Stepwise maturation of apicobasal polarity of the neuroepithelium is essential for vertebrate neurulation

Assembly, dynamics and remodeling of epithelial cell junctions throughout development

Cell junctions play key roles in epithelial integrity. During development, when epithelia undergo extensive morphogenesis, these junctions must be remodeled in order to maintain mechanochemical barriers and ensure the cohesion of the tissue. In this Review, we present a comprehensive and integrated description of junctional remodeling mechanisms in epithelial cells during development, from embryonic to adult epithelia. We largely focus on Drosophila, as quantitative analyses in this organism have provided a detailed characterization of the molecular mechanisms governing cell topologies, and discuss the conservation of these mechanisms across metazoans. We consider how changes at the molecular level translate to tissue-scale irreversible deformations, exploring the composition and assembly of cellular interfaces to unveil how junctions are remodeled to preserve tissue homeostasis during cell division, intercalation, invagination, ingression and extrusion.

Stepwise modulation of apical orientational cell adhesions for vertebrate neurulation

Neurulation transforms the neuroectoderm into the neural tube. This transformation relies on reorganising the configurational relationships between the orientations of intrinsic polarities of neighbouring cells. These orientational intercellular relationships are established, maintained, and modulated by orientational cell adhesions (OCAs). Here, using zebrafish (Danio rerio) neurulation as a major model, we propose a new perspective on how OCAs contribute to the parallel, antiparallel, and opposing intercellular relationships that underlie the neural plate-keel-rod-tube transformation, a stepwise process of cell aggregation followed by cord hollowing. We also discuss how OCAs in neurulation may be regulated by various adhesion molecules, including cadherins, Eph/Ephrins, Claudins, Occludins, Crumbs, Na+ /K+ -ATPase, and integrins. By comparing neurulation among species, we reveal that antiparallel OCAs represent a conserved mechanism for the fusion of the neural tube. Throughout, we highlight some outstanding questions regarding OCAs in neurulation. Answers to these questions will help us understand better the mechanisms of tubulogenesis of many tissues.

Orientational cell adhesions (OCAs) for tissue morphogenesis

Distinct tissue architectures arise when cells are organized in specific orientations relative to neighboring cells. These orientational intercellular relationships are established and maintained by cell adhesions. We propose the concept of 'orientational cell adhesions' (OCAs) to couple cell orientations with cell adhesions, thus offering a new perspective to study tissue morphogenesis.

Apical-basal polarity and the control of epithelial form and function

Epithelial cells are the most common cell type in all animals, forming the sheets and tubes that compose most organs and tissues. Apical-basal polarity is essential for epithelial cell form and function, as it determines the localization of the adhesion molecules that hold the cells together laterally and the occluding junctions that act as barriers to paracellular diffusion. Polarity must also target the secretion of specific cargoes to the apical, lateral or basal membranes and organize the cytoskeleton and internal architecture of the cell. Apical-basal polarity in many cells is established by conserved polarity factors that define the apical (Crumbs, Stardust/PALS1, aPKC, PAR-6 and CDC42), junctional (PAR-3) and lateral (Scribble, DLG, LGL, Yurt and RhoGAP19D) domains, although recent evidence indicates that not all epithelia polarize by the same mechanism. Research has begun to reveal the dynamic interactions between polarity factors and how they contribute to polarity establishment and maintenance. Elucidating these mechanisms is essential to better understand the roles of apical-basal polarity in morphogenesis and how defects in polarity contribute to diseases such as cancer.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Coordinated assembly and release of adhesions builds apical junctional belts during de novo polarisation of an epithelial tube

Using the zebrafish neural tube as a model, we uncover the in vivo mechanisms allowing the generation of two opposing apical epithelial surfaces within the centre of an initially unpolarised, solid organ. We show that Mpp5a and Rab11a play a dual role in coordinating the generation of ipsilateral junctional belts whilst simultaneously releasing contralateral adhesions across the centre of the tissue. We show that Mpp5a- and Rab11a-mediated resolution of cell-cell adhesions are both necessary for midline lumen opening and contribute to later maintenance of epithelial organisation. We propose that these roles for both Mpp5a and Rab11a operate through the transmembrane protein Crumbs. In light of a recent conflicting publication, we also clarify that the junction-remodelling role of Mpp5a is not specific to dividing cells.

