Zebrafish gbx1 refines the midbrain-hindbrain boundary border and mediates the Wnt8 posteriorization signal

The Oct4-related PouV gene, pou5f3, mediates isthmus development in zebrafish by directly and dynamically regulating pax2a

Using a transgenic zebrafish line harboring a heat-inducible dominant-interference pou5f3 gene (en-pou5f3), we reported that this PouV gene is involved in isthmus development at the midbrain-hindbrain boundary (MHB), which patterns the midbrain and cerebellum. Importantly, the functions of pou5f3 reportedly differ before and after the end of gastrulation. In the present study, we examined in detail the effects of en-pou5f3 induction on isthmus development during embryogenesis. When en-pou5f3 was induced around the end of gastrulation (bud stage), the isthmus was abrogated or deformed by the end of somitogenesis (24 hours post-fertilization). At this stage, the expression of MHB markers -- such as pax2a, fgf8a, wnt1, and gbx2 -- was absent in embryos lacking the isthmus structure, whereas it was present, although severely distorted, in embryos with a deformed isthmus. We further found that, after en-pou5f3 induction at late gastrulation, pax2a, fgf8a, and wnt1 were immediately and irreversibly downregulated, whereas the expression of en2a and gbx2 was reduced only weakly and slowly. Induction of en-pou5f3 at early somite stages also immediately downregulated MHB genes, particularly pax2a, but their expression was restored later. Overall, the data suggested that pou5f3 directly upregulates at least pax2a and possibly fgf8a and wnt1, which function in parallel in establishing the MHB, and that the role of pou5f3 dynamically changes around the end of gastrulation. We next examined the transcriptional regulation of pax2a using both in vitro and in vivo reporter analyses; the results showed that two upstream 1.0-kb regions with sequences conserved among vertebrates specifically drove transcription at the MHB. These reporter analyses confirmed that development of the isthmic organizer is regulated by PouV through direct regulation of pax2/pax2a in vertebrate embryos.

A role for Retinoblastoma 1 in hindbrain morphogenesis by regulating GBX family

The hindbrain, which develops from the anterior end of the neural tube expansion, can differentiate into the metencephalon and myelencephalon, with varying sizes and functions. The midbrain-hindbrain boundary (MHB) and hindbrain myelencephalon/ventral midline (HMVM) are known to be the source of the progenitors for the anterior hindbrain and myelencephalon, respectively. However, the molecular networks regulating hindbrain morphogenesis in these structures remain unclear. In this study, we show that retinoblastoma 1 (rb1) is highly expressed at the MHB and HMVM in zebrafish. Knocking out rb1 in mice and zebrafish results in an enlarged hindbrain due to hindbrain neuronal hyperproliferation. Further study reveals that Rb1 controls the hindbrain morphogenesis by suppressing the expression of Gbx1/Gbx2, essential transcription factors for hindbrain development, through its binding to E2f3/Hdac1, respectively. Interestingly, we find that Gbx1 and Gbx2 are expressed in different types of hindbrain neurons, suggesting distinct roles in hindbrain morphogenesis. In summary, our study clarifies the specific role of RB1 in hindbrain neural cell proliferation and morphogenesis by regulating the E2f3-Gbx1 axis and the Hdac1-Gbx2 axis. These findings provide a research paradigm for exploring the differential proliferation of neurons in various brain regions.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Modeling of Wnt-mediated tissue patterning in vertebrate embryogenesis

During embryogenesis, morphogens form a concentration gradient in responsive tissue, which is then translated into a spatial cellular pattern. The mechanisms by which morphogens spread through a tissue to establish such a morphogenetic field remain elusive. Here, we investigate by mutually complementary simulations and in vivo experiments how Wnt morphogen transport by cytonemes differs from typically assumed diffusion-based transport for patterning of highly dynamic tissue such as the neural plate in zebrafish. Stochasticity strongly influences fate acquisition at the single cell level and results in fluctuating boundaries between pattern regions. Stable patterning can be achieved by sorting through concentration dependent cell migration and apoptosis, independent of the morphogen transport mechanism. We show that Wnt transport by cytonemes achieves distinct Wnt thresholds for the brain primordia earlier compared with diffusion-based transport. We conclude that a cytoneme-mediated morphogen transport together with directed cell sorting is a potentially favored mechanism to establish morphogen gradients in rapidly expanding developmental systems.

