Broad-minded links cell cycle-related kinase to cilia assembly and hedgehog signal transduction

Recent findings indicate that mammalian Sonic hedgehog (Shh) signal transduction occurs within primary cilia, although the cell biological mechanisms underlying both Shh signaling and ciliogenesis have not been fully elucidated. We show that an uncharacterized TBC domain-containing protein, Broad-minded (Bromi), is required for high-level Shh responses in the mouse neural tube. We find that Bromi controls ciliary morphology and proper Gli2 localization within the cilium. By use of a zebrafish model, we further show that Bromi is required for proper association between the ciliary membrane and axoneme. Bromi physically interacts with cell cycle-related kinase (CCRK), whose Chlamydomonas homolog regulates flagellar length. Biochemical and genetic interaction data indicate that Bromi promotes CCRK stability and function. We propose that Bromi and CCRK control the structure of the primary cilium by coordinating assembly of the axoneme and ciliary membrane, allowing Gli proteins to be properly activated in response to Shh signaling.

Adaptive autoimmunity and Foxp3-based immunoregulation in zebrafish

BACKGROUND

Jawed vertebrates generate their immune-receptor repertoire by a recombinatorial mechanism that has the potential to produce harmful autoreactive lymphocytes. In mammals, peripheral tolerance to self-antigens is enforced by Foxp3(+) regulatory T cells. Recombinatorial mechanisms also operate in teleosts, but active immunoregulation is thought to be a late incorporation to the vertebrate lineage.

METHODS/PRINCIPAL FINDINGS

Here we report the characterization of adaptive autoimmunity and Foxp3-based immunoregulation in the zebrafish. We found that zebrafish immunization with an homogenate of zebrafish central nervous system (zCNS) triggered CNS inflammation and specific antibodies. We cloned the zebrafish ortholog for mammalian Foxp3 (zFoxp3) which induced a regulatory phenotype on mouse T cells and controlled IL-17 production in zebrafish embryos.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate the acquisition of active mechanisms of self-tolerance early in vertebrate evolution, suggesting that active regulatory mechanisms accompany the development of the molecular potential for adaptive autoimmunity. Moreover, they identify the zebrafish as a tool to study the molecular pathways controlling adaptive immunity.

Nestin is essential for zebrafish brain and eye development through control of progenitor cell apoptosis

Background

Nestin is expressed in neural progenitor cells (NPC) of developing brain. Despite its wide use as an NPC marker, the function of nestin in embryo development is unclear.

Methodology/Principal Findings

As nestin is conserved in zebrafish and its predicted sequence is clustered with the mammalian nestin orthologue, we used zebrafish as a model to investigate its role in embryogenesis. Injection of nestin morpholino (MO) into fertilized eggs induced time- and dose-dependent brain and eye developmental defects. Nestin morphants exhibited characteristic morphological changes including small head, small eyes and hydrocephalus. Histological examinations show reduced hind- and mid-brain size, dilated ventricle, poorly organized retina and underdeveloped lens. Injection of control nestin MO did not induce brain or eye changes. Nestin MO injection reduced expression of ascl1b (achaete-scute complex-like 1b), a marker of NPCs, without affecting its distribution. Nestin MO did not influence Elavl3/4 (Embryonic lethal, abnormal vision, Drosophila-like 3/4) (a neuronal marker), or otx2 (a midbrain neuronal marker), but severely perturbed cranial motor nerve development and axon distribution. To determine whether the developmental defects are due to excessive NPC apoptosis and/or reduced NPC proliferation, we analyzed apoptosis by TUNEL assay and acridine orange staining and proliferation by BrdU incorporation, pcna and mcm5 expressions. Excessive apoptosis was noted in hindbrain and midbrain cells. Apoptotic signals were colocalized with ascl1b. Proliferation markers were not significantly altered by nestin MO.

Conclusion/Significance

These results suggest that nestin is essential for zebrafish brain and eye development probably through control of progenitor cell apoptosis.

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.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Temporal and spatial requirements of unplugged/MuSK function during zebrafish neuromuscular development

One of the earliest events in neuromuscular junction (NMJ) development is the accumulation of acetylcholine receptor (AChR) at the center of muscle cells. The unplugged/MuSK (muscle specific tyrosine kinase) gene is essential to initiate AChR clustering but also to restrict approaching growth cones to the muscle center, thereby coordinating pre- and postsynaptic development. To determine how unplugged/MuSK signaling coordinates these two processes, we examined the temporal and spatial requirements of unplugged/MuSK in zebrafish embryos using heat-shock inducible transgenes. Here, we show that despite its expression in muscle cells from the time they differentiate, unplugged/MuSK activity is first required just prior to the appearance of AChR clusters to simultaneously induce AChR accumulation and to guide motor axons. Furthermore, we demonstrate that ectopic expression of unplugged/MuSK throughout the muscle membrane results in wildtype-like AChR prepattern and neuromuscular synapses in the central region of muscle cells. We propose that AChR prepatterning and axonal guidance are spatio-temporally coordinated through common unplugged/MuSK signals, and that additional factor(s) restrict unplugged/MuSK signaling to a central muscle zone critical for establishing mid-muscle synaptogenesis.

Cxcl12a-Cxcr4b signaling is important for proper development of the forebrain GnRH system in zebrafish

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons control pituitary gonadotropin secretion and gametogenesis. In the course of development, these neurons migrate from the olfactory placode to the hypothalamus. The precise molecular mechanism of this neuronal migration is unclear. Here, we investigated whether the chemokine receptor, Cxcr4b, and its cognate ligand, Cxcl12a, are required for proper migration of GnRH3 neurons in zebrafish. Deviated GnRH3 axonal projections and neuronal migration were detected in larvae that carry a homozygote cxcr4b mutation. Similarly, knockdown of Cxcr4b or Cxcl12a led to the appearance of abnormal GnRH3 axonal projections and cell migration, including absence of the characteristic lateral crossing of GnRH3 axons at the anterior commissure and optic chiasm. Double-labeling analysis has shown that cxcr4b and cxcl12a are expressed along the GnRH3 migration pathway (i.e. olfactory placode, terminal nerve and the optic chiasm). The results of this study suggest that the Cxcl12a-Cxcr4b ligand-receptor pair are involved in the migration of GnRH3 neurons in zebrafish, and are therefore crucial for the development of this system.

A novel role for Glucocorticoid-Induced TNF Receptor Ligand (Gitrl) in early embryonic zebrafish development

Tumour necrosis factor ligand and receptor superfamily (TNFSF and TNFRSF) members have diverse and well-studied functions in the immune system. Additional, non-immunological roles, such as in the morphogenesis of bone, tooth, hair and skin have also been described for some members. GITRL and its receptor GITR are well-described as co-regulators of the mammalian immune response. Here, we describe the identification and cloning of their zebrafish homologues and demonstrate a novel role for the ligand, but not the receptor, in early vertebrate development. The assignment of zebrafish Gitrl and Gitr was supported by homology and phylogenetic analysis. The ligand exhibited an oscillating pattern of mRNA expression during the first 36 hours post fertilization, during which time gitr mRNA was not detected, and morpholino oligonucleotide-mediated knock-down of gitrl, but not of gitr, resulted in disruption of early embryogenesis, most clearly revealed during gastrulation, which corresponded to the earliest peak in gitrl mRNA expression (5.25-10 hpf). We found Stat3 signalling to be altered in the gitrl-morphants, suggesting that one possible role for Gitrl during embryogenesis may be modulation of Jak/Stat signalling.