Notch-mediated inhibition of neurogenesis is required for zebrafish spinal cord morphogenesis

The morphogenesis of the nervous system requires coordinating the specification and differentiation of neural precursor cells, the establishment of neuroepithelial tissue architecture and the execution of specific cellular movements. How these aspects of neural development are linked is incompletely understood. Here we inactivate a major regulator of embryonic neurogenesis - the Delta/Notch pathway - and analyze the effect on zebrafish central nervous system morphogenesis. While some parts of the nervous system can establish neuroepithelial tissue architecture independently of Notch, Notch signaling is essential for spinal cord morphogenesis. In this tissue, Notch signaling is required to repress neuronal differentiation and allow thereby the emergence of neuroepithelial apico-basal polarity. Notch-mediated suppression of neurogenesis is also essential for the execution of specific morphogenetic movements of zebrafish spinal cord precursor cells. In the wild-type neural tube, cells divide at the organ midline to contribute one daughter cell to each organ half. Notch signaling deficient animals fail to display this behavior and therefore form a misproportioned spinal cord. Taken together, our findings show that Notch-mediated suppression of neurogenesis is required to allow the execution of morphogenetic programs that shape the zebrafish spinal cord.

Cdh2 coordinates Myosin-II dependent internalisation of the zebrafish neural plate

Tissue internalisation is a key morphogenetic mechanism by which embryonic tissues generate complex internal organs and a number of studies of epithelia have outlined a general view of tissue internalisation. Here we have used quantitative live imaging and mutant analysis to determine whether similar mechanisms are responsible for internalisation in a tissue that apparently does not have a typical epithelial organisation - the zebrafish neural plate. We found that although zebrafish embryos begin neurulation without a conventional epithelium, medially located neural plate cells adopt strategies typical of epithelia in order to constrict their dorsal surface membrane during cell internalisation. Furthermore, we show that Myosin-II activity is a significant driver of this transient cell remodeling which also depends on Cdh2 (N-cadherin). Abrogation of Cdh2 results in defective Myosin-II distribution, mislocalised internalisation events and defective neural plate morphogenesis. Our work suggests Cdh2 coordinates Myosin-II dependent internalisation of the zebrafish neural plate.

Apical Cell-Cell Adhesions Reconcile Symmetry and Asymmetry in Zebrafish Neurulation

The symmetric tissue and body plans of animals are paradoxically constructed with asymmetric cells. To understand how the yin-yang duality of symmetry and asymmetry are reconciled, we asked whether apical polarity proteins orchestrate the development of the mirror-symmetric zebrafish neural tube by hierarchically modulating apical cell-cell adhesions. We found that apical polarity proteins localize by a pioneer-intermediate-terminal order. Pioneer proteins establish the mirror symmetry of the neural rod by initiating two distinct types of apical adhesions: the parallel apical adhesions (PAAs) cohere cells of parallel orientation and the novel opposing apical adhesions (OAAs) cohere cells of opposing orientation. Subsequently, the intermediate proteins selectively augment the PAAs when the OAAs dissolve by endocytosis. Finally, terminal proteins are required to inflate the neural tube by generating osmotic pressure. Our findings suggest a general mechanism to construct mirror-symmetric tissues: tissue symmetry can be established by organizing asymmetric cells opposingly via adhesions.