The ubiquitin-conjugating enzyme UBE2K determines neurogenic potential through histone H3 in human embryonic stem cells

Histones modulate gene expression by chromatin compaction, regulating numerous processes such as differentiation. However, the mechanisms underlying histone degradation remain elusive. Human embryonic stem cells (hESCs) have a unique chromatin architecture characterized by low levels of trimethylated histone H3 at lysine 9 (H3K9me3), a heterochromatin-associated modification. Here we assess the link between the intrinsic epigenetic landscape and ubiquitin-proteasome system of hESCs. We find that hESCs exhibit high expression of the ubiquitin-conjugating enzyme UBE2K. Loss of UBE2K upregulates the trimethyltransferase SETDB1, resulting in H3K9 trimethylation and repression of neurogenic genes during differentiation. Besides H3K9 trimethylation, UBE2K binds histone H3 to induce its polyubiquitination and degradation by the proteasome. Notably, ubc-20, the worm orthologue of UBE2K, also regulates histone H3 levels and H3K9 trimethylation in Caenorhabditis elegans germ cells. Thus, our results indicate that UBE2K crosses evolutionary boundaries to promote histone H3 degradation and reduce H3K9me3 repressive marks in immortal cells.

Azra Fatima et al. show that ubiquitin-conjugating enzyme UBE2K regulates neurogenic potential through its target histone H3 in human embryonic stem cells. This study suggests that UBE2K promotes histone H3 degradation, reducing the H3K9me3 repressive marks in immortal cells of both worms and humans.

Wnt signaling regulates neural plate patterning in distinct temporal phases with dynamic transcriptional outputs

The process that partitions the nascent vertebrate central nervous system into forebrain, midbrain, hindbrain, and spinal cord after neural induction is of fundamental interest in developmental biology, and is known to be dependent on Wnt/β-catenin signaling at multiple steps. Neural induction specifies neural ectoderm with forebrain character that is subsequently posteriorized by graded Wnt signaling: embryological and mutant analyses have shown that progressively higher levels of Wnt signaling induce progressively more posterior fates. However, the mechanistic link between Wnt signaling and the molecular subdivision of the neural ectoderm into distinct domains in the anteroposterior (AP) axis is still not clear. To better understand how Wnt mediates neural AP patterning, we performed a temporal dissection of neural patterning in response to manipulations of Wnt signaling in zebrafish. We show that Wnt-mediated neural patterning in zebrafish can be divided into three phases: (I) a primary AP patterning phase, which occurs during gastrulation, (II) a mes/r1 (mesencephalon-rhombomere 1) specification and refinement phase, which occurs immediately after gastrulation, and (III) a midbrain-hindbrain boundary (MHB) morphogenesis phase, which occurs during segmentation stages. A major outcome of these Wnt signaling phases is the specification of the major compartment divisions of the developing brain: first the MHB, then the diencephalic-mesencephalic boundary (DMB). The specification of these lineage divisions depends upon the dynamic changes of gene transcription in response to Wnt signaling, which we show primarily involves transcriptional repression or indirect activation. We show that otx2b is directly repressed by Wnt signaling during primary AP patterning, but becomes resistant to Wnt-mediated repression during late gastrulation. Also during late gastrulation, Wnt signaling becomes both necessary and sufficient for expression of wnt8b, en2a, and her5 in mes/r1. We suggest that the change in otx2b response to Wnt regulation enables a transition to the mes/r1 phase of Wnt-mediated patterning, as it ensures that Wnts expressed in the midbrain and MHB do not suppress midbrain identity, and consequently reinforce formation of the DMB. These findings integrate important temporal elements into our spatial understanding of Wnt-mediated neural patterning and may serve as an important basis for a better understanding of neural patterning defects that have implications in human health.