Selective neuronal requirement for huntingtin in the developing zebrafish

Huntington's disease shares a common molecular basis with eight other neurodegenerative diseases, expansion of an existing polyglutamine tract. In each case, this repeat tract occurs within otherwise unrelated proteins. These proteins show widespread and overlapping patterns of expression in the brain and yet the diseases are distinguished by neurodegeneration in a specific subset of neurons that are most sensitive to the mutation. It has therefore been proposed that expansion of the polyglutamine region in these genes may result in perturbation of the normal function of the respective proteins, and that this perturbation in some way contributes to the neuronal specificity of these diseases. The normal functions of these proteins have therefore become a focus for investigation as potential pathogenic pathways. We have used synthetic antisense morpholinos to inhibit the translation of huntingtin mRNA during early zebrafish development and have previously reported the effects of huntingtin reduction on iron transport and homeostasis. Here we report an analysis of the effects of huntingtin loss-of-function on the developing nervous system, observing distinct defects in morphology of neuromasts, olfactory placode and branchial arches. The potential common origins of these defects were explored, revealing impaired formation of the anterior-most region of the neural plate as indicated by reduced pre-placodal and telencephalic gene expression with no effect on mid- or hindbrain formation. These investigations demonstrate a specific 'rate-limiting' role for huntingtin in formation of the telencephalon and the pre-placodal region, and differing levels of requirement for huntingtin function in specific nerve cell types.

Anosmin-1a is required for fasciculation and terminal targeting of olfactory sensory neuron axons in the zebrafish olfactory system

The KAL-1 gene underlies the X-linked form of Kallmann syndrome (KS), a neurological disorder that impairs the development of the olfactory and GnRH systems. KAL-1 encodes anosmin-1, a cell matrix protein that shows cell adhesion, neurite outgrowth, and axon-guidance and -branching activities. We used zebrafish embryos as model to better understand the role of this protein during olfactory system (OS) development. First, we detected the protein in olfactory sensory neurons from 22 h post-fertilization (hpf) onward, i.e. prior their pioneer axons reached presumptive olfactory bulbs (OBs). We found that anosmin-1a depletion impaired the fasciculation of olfactory axons and their terminal targeting within OBs. Last, we showed that kal1a inactivation induced a severe decrease in the number of GABAergic and dopaminergic OB neurons. Though the phenotypes induced following anosmin-1a depletion in zebrafish embryos did not match precisely the defects observed in KS patients, our results provide the first demonstration of a direct requirement for anosmin-1 in OS development in vertebrates and stress the role of OB innervation on OB neuron differentiation.

Generation and characterization of FMR1 knockout zebrafish

Fragile X syndrome (FXS) is one of the most common known causes of inherited mental retardation. The gene mutated in FXS is named FMR1, and is well conserved from human to Drosophila. In order to generate a genetic tool to study FMR1 function during vertebrate development, we generated two mutant alleles of the fmr1 gene in zebrafish. Both alleles produce no detectable Fmr protein, and produce viable and fertile progeny with lack of obvious phenotypic features. This is in sharp contrast to published results based on morpholino mediated knock-down of fmr1, reporting defects in craniofacial development and neuronal branching in embryos. These phenotypes we specifically addressed in our knock-out animals, revealing no significant deviations from wild-type animals, suggesting that the published morpholino based fmr1 phenotypes are potential experimental artifacts. Therefore, their relation to fmr1 biology is questionable and morpholino induced fmr1 phenotypes should be avoided in screens for potential drugs suitable for the treatment of FXS. Importantly, a true genetic zebrafish model is now available which can be used to study FXS and to derive potential drugs for FXS treatment.

Lgl2 executes its function as a tumor suppressor by regulating ErbB signaling in the zebrafish epidermis

Changes in tissue homeostasis, acquisition of invasive cell characteristics, and tumor formation can often be linked to the loss of epithelial cell polarity. In carcinogenesis, the grade of neoplasia correlates with impaired cell polarity. In Drosophila, lethal giant larvae (lgl), discs large (dlg), and scribble, which are components of the epithelial apico-basal cell polarity machinery, act as tumor suppressors, and orthologs of this evolutionary conserved pathway are lost in human carcinoma with high frequency. However, a mechanistic link between neoplasia and vertebrate orthologs of these tumor-suppressor genes remains to be fully explored at the organismal level. Here, we show that the pen/lgl2 mutant phenotype shares two key cellular and molecular features of mammalian malignancy: cell autonomous epidermal neoplasia and epithelial-to-mesenchymal-transition (EMT) of basal epidermal cells including the differential expression of several regulators of EMT. Further, we found that epidermal neoplasia and EMT in pen/lgl2 mutant epidermal cells is promoted by ErbB signalling, a pathway of high significance in human carcinomas. Intriguingly, EMT in the pen/lgl2 mutant is facilitated specifically by ErbB2 mediated E-cadherin mislocalization and not via canonical snail-dependent down-regulation of E-cadherin expression. Our data reveal that pen/lgl2 functions as a tumor suppressor gene in vertebrates, establishing zebrafish pen/lgl2 mutants as a valuable cancer model.

Basonuclin-2 requirements for zebrafish adult pigment pattern development and female fertility

Relatively little is known about the generation of adult form. One complex adult trait that is particularly amenable to genetic and experimental analysis is the zebrafish pigment pattern, which undergoes extensive remodeling during post-embryonic development to form adult stripes. These stripes result from the arrangement of three classes of neural crest-derived pigment cells, or chromatophores: melanophores, xanthophores, and iridophores. Here, we analyze the zebrafish bonaparte mutant, which has a normal early pigment pattern but exhibits a severe disruption to the adult stripe pattern. We show that the bonaparte mutant phenotype arises from mutations in basonuclin-2 (bnc2), encoding a highly conserved, nuclear-localized zinc finger protein of unknown function. We show that bnc2 acts non-autonomously to the melanophore lineage and is expressed by hypodermal cells adjacent to chromatophores during adult pigment pattern formation. In bonaparte (bnc2) mutants, all three types of chromatophores differentiate but then are lost by extrusion through the skin. We further show that while bnc2 promotes the development of two genetically distinct populations of melanophores in the body stripes, chromatophores of the fins and scales remain unaffected in bonaparte mutants, though a requirement of fin chromatophores for bnc2 is revealed in the absence of kit and colony stimulating factor-1 receptor activity. Finally, we find that bonaparte (bnc2) mutants exhibit dysmorphic ovaries correlating with infertility and bnc2 is expressed in somatic ovarian cells, whereas the related gene, bnc1, is expressed within oocytes; and we find that both bnc2 and bnc1 are expressed abundantly within the central nervous system. These findings identify bnc2 as an important mediator of adult pigment pattern formation and identify bonaparte mutants as an animal model for dissecting bnc2 functions.

DAZL relieves miRNA-mediated repression of germline mRNAs by controlling poly(A) tail length in zebrafish

Background

During zebrafish embryogenesis, microRNA (miRNA) miR-430 contributes to restrict Nanos1 and TDRD7 to primordial germ cells (PGCs) by inducing mRNA deadenylation, mRNA degradation, and translational repression of nanos1 and tdrd7 mRNAs in somatic cells. The nanos1 and tdrd7 3′UTRs include cis-acting elements that allow activity in PGCs even in the presence of miRNA-mediated repression.

Methodology/Principal Findings

Using a GFP reporter mRNA that was fused with tdrd7 3′UTR, we show that a germline-specific RNA-binding protein DAZ-like (DAZL) can relieve the miR-430-mediated repression of tdrd7 mRNA by inducing poly(A) tail elongation (polyadenylation) in zebrafish. We also show that DAZL enhances protein synthesis via the 3′UTR of dazl mRNA, another germline mRNA targeted by miR-430.