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Apical polarity proteins localize in a pioneer-intermediate-terminal order

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The orderly localized proteins orchestrate apical adhesion dynamics step by step

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Apical adhesions assemble asymmetric cells opposingly into a symmetric tissue

Impact of Coffee and Cacao Purine Metabolites on Neuroplasticity and Neurodegenerative Disease

Increasing evidence suggests that regular consumption of coffee, tea and dark chocolate (cacao) can promote brain health and may reduce the risk of age-related neurodegenerative disorders. However, the complex array of phytochemicals in coffee and cacao beans and tea leaves has hindered a clear understanding of the component(s) that affect neuronal plasticity and resilience. One class of phytochemicals present in relatively high amounts in coffee, tea and cacao are methylxanthines. Among such methylxanthines, caffeine has been the most widely studied and has clear effects on neuronal network activity, promotes sustained cognitive performance and can protect neurons against dysfunction and death in animal models of stroke, Alzheimer's disease and Parkinson's disease. Caffeine's mechanism of action relies on antagonism of various subclasses of adenosine receptors. Downstream xanthine metabolites, such as theobromine and theophylline, may also contribute to the beneficial effects of coffee, tea and cacao on brain health.

Microtubules, polarity and vertebrate neural tube morphogenesis

Microtubules (MTs) are key cellular components, long known to participate in morphogenetic events that shape the developing embryo. However, the links between the cellular functions of MTs, their effects on cell shape and polarity, and their role in large-scale morphogenesis remain poorly understood. Here, these relationships were examined with respect to two strategies for generating the vertebrate neural tube: bending and closure of the mammalian neural plate; and cavitation of the teleost neural rod. The latter process has been compared with 'secondary' neurulation that generates the caudal spinal cord in mammals. MTs align along the apico-basal axis of the mammalian neuroepithelium early in neural tube closure, participating functionally in interkinetic nuclear migration, which indirectly impacts on cell shape. Whether MTs play other functional roles in mammalian neurulation remains unclear. In the zebrafish, MTs are important for defining the neural rod midline prior to its cavitation, both by localizing apical proteins at the tissue midline and by orienting cell division through a mirror-symmetric MT apparatus that helps to further define the medial localization of apical polarity proteins. Par proteins have been implicated in centrosome positioning in neuroepithelia as well as in the control of polarized morphogenetic movements in the neural rod. Understanding of MT functions during early nervous system development has so far been limited, partly by techniques that fail to distinguish 'cause' from 'effect'. Future developments will likely rely on novel ways to selectively impair MT function in order to investigate the roles they play.

Coordinating cell and tissue behavior during zebrafish neural tube morphogenesis

The development of a vertebrate neural epithelium with well-organized apico-basal polarity and a central lumen is essential for its proper function. However, how this polarity is established during embryonic development and the potential influence of surrounding signals and tissues on such organization has remained less understood. In recent years the combined superior transparency and genetics of the zebrafish embryo has allowed for in vivo visualization and quantification of the cellular and molecular dynamics that govern neural tube structure. Here, we discuss recent studies revealing how co-ordinated cell-cell interactions coupled with adjacent tissue dynamics are critical to regulate final neural tissue architecture. Furthermore, new findings show how the spatial regulation and timing of orientated cell division is key in defining precise lumen formation at the tissue midline. In addition, we compare zebrafish neurulation with that of amniotes and amphibians in an attempt to understand the conserved cellular mechanisms driving neurulation and resolve the apparent differences among animals. Zebrafish neurulation not only offers fundamental insights into early vertebrate brain development but also the opportunity to explore in vivo cell and tissue dynamics during complex three-dimensional animal morphogenesis.

Establishing the plane of symmetry for lumen formation and bilateral brain formation in the zebrafish neural rod

The lumen of the zebrafish neural tube develops precisely at the midline of the solid neural rod primordium. This process depends on cell polarisation and cell rearrangements, both of which are manifest at the midline of the neural rod. The result of this cell polarisation and cell rearrangement is an epithelial tube that has overt mirror-symmetry, such that cell morphology and apicobasal polarisation are mirrored across the midline of the neural tube. This article discusses how this mirror-symmetry is established and proposes the hypothesis that positioning the cells' centrosomes to the midline of the neural rod is a key event in organising this process.