Mechanism suppressing H3K9 trimethylation in pluripotent stem cells and its demise by polyQ-expanded huntingtin mutations

Pluripotent stem cells are invaluable resources to study development and disease, holding a great promise for regenerative medicine. Here we use human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) from patients with Huntington's disease (HD-iPSCs) to shed light into the normal function of huntingtin (HTT) and its demise in disease. We find that HTT binds ATF7IP, a regulator of the histone H3 methyltransferase SETDB1. HTT inhibits the interaction of the ATF7IP-SETDB1 complex with other heterochromatin regulators and transcriptional repressors, maintaining low levels of H3K9 trimethylation (H3K9me3) in hESCs. Loss of HTT promotes global increased H3K9me3 levels and enrichment of H3K9me3 marks at distinct genes, including transcriptional regulators of neuronal differentiation. Although these genes are normally expressed at low amounts in hESCs, HTT knockdown (KD) reduces their induction during neural differentiation. Notably, mutant expanded polyglutamine repeats in HTT diminish its interaction with ATF7IP-SETDB1 complex and trigger H3K9me3 in HD-iPSCs. Conversely, KD of ATF7IP in HD-iPSCs reduces H3K9me3 alterations and ameliorates gene expression changes in their neural counterparts. Taken together, our results indicate ATF7IP as a potential target to correct aberrant H3K9me3 levels induced by mutant HTT.

4D imaging identifies dynamic migration and the fate of gbx2-expressing cells in the brain primordium of zebrafish

One of the pivotal events in neural development is compartmentalization, wherein the neural tissue divides into domains and undergoes functional differentiation. For example, midbrain-hindbrain boundary (MHB) formation and subsequent isthmus development are key steps in cerebellar development. Although several regulatory mechanisms are known to underlie this event, little is known about cellular behaviors. In this study, to examine the cellular dynamics around the MHB region, we performed confocal time-lapse imaging in zebrafish embryos to track cell populations in the neural tube via 4D analysis. We used a transgenic line wherein enhanced green fluorescent protein (EGFP) expression is driven by the gastrulation brain homeobox 2 (gbx2) enhancer, which is involved in MHB maintenance. 4D time-lapse imaging of 5-20 h revealed a novel pattern in cell migration: a dynamic ventrocaudally directed migration from the MHB region toward the hindbrain. Furthermore, in the hindbrain region, these EGFP-positive cells altered their shapes and extended the axons. Immunohistochemical analysis and retrograde labeling showed that these cells in the hindbrain were in the process of neuronal differentiation, including reticulospinal neurons. These results revealed the dynamic and two-step behavior and possible fate of the cell population, which are linked to brain compartmentalization, leading to a deeper understanding of brain development and formation of neuronal circuits.

The role of gastrulation brain homeobox 2 (gbx2) in the development of the ventral telencephalon in zebrafish embryos

During vertebrate brain development, the gastrulation brain homeobox 2 gene (gbx2) is expressed in the forebrain, but its precise roles are still unknown. In this study, we addressed this issue in zebrafish (Danio rerio) first by carefully examining gbx2 expression in the developing forebrain. We showed that gbx2 was expressed in the telencephalon during late somitogenesis, from 18h post-fertilization (hpf) to 24 hpf, and in the thalamic primordium after 26 hpf. In contrast, another gbx gene, gbx1, was expressed in the anterior-most ventral telencephalon after 36 hpf. Thus, the expression patterns of these two gbx genes did not overlap, arguing against their redundant function in the forebrain. Two-color fluorescence in situ hybridization (FISH) showed close relationships between the telencephalic expression of gbx2 and other forebrain-forming genes, suggesting that their interactions contribute to the regionalization of the telencephalon. FISH further revealed that gbx2 is expressed in the ventricular region of the telencephalon. By using transgenic fish in which gbx2 can be induced by heat shock, we found that gbx2 induction at 16 hpf repressed the expression of emx3, dlx2a, and six3b in the ventral telencephalon. Among secreted factor genes, bmp2b and wnt1 were repressed in the vicinity of the gbx2 domain in the telencephalon. The expression of forebrain-forming genes was examined in mutant embryos lacking gbx2, showing emx3 and dlx2a to be upregulated in the subpallium at 24 hpf. Taken together, these findings indicate that gbx2 contributes to the development of the subpallium through its repressive activities against other telencephalon-forming genes. We further showed that inhibiting FGF signaling and activating Wnt signaling repressed gbx2 and affected the regionalization of the telencephalon, supporting a functional link between gbx2, intracellular signaling, and telencephalon development.