Conclusions/Significance

Our present study indicated that DAZL acts as an “anti-miRNA factor” during vertebrate germ cell development. Our data also suggested that miRNA-mediated regulation can be modulated on specific target mRNAs through the poly(A) tail control.

Dynamic coupling of pattern formation and morphogenesis in the developing vertebrate retina

During embryonic development, pattern formation must be tightly synchronized with tissue morphogenesis to coordinate the establishment of the spatial identities of cells with their movements. In the vertebrate retina, patterning along the dorsal-ventral and nasal-temporal (anterior-posterior) axes is required for correct spatial representation in the retinotectal map. However, it is unknown how specification of axial cell positions in the retina occurs during the complex process of early eye morphogenesis. Studying zebrafish embryos, we show that morphogenetic tissue rearrangements during eye evagination result in progenitor cells in the nasal half of the retina primordium being brought into proximity to the sources of three fibroblast growth factors, Fgf8/3/24, outside the eye. Triple-mutant analysis shows that this combined Fgf signal fully controls nasal retina identity by regulating the nasal transcription factor Foxg1. Surprisingly, nasal-temporal axis specification occurs very early along the dorsal-ventral axis of the evaginating eye. By in vivo imaging GFP-tagged retinal progenitor cells, we find that subsequent eye morphogenesis requires gradual tissue compaction in the nasal half and directed cell movements into the temporal half of the retina. Balancing these processes drives the progressive alignment of the nasal-temporal retina axis with the anterior-posterior body axis and is controlled by a feed-forward effect of Fgf signaling on Foxg1-mediated cell cohesion. Thus, the mechanistic coupling and dynamic synchronization of tissue patterning with morphogenetic cell behavior through Fgf signaling leads to the graded allocation of cell positional identity in the eye, underlying retinotectal map formation.

Fish-specific duplicated dmrt2b contributes to a divergent function through Hedgehog pathway and maintains left-right asymmetry establishment function

Gene duplication is thought to provide raw material for functional divergence and innovation. Fish-specific dmrt2b has been identified as a duplicated gene of the dmrt2a/terra in fish genomes, but its function has remained unclear. Here we reveal that Dmrt2b knockdown zebrafish embryos display a downward tail curvature and have U-shaped somites. Then, we demonstrate that Dmrt2b contributes to a divergent function in somitogenesis through Hedgehog pathway, because Dmrt2b knockdown reduces target gene expression of Hedgehog signaling, and also impairs slow muscle development and neural tube patterning through Hedgehog signaling. Moreover, the Dmrt2b morphants display defects in heart and visceral organ asymmetry, and, some lateral-plate mesoderm (LPM) markers expressed in left side are randomized. Together, these data indicate that fish-specific duplicated dmrt2b contributes to a divergent function in somitogenesis through Hedgehog pathway and maintains the common function for left-right asymmetry establishment.

Targeted gene knockdown in zebrafish reveals distinct intraembryonic functions for insulin-like growth factor II signaling

IGF-II is the predominant IGF ligand regulating prenatal growth in all vertebrates, including humans, but its central role in placental development has confounded efforts to fully elucidate its functions within the embryo. Here we use a nonplacental model vertebrate (zebrafish) to interrogate the intraembryonic functions of IGF-II signaling. The zebrafish genome contains two coorthologs of mammalian IGF2 (igf2a, igf2b), which exhibit distinct patterns of expression during embryogenesis. Expression of igf2a mRNA is restricted to the notochord, primarily during segmentation/neurulation. By contrast, igf2b mRNA is expressed in midline tissues adjacent to the notochord, with additional sites of expression in the ventral forebrain, and the pronephros. To identify their intraembryonic functions, we suppressed the expression of each gene with morpholino oligonucleotides. Knockdown of igf2a led to defects in dorsal midline development, characterized by delayed segmentation, notochord undulations, and ventral curvature. Similarly, suppression of igf2b led to defects in dorsal midline development but also induced ectopic fusion of the nephron primordia, and defects in ventral forebrain development. Subsequent onset of severe body edema in igf2b, but not igf2a morphants, further suggested a distinct role for igf2b in development of the embryonic kidney. Simultaneous knockdown of both genes increased the severity of dorsal midline defects, confirming a conserved role for both genes in dorsal midline development. Collectively, these data provide evidence that the zebrafish orthologs of IGF2 function in dorsal midline development during segmentation/neurulation, whereas one paralog, igf2b, has evolved additional, distinct functions during subsequent organogenesis.

Cystein-serine-rich nuclear protein 1, Axud1/Csrnp1, is essential for cephalic neural progenitor proliferation and survival in zebrafish

The CSRNP (cystein-serine-rich nuclear protein) family has been conserved from Drosophila to human. Although knockout mice for each of the mammalian proteins have been generated, their function during vertebrate development has remained elusive. As an alternative to obtain insights on CSRNP's role in development, we have analysed the expression pattern and function of one member of this family, axud1, during zebrafish development. Our expression analysis indicates that axud1 is expressed from cleavage to larval stages in a dynamic pattern, becoming restricted after gastrulation to anterior regions of the developing neuraxis and later on concentrated predominantly in proliferating domains of the brain. Knockdown analysis using antisense morpholinos shows that reducing Axud1 levels impairs neural progenitor cell proliferation and survival, revealing an essential function of this gene for the growth of cephalic derivatives. The brain growth phenotypes elicited by decreasing Axud1 expression are specific and independent of anterior-posterior patterning events, initial establishment of neural progenitors, or neural differentiation occurring in this tissue. However, Axud1 is necessary for six3.1 expression and is positively regulated by sonic hedgehog. Phylogenetic examination shows that axud1 is likely to be the ortholog of the only member of this family present in Drosophila, as well as to the previously described mouse CSRNP1 and to human AXUD1 (Axin upregulated-1). Thus, we provide evidence as to the role of axud1 in brain growth in vertebrates.

The epithelial cell adhesion molecule EpCAM is required for epithelial morphogenesis and integrity during zebrafish epiboly and skin development

The aberrant expression of the transmembrane protein EpCAM is associated with tumor progression, affecting different cellular processes such as cell–cell adhesion, migration, proliferation, differentiation, signaling, and invasion. However, the in vivo function of EpCAM still remains elusive due to the lack of genetic loss-of-function studies. Here, we describe epcam (tacstd) null mutants in zebrafish. Maternal-zygotic mutants display compromised basal protrusive activity and epithelial morphogenesis in cells of the enveloping layer (EVL) during epiboly. In partial redundancy with E-cadherin (Ecad), EpCAM made by EVL cells is further required for cell–cell adhesion within the EVL and, possibly, for proper attachment of underlying deep cells to the inner surface of the EVL, thereby also affecting deep cell epiboly movements. During later development, EpCAM per se becomes indispensable for epithelial integrity within the periderm of the skin, secondarily leading to disrupted morphology of the underlying basal epidermis and moderate hyper-proliferation of skin cells. On the molecular level, EVL cells of epcam mutant embryos display reduced levels of membranous Ecad, accompanied by an enrichment of tight junction proteins and a basal extension of apical junction complexes (AJCs). Our data suggest that EpCAM acts as a partner of E-cadherin to control adhesiveness and integrity as well as plasticity and morphogenesis within simple epithelia. In addition, EpCAM is required for the interaction of the epithelia with underlying cell layers.