Fluorescently tagged Lin7c is a dynamic marker for polarity maturation in the zebrafish retinal epithelium

Development of epithelial cell polarity is a highly dynamic process, and often established by the sequential recruitment of conserved protein complexes, such as the Par or the Crumbs (Crb) complex. However, detailed insights into the refinement of polarity and the formation of the complexes are still lacking. Here, we established fluorescently tagged Lin7c, a core member of the Crb complex, as an ideal tool to follow development of polarity in zebrafish epithelia. We find that in gastrula stages, RFP-Lin7c is found in the cytosol of the enveloping layer, while Pard3-GFP is already polarized at this stage. During development of the retinal epithelium, RFP-Lin7c localization is refined from being cytosolic at 14 hours post fertilization (hpf) to almost entirely apical in cells of the eye cup at 28 hpf. This apical Lin7c localization depends on the Crb complex members Oko meduzy and Nagie oko. Thus, fluorescently tagged Lin7c can be used in a broad range of epithelia to follow polarity maturation in vivo and specifically to elucidate the sequence of events determining Crb complex-mediated polarity.

Spatial-temporal expressions of Crumbs and Nagie oko and their interdependence in zebrafish central nervous system during early development

A vast number of apicobasal polarity proteins play essential roles in the polarization and morphogenesis of the neuroepithelia. Crumbs (Crb) type I transmembrane cell-cell adhesion proteins are among these proteins. Five crb genes have been identified in zebrafish. However, their expressional and functional differences during early neural development remain to be fully elucidated. Here, we study the spatial-temporal expression patterns and functions of Crb1, Crb2a, and Crb2b in the central nervous system (CNS) during the neurulation period. We show that: 1, the optic vesicle and undifferentiated retinal neuroepithelium only express Crb2a; 2, Crb1 and Crb2a expressions overlap extensively in the undifferentiated neural tube epithelium; 3, Crb2b expression is the weakest of the three and is restricted to the ventral-most regions of the anterior CNS; and 4, Nok and Crb proteins require each other for their apical localization in neuroepithelium. The commencements of Crb1, Crb2a, and Crb2b expressions follow a spatial-temporal spread from anterior to posterior and from ventral to dorsal and lag behind that of adherens junction components, such as ZO-1 and actin bundles. Genetic and morpholino suppression analyses suggest that in regions where these Crb expressions overlap, they are functionally redundant in maintaining apicobasal polarity of the undifferentiated neuroepithelium.

Developmental time rather than local environment regulates the schedule of epithelial polarization in the zebrafish neural rod

Background

Morphogenesis requires developmental processes to occur both at the right time and in the right place. During neural tube formation in the zebrafish embryo, the generation of the apical specializations of the lumen must occur in the center of the neural rod after the neural cells have undergone convergence, invagination and interdigitation across the midline. How this coordination is achieved is uncertain. One possibility is that environmental signaling at the midline of the neural rod controls the schedule of apical polarization. Alternatively, polarization could be regulated by a timing mechanism and then independent morphogenetic processes ensure the cells are in the correct spatial location.

Results

Ectopic transplantation demonstrates the local environment of the neural midline is not required for neural cell polarization. Neural cells can self-organize into epithelial cysts in ectopic locations in the embryo and also in three-dimensional gel cultures. Heterochronic transplants demonstrate that the schedule of polarization and the specialized cell divisions characteristic of the neural rod are more strongly regulated by time than local environmental signals. The cells’ schedule for polarization is set prior to gastrulation, is stable through several rounds of cell division and appears independent of the morphogenetic movements of gastrulation and neurulation.

Conclusions

Time rather than local environment regulates the schedule of epithelial polarization in zebrafish neural rod.

Characterization of transgenic zebrafish lines that express GFP in the retina, pineal gland, olfactory bulb, hatching gland, and optic tectum

Transgenic animals are powerful tools to study gene function invivo. Here we characterize several transgenic zebrafish lines that express green fluorescent protein (GFP) under the control of the LCR(RH2)-RH2-1 or LCR(RH2)-RH2-2 green opsin regulatory elements. Using confocal immunomicroscopy, stereo-fluorescence microscopy, and Western blotting, we show that the Tg(LCR(RH2)-RH2-1:GFP)(pt112) and Tg(LCR(RH2)-RH2-2:GFP)(pt115) transgenic zebrafish lines express GFP in the pineal gland and certain types of photoreceptors. In addition, some of these lines also express GFP in the hatching gland, optic tectum, or olfactory bulb. Some of the expression patterns differ significantly from previously published similar transgenic fish lines, making them useful tools for studying the development of the corresponding tissues and organs. In addition, the variations of GFP expression among different lines corroborate the notion that transgenic expression is often subjected to position effect, thus emphasizing the need for careful verification of expression patterns when transgenic animal models are utilized for research.