Midbrain-Hindbrain Boundary Morphogenesis: At the Intersection of Wnt and Fgf Signaling

A constriction in the neural tube at the junction of the midbrain and hindbrain is a conserved feature of vertebrate embryos. The constriction is a defining feature of the midbrain-hindbrain boundary (MHB), a signaling center that patterns the adjacent midbrain and rostral hindbrain and forms at the junction of two gene expression domains in the early neural plate: an anterior otx2/wnt1 positive domain and a posterior gbx/fgf8 positive domain. otx2 and gbx genes encode mutually repressive transcription factors that create a lineage restriction boundary at their expression interface. Wnt and Fgf genes form a mutually dependent feedback system that maintains their expression domains on the otx2 or gbx side of the boundary, respectively. Constriction morphogenesis occurs after these conserved gene expression domains are established and while their mutual interactions maintain their expression pattern; consequently, mutant studies in zebrafish have led to the suggestion that constriction morphogenesis should be considered a unique phase of MHB development. We analyzed MHB morphogenesis in fgf8 loss of function zebrafish embryos using a reporter driven by the conserved wnt1 enhancer to visualize anterior boundary cells. We found that fgf8 loss of function results in a re-activation of wnt1 reporter expression posterior to the boundary simultaneous with an inactivation of the wnt1 reporter in the anterior boundary cells, and that these events correlate with relaxation of the boundary constriction. In consideration of other results that correlate the boundary constriction with Wnt and Fgf expression, we propose that the maintenance of an active Wnt-Fgf feedback loop is a key factor in driving the morphogenesis of the MHB constriction.

vox homeobox gene: a novel regulator of midbrain-hindbrain boundary development in medaka fish?

The midbrain-hindbrain boundary (MHB) is one of the key organizing centers of the vertebrate central nervous system (CNS). Its patterning is governed by a well-described gene regulatory network (GRN) involving several transcription factors, namely, pax, gbx, en, and otx, together with signaling molecules of the Wnt and Fgf families. Here, we describe the onset of these markers in Oryzias latipes (medaka) early brain development in comparison to previously known zebrafish expression patterns. Moreover, we show for the first time that vox, a member of the vent gene family, is expressed in the developing neural tube similarly to CNS markers. Overexpression of vox leads to profound changes in the gene expression patterns of individual components of MHB-specific GRN, most notably of fgf8, a crucial organizer molecule of MHB. Our data suggest that genes from the vent family, in addition to their crucial role in body axis formation, may play a role in regionalization of vertebrate CNS.

Patterning, morphogenesis, and neurogenesis of zebrafish cranial sensory placodes

Peripheral sensory organs and ganglia found in the vertebrate head arise during embryonic development from distinct ectodermal thickenings, called cranial sensory placodes (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, and otic). A series of patterning events leads to the establishment of these placodes. Subsequently, these placodes undergo specific morphogenetic movements and cell-type specification in order to shape the final placodal derivatives and to produce differentiated cell types necessary for their function. In this chapter, we will focus on recent studies in the zebrafish that have advanced our understanding of cranial sensory placode development. We will summarize the signaling events and their molecular effectors guiding the formation of the so-called preplacodal region, and the subsequent subdivision of this region along the anteroposterior axis that gives rise to specific placode identities as well as those controlling morphogenesis and neurogenesis. Finally, we will highlight the approaches used in zebrafish that have been established to precisely label cell populations, to follow their development, and/or to characterize cell fates within a specific placode.