EpCAM is a well-established marker for carcinomas of epithelial origin and a potential target for immunotherapy. In vitro analyses have implicated EpCAM in a plethora of different cellular processes, such as adhesion, motility, proliferation, differentiation, and signaling. Strikingly, depending on the context, EpCAM displayed rather opposite effects, either promoting or attenuating cell–cell adhesion versus cell migration and tissue invasion, a phenomenon described as the “double-face” of EpCAM. However, the in vivo relevance of its different effects remained largely unclear. Here, we present the first genetic analysis of EpCAM function in vivo, based on loss-of-function mutants in the zebrafish. As it is in mammals, zebrafish EpCAM is expressed in simple epithelia. Mutant embryos display defects both in epithelial morphogenesis and in epithelial integrity. Reduced epithelial morphogenesis is accompanied, and possibly caused, by an extension of apical junctional complexes and compromised basal protrusive activity. Furthermore, mutant epithelia display alterations in the relative abundance of adherence junction versus tight junction components. In addition, EpCAM tightly cooperates with E-cadherin and has a previously unrecognized trans effect on the morphogenesis and integrity of underlying cell layers. Cell differentiation and proliferation in EpCAM mutants are not, or only secondarily, affected. During later development and adulthood, EpCAM is largely dispensable, reinforcing its suitability as a target for anti-carcinoma immunotherapy with minimal side effects.

Period2Expression Pattern and its Role in the Development of the Pineal Circadian Clock in Zebrafish

In zebrafish, pineal arylalkylamine-N-acetyltransferase (zfaanat2) mRNA expression begins at 22 h post-fertilization (hpf), and the clock-controlled rhythm of its transcript begins on the third day of development. Here we describe the role of light and of the clock gene, period2 (zper2) in the development of this rhythm. In 1-day-old zebrafish embryos, zper2 expression is transiently up-regulated by light in the pineal gland and, to a lesser extent, in other areas of the brain. Expression of zper2 that was not affected by light occurred in the olfactory placode and lactotroph cells of the pituitary primordium. Circadian analysis of pineal zfaanat2 mRNA expression indicated that light exposure is required for proper development of the circadian clock-controlled rhythmic expression of this gene. Knockdown of zPER2 using antisense technology abolished the effect of light on development of the zfaanat2 rhythm in the pineal gland, corroborating the role of zper2 in light entrainment of the circadian oscillator in zebrafish. Further analysis of zper2 expression at earlier stages of development revealed that light exposure at the blastula to mid-segmentation stages also caused a transient increase in zper2 expression. At mid-segmentation, before pineal differentiation, light-induced zper2 expression was enhanced in pineal progenitor cells. Thus, a possible role for early photoreception and light-induced zper2 expression in the development of clock-controlled rhythms remains to be investigated.

Regulation of embryonic cell adhesion by the prion protein

Prion proteins (PrPs) are key players in fatal neurodegenerative disorders, yet their physiological functions remain unclear, as PrP knockout mice develop rather normally. We report a strong PrP loss-of-function phenotype in zebrafish embryos, characterized by the loss of embryonic cell adhesion and arrested gastrulation. Zebrafish and mouse PrP mRNAs can partially rescue this knockdown phenotype, indicating conserved PrP functions. Using zebrafish, mouse, and Drosophila cells, we show that PrP: (1) mediates Ca⁺²-independent homophilic cell adhesion and signaling; and (2) modulates Ca⁺²-dependent cell adhesion by regulating the delivery of E-cadherin to the plasma membrane. In vivo time-lapse analyses reveal that the arrested gastrulation in PrP knockdown embryos is due to deficient morphogenetic cell movements, which rely on E-cadherin–based adhesion. Cell-transplantation experiments indicate that the regulation of embryonic cell adhesion by PrP is cell-autonomous. Moreover, we find that the local accumulation of PrP at cell contact sites is concomitant with the activation of Src-related kinases, the recruitment of reggie/flotillin microdomains, and the reorganization of the actin cytoskeleton, consistent with a role of PrP in the modulation of cell adhesion via signaling. Altogether, our data uncover evolutionarily conserved roles of PrP in cell communication, which ultimately impinge on the stability of adherens cell junctions during embryonic development.

Unlike conventional pathogens, prions are infectious particles devoid of nucleic acids and composed entirely of a misfolded host protein, PrP. It is widely assumed that the neurodegeneration observed in prion disorders may be related to an aberrant function of PrP in the misfolded state. However, the normal physiological function of PrP remains poorly understood, mainly owing to the absence of clear phenotypes in mice lacking PrP. Here, we show that when PrP is depleted in zebrafish embryos, dramatic phenotypes ensue, severely affecting the development of early and late (neural) structures. We examined the mechanisms responsible for some of these defects, and found that fish and mammalian PrPs play conserved roles in cell–cell communication, by directly mediating cell adhesion and by triggering cellular signals that further modulate the function of other adhesion molecules. In the early zebrafish embryo, these activities control not only tissue integrity and cell morphology, but also the complex cellular movements that give rise to germ layers. This study describes—to our knowledge—the first known in vivo function of PrP and its molecular cellular basis, which may provide helpful insights into the role of PrP in the adult brain and its proposed connections to prion-induced neurotoxicity.

Knockdown experiments in zebrafish embryos reveal that prion proteins, otherwise known for their involvement in lethal neurodegenerative disease, play important roles in cell-cell adhesion and signaling during embryonic development.

Synaptojanin1 is required for temporal fidelity of synaptic transmission in hair cells

To faithfully encode mechanosensory information, auditory/vestibular hair cells utilize graded synaptic vesicle (SV) release at specialized ribbon synapses. The molecular basis of SV release and consequent recycling of membrane in hair cells has not been fully explored. Here, we report that comet, a gene identified in an ENU mutagenesis screen for zebrafish larvae with vestibular defects, encodes the lipid phosphatase Synaptojanin 1 (Synj1). Examination of mutant synj1 hair cells revealed basal blebbing near ribbons that was dependent on Cav1.3 calcium channel activity but not mechanotransduction. Synaptojanin has been previously implicated in SV recycling; therefore, we tested synaptic transmission at hair-cell synapses. Recordings of post-synaptic activity in synj1 mutants showed relatively normal spike rates when hair cells were mechanically stimulated for a short period of time at 20 Hz. In contrast, a sharp decline in the rate of firing occurred during prolonged stimulation at 20 Hz or stimulation at a higher frequency of 60 Hz. The decline in spike rate suggested that fewer vesicles were available for release. Consistent with this result, we observed that stimulated mutant hair cells had decreased numbers of tethered and reserve-pool vesicles in comparison to wild-type hair cells. Furthermore, stimulation at 60 Hz impaired phase locking of the postsynaptic activity to the mechanical stimulus. Following prolonged stimulation at 60 Hz, we also found that mutant synj1 hair cells displayed a striking delay in the recovery of spontaneous activity. Collectively, the data suggest that Synj1 is critical for retrieval of membrane in order to maintain the quantity, timing of fusion, and spontaneous release properties of SVs at hair-cell ribbon synapses.

Ribbon synapses are found in the ear and eye and facilitate the transmission of sensory information to the brain. In hair cells of the ear, the molecules required for ribbon function have not been fully explored. Zebrafish are ideal for investigating molecular components of these specialized synapses because of the ability to study ribbon function using genetic, cellular, and physiological methods. Here, we explore the role of the lipid phosphatase Synaptojanin at the hair cell synapse. Synaptojanin has been previously implicated in synaptic vesicle recycling in conventional synapses, and we also find that the number of synaptic vesicles are reduced in mutant synaptojanin hair cells. Mutant synaptojanin larvae have obvious equilibrium defects, and our electrophysiological recordings revealed that synaptic transmission from hair cells to neurons projecting to the brain is impaired in terms of both rate and accuracy. When stimulated at high frequency or for prolonged periods, mutant synaptojanin hair cells release vesicles out of phase with mechanical stimuli, thus compromising the transfer of sensory information to the brain.