Neurogenesis in zebrafish - from embryo to adult

Neurogenesis in the developing central nervous system consists of the induction and proliferation of neural progenitor cells and their subsequent differentiation into mature neurons. External as well as internal cues orchestrate neurogenesis in a precise temporal and spatial way. In the last 20 years, the zebrafish has proven to be an excellent model organism to study neurogenesis in the embryo. Recently, this vertebrate has also become a model for the investigation of adult neurogenesis and neural regeneration. Here, we summarize the contributions of zebrafish in neural development and adult neurogenesis.

Apicobasal polarity and neural tube closure

During development, a flat neural plate rolls up and closes to form a neural tube. This process, called neural tube closure, is complex and requires morphogenetic events to occur along multiple axes of the neural plate. Recent studies suggest that cell and tissue polarity play a major role in neural tube morphogenesis. While the planar cell polarity pathway is known to be involved in this process, a role for the apicobasal polarity pathway has only recently begun to be elucidated. These studies show that bone morphogenetic proteins can regulate the apicobasal polarity pathway in the neural plate in a cell cycle dependent manner. This dynamically modulates apical junctions in the neural plate, resulting in cell and tissue shape changes that help bend, shape and close the neural tube.

Mirror-symmetric microtubule assembly and cell interactions drive lumen formation in the zebrafish neural rod

By analysing the cellular and subcellular events that occur in the centre of the developing zebrafish neural rod, we have uncovered a novel mechanism of cell polarisation during lumen formation. Cells from each side of the neural rod interdigitate across the tissue midline. This is necessary for localisation of apical junctional proteins to the region where cells intersect the tissue midline. Cells assemble a mirror-symmetric microtubule cytoskeleton around the tissue midline, which is necessary for the trafficking of proteins required for normal lumen formation, such as partitioning defective 3 and Rab11a to this point. This occurs in advance and is independent of the midline cell division that has been shown to have a powerful role in lumen organisation. To our knowledge, this is the first example of the initiation of apical polarisation part way along the length of a cell, rather than at a cell extremity. Although the midline division is not necessary for apical polarisation, it confers a morphogenetic advantage by efficiently eliminating cellular processes that would otherwise bridge the developing lumen.

Structural disorganization of pronephric glomerulus in zebrafish mpp5a/nagie oko mutant

BACKGROUND

The podocyte slit diaphragm (SD) is an essential component of the selective filtration barrier in the glomerulus. Several structural proteins required for formation and maintenance of SD have been identified; however, molecular mechanisms regulating these proteins are still limited.

RESULTS

Here, we demonstrate that MAGUK p55 subfamily member 5a (Mpp5a)/Nagie oko, a component of the Crb multi-protein complex, was colocalized with an SD-associated protein ZO-1 in the zebrafish pronephric glomerulus. To characterize the function of Mpp5a, zebrafish mpp5a(m520) mutant embryos, which are known to have defects in cardiac and neuronal morphogenesis, were analyzed. These mutants failed to merge the bilateral glomerular primordia and to form the glomerular capillary and mesangium, but the foot processes and SD showed normal appearance. The structural disorganization in the mpp5a(m520) mutant glomerulus was quite similar to that of a cardiac troponin T2a/tnnt2a/silent heart knockdown zebrafish, which exhibited circulatory failure due to lack of heart beating.

CONCLUSIONS

Mpp5a is not prerequisite to form podocyte slit diaphragm in the pronephric glomerular development in zebrafish. The structural disorganization of the pronephric glomerulus in the mpp5a(m520) mutant is likely to result from circulatory failure, rather than the anomaly of Mpp5a protein in the glomerulus.