Toxicological assessment of trace β-diketone antibiotic mixtures on zebrafish (Danio rerio) by proteomic analysis

β-Diketone antibiotics (DKAs) can produce chronic toxicity in aquatic ecosystems due to their pseudo-persistent in the environment. In this study, after long-term DKA exposure to zebrafish (Danio rerio), 47 protein spots had greater than 2-fold differential expression as compared to the control; there were 26 positive proteins with 14 up-regulated and 12 down-regulated. The main functions of the differentially expressed proteins were related to signal transduction mechanisms and the cytoskeleton. Of the 26 target genes, 11 genes were consistent between their transcriptional and translational levels. Low dose DKA exposure (4.69 and 9.38 mg/L) stimulated spontaneous movement in zebrafish. Changes in both creatine kinase activity and creatine concentration showed a similar trend to zebrafish activity. There was no obvious change in SV-BA after DKA exposure, while a reduction of heart rate was concomitant with increasing DKA concentrations. DKAs also induced severe histopathological changes in zebrafish heart tissue, such as dissolution of cristae and vacuolation of mitochondria. These results demonstrated that trace-level DKA exposure affects a variety of cellular and biological processes in zebrafish.

Dynamic association with donor cell filopodia and lipid-modification are essential features of Wnt8a during patterning of the zebrafish neuroectoderm

BACKGROUND

Wnt proteins are conserved signaling molecules that regulate pattern formation during animal development. Many Wnt proteins are post-translationally modified by addition of lipid adducts. Wnt8a provides a crucial signal for patterning the anteroposterior axis of the developing neural plate in vertebrates. However, it is not clear how this protein propagates from its source, the blastoderm margin, to the target cells in the prospective neural plate, and how lipid-modifications might influence Wnt8a propagation and activity.

RESULTS

We have dynamically imaged biologically active, fluorescently tagged Wnt8a in living zebrafish embryos. We find that Wnt8a localizes to membrane-associated, punctate structures in live tissue. In Wnt8a expressing cells, these puncta are found on filopodial cellular processes, from where the protein can be released. In addition, Wnt8a is found colocalized with Frizzled receptor-containing clusters on signal receiving cells. Combining in vitro and in vivo assays, we compare the roles of conserved Wnt8a residues in cell and non-cell-autonomous signaling activity and secretion. Non-signaling Wnt8 variants show these residues can regulate Wnt8a distribution in producing cell membranes and filopodia as well as in the receiving tissue.

CONCLUSIONS

Together, our results show that Wnt8a forms dynamic clusters found on filopodial donor cell and on signal receiving cell membranes. Moreover, they demonstrate a differential requirement of conserved residues in Wnt8a protein for distribution in producing cells and receiving tissue and signaling activity during neuroectoderm patterning.

Cerebellar development in the absence of Gbx function in zebrafish

The midbrain-hindbrain boundary (MHB) is a well-known organizing center during vertebrate brain development. The MHB forms at the expression boundary of Otx2 and Gbx2, mutually repressive homeodomain transcription factors expressed in the midbrain/forebrain and anterior hindbrain, respectively. The genetic hierarchy of gene expression at the MHB is complex, involving multiple positive and negative feedback loops that result in the establishment of non-overlapping domains of Wnt1 and Fgf8 on either side of the boundary and the consequent specification of the cerebellum. The cerebellum derives from the dorsal part of the anterior-most hindbrain segment, rhombomere 1 (r1), which undergoes a distinctive morphogenesis to give rise to the cerebellar primordium within which the various cerebellar neuron types are specified. Previous studies in the mouse have shown that Gbx2 is essential for cerebellar development. Using zebrafish mutants we show here that in the zebrafish gbx1 and gbx2 are required redundantly for morphogenesis of the cerebellar primordium and subsequent cerebellar differentiation, but that this requirement is alleviated by knocking down Otx. Expression of fgf8, wnt1 and the entire MHB genetic program is progressively lost in gbx1-;gbx2- double mutants but is rescued by Otx knock-down. This rescue of the MHB genetic program depends on rescued Fgf signaling, however the rescue of cerebellar primordium morphogenesis is independent of both Gbx and Fgf. Based on our findings we propose a revised model for the role of Gbx in cerebellar development.