Novel and unexpected functions of zebrafish CCAAT box binding transcription factor (NF-Y) B subunit during cartilages development

We used zebrafish as a model to study the biological functions of NF-YB during early development. Both RT-PCR and whole-mount in situ hybridization experiments revealed that nf-yb was a maternally inherited gene. Later, its expression was restricted in the future head cartilages as well as in the developing notochord. Embryos after injection with nf-yb-morpholino displayed reduced-head phenotypes, including smaller head (WT, length of head, L: 0.515+/-0.019 mm, width of head, W: 0.323+/-0.077 mm; nf-yb-morphant, L: 0.347+/-0.037 mm; W: 0.266+/-0.018 mm), sharpen Meckel's cartilage, loss of ceratobranchial, and enlarged angles of ceratohyal (WT: 72.6+/-9.4 degrees ; nf-yb-morphant: 110.0+/-32.5 degrees ). Subsequently, those abnormalities can be rescued after injection with capped nf-yb mRNA. TUNEL assay suggested that large amounts of cell apoptosis appeared in the head region of nf-yb-morphants. Staining with digoxigenin-labeled dlx2a, sox9a, runx2b and col2a1 riboprobes showed that nf-yb-morphants displayed reduced amounts of cranial neural crest cells which are required for mandibular and branchial arches formation. These observations clearly indicate that knockdown of nf-yb translation induced parts of cranial neural crest cells apoptosis, affected cartilages formation and consequently caused reduced-head phenotypes. These findings uncover a novel and unexpected role for NF-YB as a critical modulator of neural crest cell's gene expression governing embryonic cartilage growth.

Both foxj1a and foxj1b are implicated in left-right asymmetric development in zebrafish embryos

The HNF-3/HFH-4/Foxj1, a transcription factor, has been reported to be involved in systemic autoimmunity and cilia genesis in vertebrates. The zebrafish genome expressed two paralogous foxj1 genes, foxj1a and foxj1b. In this study, we demonstrate that down-regulation of either foxj1a or foxj1b by injecting antisense morpholino at the one-cell stage results in randomized expression of the early left-right (LR) asymmetric markers lefty2, southpaw, pitx2c and the later internal organ markers tpm4-tv1, cmlc2, cp in zebrafish embryos. Overexpression of foxj1a and foxj1b by injecting synthetic mRNAs also disrupts normal LR asymmetries. These data indicate that the two foxj1 genes are required for normal laterality development in zebrafish embryos. In contrast to foxj1b knockdown exclusively in dorsal forerunner cells (DFCs) that has little effect on laterality, foxj1a knockdown in DFCs randomizes the LR patterns of the markers. Thus, foxj1a regulates asymmetric development through DFCs in a cell-autonomous fashion but foxj1b functions indirectly.

SOCS1 and SOCS3 are the main negative modulators of the somatotrophic axis in liver of homozygous GH-transgenic zebrafish (Danio rerio)

Homozygote individuals (HO) of the GH-transgenic zebrafish lineage (F0104), despite expressing double the amount of growth hormone (GH) in relation to the hemizygote (HE) individuals, presented smaller growth in relation to the last, and similar to the non-transgenic (NT) group. Through the analysis of the expression of genes of the somatotrophic axis in the livers of HO and NT individuals, it was verified that GHR, JAK2 and STAT5.1 did not present significant differences among the analyzed genotypes (NT and HO). However, in the IGF-I gene expression, an accentuated decrease was observed in group HO (p<0.01), suggesting a resistance effect to excess GH. This resistance could be related to the insufficient amount of energy for supporting the accelerated metabolic demand caused by excess circulating GH. Analysis of the genes involved in the regulation of GH signalization by dephosphorylation (PTP-H1 and PTP-1B) did not show any significant alteration when comparing groups HO and NT. However, the analysis of the SOCS1 and SOCS3 genes showed an induction in homozygotes of 2.5 times (p<0.01) and 4.3 times (p<0.05), respectively, in relation to non-transgenics. The results of the present work demonstrate that, in homozygotes, GH signaling is reduced by the action of the SOCS1 and SOCS3 proteins.

Involvement of estradiol-17beta and its membrane receptor, G protein coupled receptor 30 (GPR30) in regulation of oocyte maturation in zebrafish, Danio rario

The orphan G protein coupled receptor, GPR30, has the characteristics of a high affinity, specific estrogen membrane receptor on Atlantic croaker oocytes and mediates estrogen inhibition of oocyte maturation in this perciform fish. In order to determine the broad applicability of these findings to other teleosts, similar experiments were conducted in a cyprinid fish, zebrafish, in the present study. GPR30 mRNA expression was detected in zebrafish oocytes but not in the ovarian follicular cells. Both spontaneous and 17, 20beta-dihyroxy-4-pregnen-3-one (DHP)-induced maturation of follicle-enclosed zebrafish oocytes was significantly decreased when they were incubated with either estradiol-17beta, or the GPR30 agonists, ICI 182 780 and tamoxifen, or with the GPR30 specific agonist G-1. On the other hand spontaneous oocyte maturation increased two-fold when zebrafish ovarian follicles were incubated with an aromatase inhibitor, ATD. Moreover, the stimulatory effects of ATD on germinal vesicle breakdown (GVBD) were partially reversed by co-treatment with 100 nM of E2 or G-1. These results suggest that endogenous estrogens acting through GPR30 are involved in maintaining meiotic arrest of zebrafish oocytes.

Decreased BDNF levels are a major contributor to the embryonic phenotype of huntingtin knockdown zebrafish

Huntington's disease (HD) is an autosomal dominant, neurodegenerative condition caused by a CAG trinucleotide repeat expansion that is translated into an abnormally long polyglutamine tract in the protein huntingtin. Genetic and transgenic studies suggest that the mutation causes disease predominantly via gain-of-function mechanisms. However, loss of normal huntingtin function resulting from the polyglutamine expansion might also contribute to the pathogenesis of HD. Here, we have studied the effects of huntingtin knockdown in zebrafish using morpholino antisense oligonucleotides, as its huntingtin orthologue has 70% amino acid identity with the human protein. Reduced huntingtin levels did not impact on gastrulation and early development, but caused massive apoptosis of neuronal cells by 24 hpf. This was accompanied by impaired neuronal development, resulting in small eyes and heads and enlargement of brain ventricles. Older huntingtin knockdown fish developed lower jaw abnormalities with most branchial arches missing. Molecular analysis revealed that BDNF expression was reduced by approximately 50%. Reduction of BDNF levels by injection of a BDNF morpholino resulted in phenotypes very similar to those seen in huntingtin knockdown zebrafish. The phenotypes of both huntingtin- and BDNF-knockdown zebrafish showed significant rescue when treated with exogenous BDNF protein. This underscores the physiological importance of huntingtin as a regulator of BDNF production and suggests that loss of BDNF is a major cause of the developmental abnormalities seen with huntingtin knockdown in zebrafish. Increasing BDNF expression may represent a useful strategy for Huntington's disease treatment.