Crb apical polarity proteins maintain zebrafish retinal cone mosaics via intercellular binding of their extracellular domains

Cone photoreceptors are assembled by unknown mechanisms into geometrically regular mosaics in many vertebrate species. The formation and maintenance of photoreceptor mosaics are speculated to require differential cell-cell adhesion. However, the molecular basis for this theory has yet to be identified. The retina and many other tissues express Crumbs (Crb) polarity proteins. The functions of the extracellular domains of Crb proteins remain to be understood. Here we report cell-type-specific expression of the crb2a and crb2b genes at the cell membranes of photoreceptor inner segments and Müller cell apical processes in the zebrafish retina. We demonstrate that the extracellular domains of Crb2a and Crb2b mediate a cell-cell adhesion function, which plays an essential role in maintaining the integrity of photoreceptor layer and cone mosaics. Because Crb proteins are expressed in many types of epithelia, the Crb-based cell-cell adhesion may underlie cellular patterning in other epithelium-derived tissues as well.

Association study of PARD3 gene polymorphisms with neural tube defects in a Chinese Han population

Partitioning defective 3 homolog (PARD3) is an attractive candidate gene for screening neural tube defect (NTD) risk. To investigate the role of genetic variants in PARD3 on NTD risk, a case-control study was performed in a region of China with a high prevalence of NTDs. Total 53 single-nucleotide polymorphisms (SNPs) in PARD3 were genotyped in 224 fetuses with NTDs and in 253 normal fetuses. We found that 6 SNPs (rs2496720, rs2252655, rs3851068, rs118153230, rs10827337, and rs12218196) were statistically associated with NTDs (P < .05). After stratifying participants by NTD phenotypes, the significant association only existed in cases with anencephaly rather than spina bifida. Further haplotype analysis confirmed the association between PARD3 polymorphisms and NTD risk (global test P = 3.41e-008). Our results suggested that genetic variants in PARD3 were associated with susceptibility to NTDs in a Chinese Han population, and this association was affected by NTD phenotypes.

Bone morphogenetic proteins regulate neural tube closure by interacting with the apicobasal polarity pathway

During neural tube closure, specialized regions called hinge points (HPs) display dynamic and polarized cell behaviors necessary for converting the neural plate into a neural tube. The molecular bases of such cell behaviors (e.g. apical constriction, basal nuclear migration) are poorly understood. We have identified a two-dimensional canonical BMP activity gradient in the chick neural plate that results in low and temporally pulsed BMP activity at the ventral midline/median hinge point (MHP). Using in vivo manipulations, high-resolution imaging and biochemical analyses, we show that BMP attenuation is necessary and sufficient for MHP formation. Conversely, BMP overexpression abolishes MHP formation and prevents neural tube closure. We provide evidence that BMP modulation directs neural tube closure via the regulation of apicobasal polarity. First, BMP blockade produces partially polarized neural cells, which retain contact with the apical and basal surfaces but where basolateral proteins (LGL) become apically localized and apical junctional proteins (PAR3, ZO1) become targeted to endosomes. Second, direct LGL misexpression induces ectopic HPs identical to those produced by noggin or dominant-negative BMPR1A. Third, BMP-dependent biochemical interactions occur between the PAR3-PAR6-aPKC polarity complex and phosphorylated SMAD5 at apical junctions. Finally, partially polarized cells normally occur at the MHP, their frequencies inversely correlated with the BMP activity gradient in the neural plate. We propose that spatiotemporal modulation of the two-dimensional BMP gradient transiently alters cell polarity in targeted neuronal cells. This ensures that the neural plate is flexible enough to be focally bent and shaped into a neural tube, while retaining overall epithelial integrity.

Essential genes for astroglial development and axon pathfinding during zebrafish embryogenesis

The formation of the central nervous system depends on the coordinated development of neural and glial cell types that arise from a common precursor. Using an existing group of zebrafish mutants generated by viral insertion, we performed a "shelf-screen" to identify genes necessary for astroglial development and axon scaffold formation. We screened 274 of 315 viral insertion lines using antibodies that label axons (anti-Acetylated Tubulin) and astroglia (anti-Gfap) and identified 25 mutants with defects in gliogenesis, glial patterning, neurogenesis, and axon guidance. We also identified a novel class of mutants affecting radial glial cell numbers. Defects in astroglial patterning were always associated with axon defects, supporting an important role for axon-glial interactions during axon scaffold development. The genes disrupted in these viral lines have all been identified, providing a powerful new resource for the study of axon guidance, glio- and neurogenesis, and neuron-glial interactions during development of the vertebrate CNS.