The evolutionary and genetic origins of consciousness in the Cambrian Period over 500 million years ago

Vertebrates evolved in the Cambrian Period before 520 million years ago, but we do not know when or how consciousness arose in the history of the vertebrate brain. Here we propose multiple levels of isomorphic or somatotopic neural representations as an objective marker for sensory consciousness. All extant vertebrates have these, so we deduce that consciousness extends back to the group's origin. The first conscious sense may have been vision. Then vision, coupled with additional sensory systems derived from ectodermal placodes and neural crest, transformed primitive reflexive systems into image forming brains that map and perceive the external world and the body's interior. We posit that the minimum requirement for sensory consciousness and qualia is a brain including a forebrain (but not necessarily a developed cerebral cortex/pallium), midbrain, and hindbrain. This brain must also have (1) hierarchical systems of intercommunicating, isomorphically organized, processing nuclei that extensively integrate the different senses into representations that emerge in upper levels of the neural hierarchy; and (2) a widespread reticular formation that integrates the sensory inputs and contributes to attention, awareness, and neural synchronization. We propose a two-step evolutionary history, in which the optic tectum was the original center of multi-sensory conscious perception (as in fish and amphibians: step 1), followed by a gradual shift of this center to the dorsal pallium or its cerebral cortex (in mammals, reptiles, birds: step 2). We address objections to the hypothesis and call for more studies of fish and amphibians. In our view, the lamprey has all the neural requisites and is likely the simplest extant vertebrate with sensory consciousness and qualia. Genes that pattern the proposed elements of consciousness (isomorphism, neural crest, placodes) have been identified in all vertebrates. Thus, consciousness is in the genes, some of which are already known.

The homeodomain factor Gbx1 is required for locomotion and cell specification in the dorsal spinal cord

Dorsal horn neurons in the spinal cord integrate and relay sensory information to higher brain centers. These neurons are organized in specific laminae and different transcription factors are involved in their specification. The murine homeodomain Gbx1 protein is expressed in the mantle zone of the spinal cord at E12.5-13.5, correlating with the appearance of a discernable dorsal horn around E14 and eventually defining a narrow layer in the dorsal horn around perinatal stages. At postnatal stages, Gbx1 identifies a specific subpopulation of GABAergic neurons in the dorsal spinal cord. We have generated a loss of function mutation for Gbx1 and analyzed its consequences during spinal cord development. Gbx1^−/− mice are viable and can reproduce as homozygous null mutants. However, the adult mutant mice display an altered gait during forward movement that specifically affects the hindlimbs. This abnormal gait was evaluated by a series of behavioral tests, indicating that locomotion is impaired, but not muscle strength or motor coordination. Molecular analysis showed that the development of the dorsal horn is not profoundly affected in Gbx1^−/− mutant mice. However, analysis of terminal neuronal differentiation revealed that the proportion of GABAergic inhibitory interneurons in the superficial dorsal horn is diminished. Our study unveiled a role for Gbx1 in specifying a subset of GABAergic neurons in the dorsal horn of the spinal cord involved in the control of posterior limb movement.

Identification and expression analysis of the zebrafish homologs of the ceramide synthase gene family

BACKGROUND

Sphingolipids represent a major class of lipids which both serve as structural components of membranes and as bioactive molecules involved in lipid signaling. Ceramide synthases (cers) reside in the center of sphingolipid metabolism by producing ceramide through de novo synthesis or degradative pathways. While the six mammalian cers family members have been extensively studied in cell culture and in adult tissues, a systematic analysis of cers expression and function during embryogenesis is still lacking.

RESULTS

Using bioinformatic and phylogenetic analysis, we identified nine highly conserved homologs of the vertebrate cers gene family in the zebrafish genome. A systematic expression analysis throughout five developmental stages indicates that, whereas until 48 hours post fertilization most zebrafish cers homologs are expressed in distinct patterns, e.g., in the intermediate cell mass and the pronephric duct, they show a highly overlapping expression during later stages of embryonic development, mostprominently in the developing brain.

CONCLUSIONS

In this study, the expression of the cers gene homologs is comprehensively analyzed for the first time during vertebrate embryogenesis. Our data indicate that each embryonic tissue has a unique profile of cers expression during zebrafish embryogenesis suggesting tissue-specific profiles of ceramides and their derivatives.

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.