The tumor suppressor PRDM5 regulates Wnt signaling at early stages of zebrafish development

PRDM genes are a family of transcriptional regulators that modulate cellular processes such as differentiation, cell growth and apoptosis. Some family members are involved in tissue or organ maturation, and are differentially expressed in specific phases of embryonic development. PRDM5 is a recently identified family member that functions as a transcriptional repressor and behaves as a putative tumor suppressor in different types of cancer. Using gene expression profiling, we found that transcriptional targets of PRDM5 in human U2OS cells include critical genes involved in developmental processes, and specifically in regulating wnt signaling. We therefore assessed PRDM5 function in vivo by performing loss-of-function and gain-of-function experiments in zebrafish embryos. Depletion of prdm5 resulted in impairment of morphogenetic movements during gastrulation and increased the occurrence of the masterblind phenotype in axin+/- embryos, characterized by the loss of eyes and telencephalon. Overexpression of PRDM5 mRNA had opposite effects on the development of anterior neural structures, and resulted in embryos with a shorter body axis due to posterior truncation, a bigger head and abnormal somites. In situ hybridization experiments aimed at analyzing the integrity of wnt pathways during gastrulation at the level of the prechordal plate revealed inhibition of non canonical PCP wnt signaling in embryos overexpressing PRDM5, and over-activation of wnt/beta-catenin signaling in embryos lacking Prdm5. Our data demonstrate that PRDM5 regulates the expression of components of both canonical and non canonical wnt pathways and negatively modulates wnt signaling in vivo.

Nkx2.7 and Nkx2.5 function redundantly and are required for cardiac morphogenesis of zebrafish embryos

Background

Nkx2.7 is the tinman-related gene, as well as orthologs of Nkx2.5 and Nkx-2.3. Nkx2.7 and Nkx2.5 express in zebrafish heart fields of lateral plate mesoderm. The temporal and spatial expression patterns of Nkx2.7 are similar to those of Nkx2.5, but their functions during cardiogenesis remain unclear.

Methodology/Principal Findings

Here, Nkx2.7 is demonstrated to compensate for Nkx2.5 loss of function and play a predominant role in the lateral development of the heart, including normal cardiac looping and chamber formation. Knocking down Nkx2.5 showed that heart development was normal from 24 to 72 hpf. However, when knocking down either Nkx2.7 or Nkx2.5 together with Nkx2.7, it appeared that the heart failed to undergo looping and showed defective chambers, although embryos developed normally before the early heart tube stage. Decreased ventricular myocardium proliferation and defective myocardial differentiation appeared to result from late-stage up-regulation of bmp4, versican, tbx5 and tbx20, which were all expressed normally in hearts at an early stage. We also found that tbx5 and tbx20 were modulated by Nkx2.7 through the heart maturation stage because an inducible overexpression of Nkx2.7 in the heart caused down-regulation of tbx5 and tbx20. Although heart defects were induced by overexpression of an injection of 150-pg Nkx2.5 or 5-pg Nkx2.7 mRNA, either Nkx2.5 or Nkx2.7 mRNA rescued the defects induced by Nkx2.7-morpholino(MO) and Nkx2.5-MO with Nkx2.7-MO.

Conclusions and Significance

Therefore, we conclude that redundant activities of Nkx2.5 and Nkx2.7 are required for cardiac morphogenesis, but that Nkx2.7 plays a more critical function, specifically indicated by the gain-of-function and loss-of- function experiments where Nkx2.7 is observed to regulate the expressions of tbx5 and tbx20 through the maturation stage.

Zebrafish Jak2a plays a crucial role in definitive hematopoiesis and blood vessel formation

We examined the role of zebrafish (Danio rerio) Jak2a, a homolog of mammalian Jak2, in the developing embryo by injecting in vitro synthesized Jak2a shRNA into zebrafish zygotes. Blood circulation was suppressed in Jak2a shRNA-injected embryos from 24hours post fertilization (hpf) and all embryos died with enlarged pericardium, shortened body lengths, and defects in some vasculature within 8 days post fertilization. O-dianisidine staining of red blood cells revealed normal blood island formation with no circulating red blood cells. As in Jak2(-/-) transgenic mice, expression of definitive Ba1 globin was significantly reduced in Jak2a knockdown embryos at 36hpf, whereas expression of other hematopoietic markers, primitive be1 globin, gata-1, and scl, were unaffected. More importantly, blood vessel formation was disturbed in Jak2a knockdown embryos as revealed by alkaline phosphatase staining at 72hpf. Thus, our data indicate that zebrafish Jak2a is important in both definitive hematopoiesis and blood vessel formation.

Mutation of the zebrafish nucleoporin elys sensitizes tissue progenitors to replication stress

The recessive lethal mutation flotte lotte (flo) disrupts development of the zebrafish digestive system and other tissues. We show that flo encodes the ortholog of Mel-28/Elys, a highly conserved gene that has been shown to be required for nuclear integrity in worms and nuclear pore complex (NPC) assembly in amphibian and mammalian cells. Maternal elys expression sustains zebrafish flo mutants to larval stages when cells in proliferative tissues that lack nuclear pores undergo cell cycle arrest and apoptosis. p53 mutation rescues apoptosis in the flo retina and optic tectum, but not in the intestine, where the checkpoint kinase Chk2 is activated. Chk2 inhibition and replication stress induced by DNA synthesis inhibitors were lethal to flo larvae. By contrast, flo mutants were not sensitized to agents that cause DNA double strand breaks, thus showing that loss of Elys disrupts responses to selected replication inhibitors. Elys binds Mcm2-7 complexes derived from Xenopus egg extracts. Mutation of elys reduced chromatin binding of Mcm2, but not binding of Mcm3 or Mcm4 in the flo intestine. These in vivo data indicate a role for Elys in Mcm2-chromatin interactions. Furthermore, they support a recently proposed model in which replication origins licensed by excess Mcm2-7 are required for the survival of human cells exposed to replication stress.

DNA replication is a complex process that requires activation of cell cycle checkpoints and DNA repair pathways. Genetic analyses in fungi have suggested that nucleoporins, the proteins that make up the nuclear pore complex (NPC), play a role in the cellular response to agents that disrupt cell proliferation or damage DNA. Here we show that mutation of the Elys nucleoporin causes widespread apoptosis in the intestine and other tissues of zebrafish flotte lotte (flo) mutants. Intestinal apoptosis occurs in the absence of the DNA damage marker γH2X, and levels of chromatin bound Mcm2, a component of the DNA replication helicase, were also reduced in flo mutants. These findings suggested that flo intestinal cells cannot repair endogenous replication errors. Consistent with this idea, flo mutants were highly sensitized to treatment with DNA replication inhibitors such as hydroxyurea, UV irradiation, or cisplatin, but not agents that cause DNA double strand breaks, such as γ-irradiation or camptothecin. These data point to a conserved role for nucleoporins in the cellular response to replication stress in eukaryote cells.

Diaphanous-related formin 2 and profilin I are required for gastrulation cell movements

Intensive cellular movements occur during gastrulation. These cellular movements rely heavily on dynamic actin assembly. Rho with its associated proteins, including the Rho-activated formin, Diaphanous, are key regulators of actin assembly in cellular protrusion and migration. However, the function of Diaphanous in gastrulation cell movements remains unclear. To study the role of Diaphanous in gastrulation, we isolated a partial zebrafish diaphanous-related formin 2 (zdia2) clone with its N-terminal regulatory domains. The GTPase binding domain of zDia2 is highly conserved compared to its mammalian homologues. Using a yeast two-hybrid assay, we showed that zDia2 interacts with constitutively-active RhoA and Cdc42. The zdia2 mRNAs were ubiquitously expressed during early embryonic development in zebrafish as determined by RT-PCR and whole-mount in situ hybridization analyses. Knockdown of zdia2 by antisense morpholino oligonucleotides (MOs) blocked epiboly formation and convergent extension in a dose-dependent manner, whereas ectopic expression of a human mdia gene partially rescued these defects. Time-lapse recording further showed that bleb-like cellular processes of blastoderm marginal deep marginal cells and pseudopod-/filopod-like processes of prechordal plate cells and lateral cells were abolished in the zdia2 morphants. Furthermore, zDia2 acts cell-autonomously since transplanted zdia2-knockdown cells exhibited low protrusive activity with aberrant migration in wild type host embryos. Lastly, co-injection of antisense MOs of zdia2 and zebrafish profilin I (zpfn 1), but not zebrafish profilin II, resulted in a synergistic inhibition of gastrulation cell movements. These results suggest that zDia2 in conjunction with zPfn 1 are required for gastrulation cell movements in zebrafish.

