In the present study, we investigated the neuron-specific expression of atp6v0c2, a novel isoform of the V-ATPase c-subunit, during the development of the zebrafish CNS. Zebrafish atp6v0c2 was isolated from a genome-wide analysis of the zebrafish mib(ta52b) mutant designed to identify genes differentially regulated by Notch signaling. Whole-mount in situ hybridization revealed that atp6v0c2 is expressed in a subset of CNS neurons beginning several hours after the emergence of post-mitotic neurons. The ATP6V0C2 protein is co-localized with the presynaptic vesicle marker, SV2, suggesting that it is involved in neurotransmitter storage and/or secretion in neurons. In addition, the loss-of-function experiment suggests that ATP6V0C2 is involved in the control of neuronal excitability.

Delaying Gal4-driven gene expression in the zebrafish with morpholinos and Gal80

The modular Gal4/UAS gene expression system has become an indispensable tool in modern biology. Several large-scale gene- and enhancer-trap screens in the zebrafish have generated hundreds of transgenic lines expressing Gal4 in unique patterns. However, the early embryonic expression of the Gal4 severely limits their use for studies on regeneration or behavior because UAS-driven effectors could disrupt normal organogenesis. To overcome this limitation, we explored the use of the Gal4 repressor Gal80 in transient assays and with stable transgenes to temporally control Gal4 activity. We also validated a strategy to delay Gal4-driven gene expression using a morpholino targeted to Gal4. The first approach is limited to transgenes expressing the native Gal4. The morphant approach can also be applied to transgenic lines expressing the Gal4-VP16 fusion protein. It promises to become a standard approach to delay Gal4-driven transgene expression and enhance the genetic toolkit for the zebrafish.

Notch3 activation promotes invasive glioma formation in a tissue site-specific manner

Although Notch signaling has been widely implicated in neoplastic growth, direct evidence for in vivo initiation of neoplasia by the pathway in murine models has been limited to tumors of lymphoid, breast, and choroid plexus cells. To examine tumorigenic potential in the eye and brain, we injected retroviruses encoding activated forms of Notch1, Notch2, or Notch3 into embryonic mice. Interestingly, the majority of animals infected with active Notch3 developed proliferative lesions comprised of pigmented ocular choroid cells, retinal and optic nerve glia, and lens epithelium. Notch3-induced lesions in the choroid, retina, and optic nerve were capable of invading adjacent tissues, suggesting that they were malignant tumors. Although Notch3 activation induced choroidal tumors in up to 67% of eyes, Notch1 or Notch2 activation never resulted in such tumors. Active forms of Notch1 and Notch2 did generate a few small proliferative glial nodules in the retina and optic nerve, whereas Notch3 was 10-fold more efficient at generating growths, many of which were large invasive gliomas. Expression of active Notch1/Notch3 chimeric receptors implicated the RBPjk-association molecule and transactivation domains of Notch3 in generating choroidal and glial tumors, respectively. In contrast to our findings in the optic nerve and retina, introduction of active Notch receptors, including Notch3, into the brain never caused glial tumors. Our results highlight the differential ability of Notch receptor paralogs to initiate malignant tumor formation, and suggest that glial precursors of the optic nerve, but not the brain, are susceptible to transformation by Notch3.

Non-canonical activation of Notch signaling/target genes in vertebrates

Evolutionarily conserved Notch signaling orchestrates diverse physiological mechanisms during metazoan development and homeostasis. Classically, ligand-activated Notch receptors transduce the signaling cascade through the interaction of DNA-bound CBF1-co-repressor complex. However, recent reports have demonstrated execution of a CBF1-independent Notch pathway through signaling cross-talks in various cells/tissues. Here, we have tried to congregate the reports that describe the non-canonical/CBF1-independent Notch signaling and target gene activation in vertebrates with specific emphasis on their functional relevance.

Deletion of the WD40 domain of LRRK2 in Zebrafish causes Parkinsonism-like loss of neurons and locomotive defect

LRRK2 plays an important role in Parkinson's disease (PD), but its biological functions are largely unknown. Here, we cloned the homolog of human LRRK2, characterized its expression, and investigated its biological functions in zebrafish. The blockage of zebrafish LRRK2 (zLRRK2) protein by morpholinos caused embryonic lethality and severe developmental defects such as growth retardation and loss of neurons. In contrast, the deletion of the WD40 domain of zLRRK2 by morpholinos targeting splicing did not induce severe embryonic developmental defects; rather it caused Parkinsonism-like phenotypes, including loss of dopaminergic neurons in diencephalon and locomotion defects. These neurodegenerative and locomotion defects could be rescued by over-expressing zLRRK2 or hLRRK2 mRNA. The administration of L-dopa could also rescue the locomotion defects, but not the neurodegeneration. Taken together, our results demonstrate that zLRRK2 is an ortholog of hLRRK2 and that the deletion of WD40 domain of zLRRK2 provides a disease model for PD.