Elongation factor 1 alpha1 and genes associated with Usher syndromes are downstream targets of GBX2

Gbx2 encodes a DNA-binding transcription factor that plays pivotal roles during embryogenesis. Gain-and loss-of-function studies in several vertebrate species have demonstrated a requirement for Gbx2 in development of the anterior hindbrain, spinal cord, inner ear, heart, and neural crest cells. However, the target genes through which GBX2 exerts its effects remain obscure. Using chromatin immunoprecipitation coupled with direct sequencing (ChIP-Seq) analysis in a human prostate cancer cell line, we identified cis-regulatory elements bound by GBX2 to provide insight into its direct downstream targets. The analysis revealed more than 286 highly significant candidate target genes, falling into various functional groups, of which 51% are expressed in the nervous system. Several of the top candidate genes include EEF1A1, ROBO1, PLXNA4, SLIT3, NRP1, and NOTCH2, as well as genes associated with the Usher syndrome, PCDH15 and USH2A, and are plausible candidates contributing to the developmental defects in Gbx2(-/-) mice. We show through gel shift analyses that sequences within the promoter or introns of EEF1A1, ROBO1, PCDH15, USH2A and NOTCH2, are directly bound by GBX2. Consistent with these in vitro results, analyses of Gbx2(-/-) embryos indicate that Gbx2 function is required for migration of Robo1-expressing neural crest cells out of the hindbrain. Furthermore, we show that GBX2 activates transcriptional activity through the promoter of EEF1A1, suggesting that GBX2 could also regulate gene expression indirectly via EEF1A. Taken together, our studies show that GBX2 plays a dynamic role in development and diseases.

Development and evolution of cerebellar neural circuits

The cerebellum controls smooth and skillful movements and it is also involved in higher cognitive and emotional functions. The cerebellum is derived from the dorsal part of the anterior hindbrain and contains two groups of cerebellar neurons: glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons. Purkinje cells are GABAergic and granule cells are glutamatergic. Granule and Purkinje cells receive input from outside of the cerebellum from mossy and climbing fibers. Genetic analysis of mice and zebrafish has revealed genetic cascades that control the development of the cerebellum and cerebellar neural circuits. During early neurogenesis, rostrocaudal patterning by intrinsic and extrinsic factors, such as Otx2, Gbx2 and Fgf8, plays an important role in the positioning and formation of the cerebellar primordium. The cerebellar glutamatergic neurons are derived from progenitors in the cerebellar rhombic lip, which express the proneural gene Atoh1. The GABAergic neurons are derived from progenitors in the ventricular zone, which express the proneural gene Ptf1a. The mossy and climbing fiber neurons originate from progenitors in the hindbrain rhombic lip that express Atoh1 or Ptf1a. Purkinje cells exhibit mediolateral compartmentalization determined on the birthdate of Purkinje cells, and linked to the precise neural circuitry formation. Recent studies have shown that anatomy and development of the cerebellum is conserved between mammals and bony fish (teleost species). In this review, we describe the development of cerebellar neurons and neural circuitry, and discuss their evolution by comparing developmental processes of mammalian and teleost cerebellum.

The peripheral sensory nervous system in the vertebrate head: a gene regulatory perspective

In the vertebrate head, crucial parts of the sense organs and sensory ganglia develop from special regions, the cranial placodes. Despite their cellular and functional diversity, they arise from a common field of multipotent progenitors and acquire distinct identity later under the influence of local signalling. Here we present the gene regulatory network that summarises our current understanding of how sensory cells are specified, how they become different from other ectodermal derivatives and how they begin to diversify to generate placodes with different identities. This analysis reveals how sequential activation of sets of transcription factors subdivides the ectoderm over time into smaller domains of progenitors for the central nervous system, neural crest, epidermis and sensory placodes. Within this hierarchy the timing of signalling and developmental history of each cell population is of critical importance to determine the ultimate outcome. A reoccurring theme is that local signals set up broad gene expression domains, which are further refined by mutual repression between different transcription factors. The Six and Eya network lies at the heart of sensory progenitor specification. In a positive feedback loop these factors perpetuate their own expression thus stabilising pre-placodal fate, while simultaneously repressing neural and neural crest specific factors. Downstream of the Six and Eya cassette, Pax genes in combination with other factors begin to impart regional identity to placode progenitors. While our review highlights the wealth of information available, it also points to the lack information on the cis-regulatory mechanisms that control placode specification and of how the repeated use of signalling input is integrated.