Teasing out T-box targets in early mesoderm

T-box transcription factor genes are widely conserved in metazoan development and widely involved in developmental processes. With the phase of T-box gene discovery winding down, the phase of transcriptional target discovery for T-box transcription factors is finally taking off and yielding rich rewards. Mutant phenotypes in mouse and zebrafish as well as morpholino studies in zebrafish have helped to link the T-box genes to a variety of signaling pathways through diverse target genes and feedback loops. Particularly in early mesoderm development, it is emerging that a network of T-box genes interacts with Wnt/beta-catenin and Notch/Delta signaling pathways, among others, to control the important processes of mesoderm specification, somite segmentation, and left/right body axis determination.

Brd4 associates with mitotic chromosomes throughout early zebrafish embryogenesis

Brd4 is a member of the BET (bromodomains and extraterminal) subfamily of bromodomain proteins that includes chromatin-modifying proteins and transcriptional regulators. Brd4 has a role in cell cycle progression, making it indispensable in mouse embryos and cultured cells. The N-terminal domain of Brd4 participates in a fusion oncogene. Brd4 associates with acetylated histones in chromatin, and this association persists during mitosis implicating Brd4 in epigenetic memory. Brd4 sequence, particularly the bromodomains and ET domain, is conserved in the zebrafish and Xenopus laevis proteins reported here. Brd4 is expressed and localized on mitotic chromosomes in early zebrafish embryos before and after the midblastula transition (MBT), indicating that the Brd4-chromosome association is a conserved property that is maintained even before zygotic transcription. The association of Brd4 with acetylated histones may also be conserved in early embryos as we found that histones H3 and H4 are already acetylated during pre-MBT stages.

The orphan G protein-coupled receptor 161 is required for left-right patterning

Gpr161 (also known as RE2) is an orphan G protein-coupled receptor (GPCR) that is expressed during embryonic development in zebrafish. Determining its biological function has proven difficult due to lack of knowledge regarding its natural or synthetic ligands. Here, we show that targeted knockdown of gpr161 disrupts asymmetric gene expression in the lateral plate mesoderm, resulting in aberrant looping of the heart tube. This is associated with elevated Ca(2+) levels in cells lining the Kupffer's vesicle and normalization of Ca(2+) levels, by over-expression of ncx1 or pmca-RNA, is able to partially rescue the cardiac looping defect in gpr161 knockdown embryos. Taken together, these data support a model in which gpr161 plays an essential role in left-right (L-R) patterning by modulating Ca(2+) levels in the cells surrounding the Kupffer's vesicle.

Regulation of zebrafish skeletogenesis by ext2/dackel and papst1/pinscher

Mutations in human Exostosin genes (EXTs) confer a disease called Hereditary Multiple Exostoses (HME) that affects 1 in 50,000 among the general population. Patients with HME have a short stature and develop osteochondromas during childhood. Here we show that two zebrafish mutants, dackel (dak) and pinscher (pic), have cartilage defects that strongly resemble those seen in HME patients. We have previously determined that dak encodes zebrafish Ext2. Positional cloning of pic reveals that it encodes a sulphate transporter required for sulphation of glycans (Papst1). We show that although both dak and pic are required during cartilage morphogenesis, they are dispensable for chondrocyte and perichondral cell differentiation. They are also required for hypertrophic chondrocyte differentiation and osteoblast differentiation. Transplantation analysis indicates that dak^−/− cells are usually rescued by neighbouring wild-type chondrocytes. In contrast, pic^−/− chondrocytes always act autonomously and can disrupt the morphology of neighbouring wild-type cells. These findings lead to the development of a new model to explain the aetiology of HME.

Hereditary Multiple Exostoses is a disease that causes the formation of benign bone tumours in children. Besides causing severe skeletal deformity, the bone tumours can compress nerves or other tissue resulting in chronic pain. Although the tumours can usually be surgically removed, they sometimes recur or are in positions that prevent surgery. We have identified two strains of zebrafish whose offspring have skeletal defects that resemble those of patients with Hereditary Multiple Exostoses. We have found that each strain carries a mutated form of an essential gene. Importantly, these two genes are also found in humans, and thus by analysing their function in zebrafish, we may shed light on their role in humans. Our study has elucidated the roles of these genes during normal skeletal development and has allowed us to generate a model for how genetic changes give rise to bone tumours in humans.

Cadherin-6 function in zebrafish retinal development

Cadherin cell-adhesion molecules play crucial roles in vertebrate development including the development of the visual system. Most studies have focused on examining functions of classical type I cadherins (e.g., cadherin-2) in visual system development. There is little information on the function of classical type II cadherins (e.g., cadherin-6) in the development of the vertebrate visual system. To gain insight into cadherin-6 role in the formation of the retina, we analyzed differentiation of retinal ganglion cells (RGCs), amacrine cells, and photoreceptors in zebrafish embryos injected with cadherin-6 specific antisense morpholino oligonucleotides. Differentiation of the retinal neurons in cadherin-6 knockdown embryos (cdh6 morphants) was analyzed using multiple markers. We found that expression of transcription factors important for retinal development was greatly reduced, and expression of Notch-Delta genes and proneural gene ath5 was altered in the cdh6 morphant retina. The retinal lamination was present in the morphants, although the morphant eyes were significantly smaller than control embryos due mainly to decreased cell proliferation. Differentiation of the RGCs, amacrine cells, and photoreceptors was severely disrupted in the cdh6 morphants due to a significant delay in neural differentiation. Our results suggest that cadherin-6 plays an important role in the normal formation of the zebrafish retina. (c) 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2008.

Zebrafish orthologue of mental retardation protein IL1RAPL1 regulates presynaptic differentiation

IL1-receptor accessory protein-like 1 (IL1RAPL1), a member of interleukin-1/toll receptor (TIR) family, is responsible for a nonsyndromic form of mental retardation (MR). The zebrafish orthologue of mammalian IL1RAPL1, designated as Il1rapl1b, was expressed widely in the brain and in the olfactory placode. We employed an olfactory sensory neuron-specific gene manipulation system in combination with in vivo imaging of transparent zebrafish embryos to examine the functional role of Il1rapl1b in synaptic vesicle accumulation and subsequent morphological remodeling of axon terminals, the characteristic features of presynaptic differentiation of zebrafish olfactory sensory neurons during synapse formation. Antisense morpholino oligonucleotide against il1rapl1b suppressed both the synaptic vesicle accumulation and axon terminal remodeling. Consistently, the overexpression of Il1rapl1b stimulated synaptic vesicle accumulation. Swapping the carboxyl-terminal domain of Il1rapl1b with that of mouse IL-1 receptor accessory protein abolished the stimulatory effect. On the other hand, a substitution mutation in the TIR domain suppressed the morphological remodeling of axon terminals. Thus, the regulation of synaptic vesicle accumulation and subsequent morphological remodeling by Il1rapl1b appeared to be mediated by distinct domains. These results suggest that Il1rapl1b plays an important role in presynaptic differentiation during synapse formation.