Parkinson's disease (PD) is a degenerative disease of the brain (central nervous system) that often impairs motor skills, speech, and other functions. PD was long thought to be caused by environmental factors, but the discovery of several gene mutations in the patients (mostly with familial form of PD) clearly demonstrated the involvement of genetic factors in the development of PD. Among the identified genes, LRRK2 was discovered to be one of the most important genetic causes of PD. The biological function of LRRK2 was, however, largely unknown. In this study, we studied the function of LRRK2 in zebrafish by blocking the normal function of LRRK2. The zebrafish showed features of neurodegeneration and locomotion defects, similar to those of PD patients. The defects of the fish could be rescued by expressing the normal protein of LRRK2, and the locomotion defect could also be rescued by the administration of L-dopa that is commonly used for treating PD patients. We have therefore developed a zebrafish model for PD that can be used for understanding the mechanism underlying the development of PD and will be helpful for future screening of new drugs to treat PD.

Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina

It has been reported that the loss of apicobasal cell polarity and the disruption of adherens junctions induce hyperplasia in the mouse developing brain. However, it is not fully understood whether hyperplasia is caused by an enhanced cell proliferation, an inhibited neurogenesis, or both. In this study, we found that the ratio of the number of proliferating progenitor cells to the total number of retinal cells increases in the neurogenic stages in zebrafish n-cadherin (ncad) and nagie oko (nok) mutants, in which the apicobasal cell polarity and adherens junctions in the retinal epithelium are disrupted. The cell-cycle progression was not altered in the ncad and nok mutants. Rather, the ratio of the number of cells undergoing neurogenic cell division to the total number of cells undergoing mitosis decreased in the ncad and nok mutant retinas, suggesting that the switching from proliferative cell division to neurogenic cell division was compromised in these mutant retinas. These findings suggest that the inhibition of neurogenesis is a primary defect that causes hyperplasia in the ncad and nok mutant retinas. The Hedgehog-protein kinase A signaling pathway and the Notch signaling pathway regulate retinal neurogenesis in zebrafish. We found that both signaling pathways are involved in the generation of neurogenic defects in the ncad and nok mutant retinas. Taken together, these findings suggest that apicobasal cell polarity and epithelial integrity are essential for retinal neurogenesis in zebrafish.

Progenitor cell maturation in the developing vertebrate retina

Progenitor cells in the developing retina initially divide so that each division produces two cells that divide again. Subsequently, progenitor cells change their mode of division so that one or both cells produced by a division can withdraw from the mitotic cycle and differentiate. We asked how these two progenitor cell stages differ molecularly and what controls the switch in the mode of division. We show that early preneurogenic progenitor cells express the transcription factor, Sox2, and the Notch ligand, Delta1. More mature neurogenic progenitor cells express Sox2 and the bHLH transcription factor, E2A, and not Delta1. Notch signaling maintains progenitor cells in the preneurogenic state. Sonic hedgehog expressed by newly differentiating cells initiates maturation of progenitor cells from preneurogenic to neurogenic at the neurogenic front, possibly by down-regulating Delta1 expression. Our results show that the preneurogenic-to-neurogenic transition is a highly organized unidirectional step made in unison by neighboring cells.

Delayed and restricted expression of UAS-regulated GFP gene in early transgenic zebrafish embryos by using the GAL4/UAS system

A stable Tg(UAS:GFP) zebrafish line was generated and crossed with Tg(hsp70:GAL4) line, in which the GAL4 gene is under the control of an inducible zebrafish promoter derived from the heat shock 70 protein gene (hsp70). The dynamic green fluorescent protein (GFP) expression in early zebrafish embryos in the GAL4/UAS binary system was then investigated. We found that, at early developmental stages, expression of GFP effector gene was restricted and required a long recovery time to reach a detectable level. At later developmental stage (after 2 days postfertilization), GFP could be activated in multiple tissues in a shorter time, apparently due to a higher level of GAL4 messenger RNA induction. It appears that the type of tissues expressing GFP was dependent on whether they had been developed at the time of heat shock. Therefore, the delayed and restricted transgene expression should be taken into consideration when GAL4/UAS system is used to study transgene expression in early developmental stages.

Neurogenesis

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

Analysis of the retina in the zebrafish model

The zebrafish is one of the leading models for the analysis of the vertebrate visual system. A wide assortment of molecular, genetic, and cell biological approaches is available to study zebrafish visual system development and function. As new techniques become available, genetic analysis and imaging continue to be the strengths of the zebrafish model. In particular, recent developments in the use of transposons and zinc finger nucleases to produce new generations of mutant strains enhance both forward and reverse genetic analysis. Similarly, the imaging of developmental and physiological processes benefits from a wide assortment of fluorescent proteins and the ways to express them in the embryo. The zebrafish is also highly attractive for high-throughput screening of small molecules, a promising strategy to search for compounds with therapeutic potential. Here we discuss experimental approaches used in the zebrafish model to study morphogenetic transformations, cell fate decisions, and the differentiation of fine morphological features that ultimately lead to the formation of the functional vertebrate visual system.