Wnt/PCP signaling controls intracellular position of MTOCs during gastrulation convergence and extension movements

During vertebrate gastrulation, convergence and extension cell movements are coordinated with the anteroposterior and mediolateral embryonic axes. Wnt planar cell polarity (Wnt/PCP) signaling polarizes the motile behaviors of cells with respect to the anteroposterior embryonic axis. Understanding how Wnt/PCP signaling mediates convergence and extension (C&E) movements requires analysis of the mechanisms employed to alter cell morphology and behavior with respect to embryonic polarity. Here, we examine the interactions between the microtubule cytoskeleton and Wnt/PCP signaling during zebrafish gastrulation. First, we assessed the location of the centrosome/microtubule organizing center (MTOC) relative to the cell nucleus and the body axes, as a marker of cell polarity. The intracellular position of MTOCs was polarized, perpendicular to the plane of the germ layers, independently of Wnt/PCP signaling. In addition, this position became biased posteriorly and medially within the plane of the germ layers at the transition from mid- to late gastrulation and from slow to fast C&E movements. This depends on intact Wnt/PCP signaling through Knypek (Glypican4/6) and Dishevelled components. Second, we tested whether microtubules are required for planar cell polarization. Once the planar cell polarity is established, microtubules are not required for accumulation of Prickle at the anterior cell edge. However, microtubules are needed for cell-cell contacts and initiation of its anterior localization. Reciprocal interactions occur between Wnt/PCP signaling and microtubule cytoskeleton during C&E gastrulation movements. Wnt/PCP signaling influences the polarity of the microtubule cytoskeleton and, conversely, microtubules are required for the asymmetric distribution of Wnt/PCP pathway components.

Claudin-5a in developing zebrafish brain barriers: another brick in the wall

Claudins serve essential roles in regulating paracellular permeability properties within occluding junctions. Recent studies have begun to elucidate developmental roles of claudins within immature tissues. This work has uncovered an involvement of several claudins in determining tight junction properties that have an effect on embryonic morphogenesis and physiology. During zebrafish brain morphogenesis, Claudin-5a determines the paracellular permeability of tight junctions within a transient neuroepithelial-ventricular barrier that maintains the hydrostatic fluid pressure required for brain ventricular lumen expansion. However, the roles of Claudins in development may well extend beyond being mere junctional components. Several post-translational modifications of Claudins have been characterized that indicate a direct regulation by developmental signals. This review focuses on the involvement of Claudin-5a in cerebral barrier formation in the zebrafish embryo and includes some speculations about possible modes of regulation.

Pax2 coordinates epithelial morphogenesis and cell fate in the inner ear

Crucial components of the vertebrate eye, ear and nose develop from discrete patches of surface epithelium, called placodes, which fold into spheroids and undergo complex morphogenesis. Little is known about how the changes in cell and tissue shapes are coordinated with the acquisition of cell fates. Here we explore whether these processes are regulated by common transcriptional mechanisms in the developing ear. After specification, inner ear precursors elongate to form the placode, which invaginates and is transformed into the complex structure of the adult ear. We show that the transcription factor Pax2 plays a key role in coordinating otic fate and placode morphogenesis, but appears to regulate each process independently. In the absence of Pax2, otic progenitors not only lose otic marker expression, but also fail to elongate due to the loss of apically localised N-cadherin and N-CAM. In the absence of either N-cadherin or N-CAM otic cells lose apical cell–cell contact and their epithelial shape. While misexpression of Pax2 leads to ectopic activation of both adhesion molecules, it is not sufficient to confer otic identity. These observations suggest that Pax2 controls cell shape independently from cell identity and thus acts as coordinator for these processes.

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.