Specification and regionalisation of the neural plate border

During early vertebrate development, the embryonic ectoderm becomes subdivided into neural, neural plate border (border) and epidermal regions. The nervous system is derived from the neural and border domains which, respectively, give rise to the central and peripheral nervous systems. To better understand the functional nervous system we need to know how individual neurons are specified and connected. Our understanding of the early development of the peripheral nervous system has been lagging compared to knowledge regarding central nervous system and epidermal cell lineage decision. Recent advances have shown when and how the specification of border cells is initiated. One important insight is that border specification is already initiated at blastula stages, and can be molecularly and temporally distinguished from rostrocaudal regionalisation of the border. From findings in several species, it is clear that Wnt, Bone Morphogenetic Protein and Fibroblast Growth Factor signals play important roles during the specification and regionalisation of the border. In this review, we highlight the individual roles of these signals and compare models of border specification, including a new model that describes how temporal coordination and epistatic interactions of extracellular signals result in the specification and regionalisation of border cells.

Midbrain-hindbrain boundary patterning and morphogenesis are regulated by diverse grainy head-like 2-dependent pathways

The isthmic organiser located at the midbrain-hindbrain boundary (MHB) is the crucial developmental signalling centre responsible for patterning mesencephalic and metencephalic regions of the vertebrate brain. Formation and maintenance of the MHB is characterised by a hierarchical program of gene expression initiated by fibroblast growth factor 8 (Fgf8), coupled with cellular morphogenesis, culminating in the formation of the tectal-isthmo-cerebellar structures. Here, we show in zebrafish that one orthologue of the transcription factor grainy head-like 2 (Grhl2), zebrafish grhl2b plays a central role in both MHB maintenance and folding by regulating two distinct, non-linear pathways. Loss of grhl2b expression induces neural apoptosis and extinction of MHB markers, which are rescued by re-expression of engrailed 2a (eng2a), an evolutionarily conserved target of the Grhl family. Co-injection of sub-phenotypic doses of grhl2b and eng2a morpholinos reproduces the apoptosis and MHB marker loss, but fails to substantially disrupt formation of the isthmic constriction. By contrast, a novel direct grhl2b target, spec1, identified by phylogenetic analysis and confirmed by ChIP, functionally cooperates with grhl2b to induce MHB morphogenesis, but plays no role in apoptosis or maintenance of MHB markers. Collectively, these data show that MHB maintenance and morphogenesis are dissociable events regulated by grhl2b through diverse transcriptional targets.

Neural crest induction at the neural plate border in vertebrates

The neural crest is a transient and multipotent cell population arising at the edge of the neural plate in vertebrates. Recent findings highlight that neural crest patterning is initiated during gastrulation, i.e. earlier than classically described, in a progenitor domain named the neural border. This chapter reviews the dynamic and complex molecular interactions underlying neural border formation and neural crest emergence.

Evolutionarily conserved function of Gbx2 in anterior hindbrain development

The amino acid sequence across the DNA-binding homeodomain of Gbx2 is highly conserved across multiple species. In mice, Gbx2 is essential for establishment of the midbrain-hindbrain boundary (MHB), and in development of anterior hindbrain structures, rhombomeres (r) 1-r3, and the r2/r3-derived cranial nerve V. In contrast, studies in zebrafish have implicated gbx1 in establishment of the MHB. Therefore, we tested potential roles for gbx2 in anterior hindbrain development in zebrafish. gbx2 knockdown with antisense morpholino results in increased cell death in r2, r3, and r5 and a truncation of the anterior hindbrain, similar to the defect in Gbx2(-/-) mice. Moreover, there is abnormal clustering of cranial nerve V cell bodies in r2 and r3 indicative of defects in aspects of anterior hindbrain patterning. These phenotypes can be rescued by expression of the mouse GBX2 protein. These results suggest that gbx2/Gbx2 has an evolutionarily conserved role in anterior hindbrain development.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.