Regulation of canonical Wnt signaling by Brachyury is essential for posterior mesoderm formation

The T box transcription factor Brachyury is essential for the formation of the posterior body in all vertebrates, although its critical transcriptional targets have remained elusive. Loss-of-function studies of mouse Brachyury and the zebrafish Brachyury ortholog Ntl indicated that Brachyury plays a more significant role in higher vertebrates than lower vertebrates. We have identified a second zebrafish Brachyury ortholog (Bra), and show that a combined loss of Ntl and Bra recapitulates the mouse phenotype, demonstrating an ancient role for Brachyury in patterning all but the most anterior somites. Using cell transplantation, we show that the only essential role for Brachyury during somite formation is non-cell autonomous, and demonstrate that Ntl and Bra are required for and can induce expression of the canonical Wnts wnt8 and wnt3a. We propose that a positive autoregulatory loop between Ntl/Bra and canonical Wnt signaling maintains the mesodermal progenitors to facilitate posterior somite development in chordates.

Notch signalling synchronizes the zebrafish segmentation clock but is not needed to create somite boundaries

Somite segmentation depends on a gene expression oscillator or clock in the posterior presomitic mesoderm (PSM) and on read-out machinery in the anterior PSM to convert the pattern of clock phases into a somite pattern. Notch pathway mutations disrupt somitogenesis, and previous studies have suggested that Notch signalling is required both for the oscillations and for the read-out mechanism. By blocking or overactivating the Notch pathway abruptly at different times, we show that Notch signalling has no essential function in the anterior PSM and is required only in the posterior PSM, where it keeps the oscillations of neighbouring cells synchronized. Using a GFP reporter for the oscillator gene her1, we measure the influence of Notch signalling on her1 expression and show by mathematical modelling that this is sufficient for synchronization. Our model, in which intracellular oscillations are generated by delayed autoinhibition of her1 and her7 and synchronized by Notch signalling, explains the observations fully, showing that there are no grounds to invoke any additional role for the Notch pathway in the patterning of somite boundaries in zebrafish.

The somites—the embryonic segments of the vertebrate body—form one after another from tissue at the tail end of the embryo. A gene expression oscillator, the somite segmentation clock, operating in this tail region, marks out a periodic spatial pattern and so controls the segmentation process. Evidence from mutants shows that the Notch cell-cell signalling pathway has a critical role in the clock mechanism. However, when we switch on a blockade of Notch signalling, by immersing zebrafish embryos in the chemical inhibitor DAPT, the next ∼12 somites form normally, and only after that do disrupted somites appear. We show that this is because Notch signalling is needed only to maintain synchrony between the clocks of individual cells. The cells take about seven cycles to drift out of synchrony when Notch-mediated communication is blocked, and then a further five cycles to pass from the site where the tissue receives its “time-stamp” to the site where overt segmentation begins. By mathematical modelling, backed up with measurements on transgenic embryos, we show how Notch signalling may act at a molecular level to synchronise the intracellular oscillators of adjacent individual cells.

Overexpression of Myostatin2 in zebrafish reduces the expression of dystrophin associated protein complex (DAPC) which leads to muscle dystrophy

Myostatin, a member of the TGF-beta superfamily, is a potent negative regulator of skeletal muscle and growth. Previously, we reported Mstn1 from zebrafish and studied its influence on muscle development. In this study, we identified another form of Myostatin protein which is referred to as Mstn2. The size of Mstn2 cDNA is 1342 bp with 109 and 132 bp of 5' and 3'-untranslated regions (UTRs), respectively. The coding region is 1101 bp encoding 367 amino acids. The identity between zebrafish Mstn1 and 2 is 66%. The phylogenetic tree revealed that the Mstn2 is an ancestral form of Mstn1. To study the functional aspects, we overexpressed mstn2 and noticed that embryos became less active and the juveniles with bent and curved phenotypes when compared to the control. The RT-PCR and in situ hybridization showed concurrent reduction of dystrophin associated protein complex (DAPC). In cryosection and in situ hybridization, we observed the disintegration of somites, lack of transverse myoseptum and loss of muscle integrity due to the failure of muscle attachment in mstn2 overexpressed embryos. Immunohistochemistry and western blot showed that there was a reduction of dystrophin, dystroglycan and sarcoglycan at translational level in overexpressed embryos. Taken together, these results indicate the suitability of zebrafish as an excellent animal model and our data provide the first in vivo evidence of muscle attachment failure by the overexpression of mstn2 and it leads to muscle loss which results in muscle dystrophy that may contribute to Duchenne syndrome and other muscle related diseases.

Tjp3/zo-3 is critical for epidermal barrier function in zebrafish embryos

TJP3/ZO-3 is a scaffolding protein that tethers tight junction integral membrane proteins to the actin cytoskeleton and links the conserved Crumbs polarity complex to tight junctions. The physiological function of TJP3/ZO-3 is not known and mice lacking TJP3/ZO-3 show no apparent phenotype. Here we show that Tjp3/Zo-3 is a component of tight junctions present in the enveloping cell layer of zebrafish embryos. Silencing tjp3/zo-3 using morpholinos leads to edema, loss of blood circulation and tail fin malformations in the embryos. The ultrastructure of tight junctions of the enveloping cell layer is disrupted, without affecting the asymmetric distribution of plasma membrane proteins. Morphants show a loss of the epidermal barrier, as assessed by an increased permeability of the enveloping cell layer to low molecular weight tracers and a higher sensitivity of the embryos to osmotic stress. Subjecting wild-type embryos to osmotic stress mimicks the morphant phenotype, consistent with the phenotype being a direct consequence of failed osmoregulation. Thus, Tjp3/Zo-3 is critical for barrier function of the enveloping cell layer and osmoregulation in early stages of zebrafish development.

Zebrafish homologue irx1a is required for the differentiation of serotonergic neurons

Serotonergic (5HT) neurons produce neurotransmitter serotonin, which modulates various neuronal circuits. The specification and differentiation of 5HT neurons require both extrinsic signals such as Shh and Fgf, as well as intrinsic transcription factors such as nkx2.2, mash1, phox2b, Gata2, and pet1. In this study, we show that iroquois homeodomain factor irx1a, but not irx1b, is expressed in the 5HT neurons. Knockdown of irx1a by antisense morpholino nucleotides reveals that it is a critical determinant for the differentiation of 5HT neurons in the hindbrain. However, irx1a morphants do not show a reduction of the progenitors of 5HT neurons. Hence, irx1a is not required for the initial specification but it is required for the complete differentiation of 5HT neurons.

Monitoring Notch1 activity in development: evidence for a feedback regulatory loop

Signaling through Notch receptors, which regulate cell fate decisions and embryonic patterning, requires ligand-induced receptor cleavage to generate the signaling active Notch intracellular domain (NICD). Here, we show an analysis at specific developmental stages of the distribution of active mouse Notch1. We use an antibody that recognizes N1ICD, and a highly sensitive staining technique. The earliest N1ICD expression was observed in the mesoderm and developing heart, where we detected expression in nascent endocardium, presumptive cardiac valves, and ventricular and atrial endocardium. During segmentation, N1ICD was restricted to the presomitic mesoderm. N1ICD expression was also evident in arterial endothelium, and in kidney and endodermal derivatives such as pancreas and thymus. Ectodermal N1ICD expression was found in central nervous system and sensory placodes. We found that Notch1 transcription and activity was severely reduced in zebrafish and mouse Notch pathway mutants, suggesting that vertebrate Notch1 expression is regulated by a positive feedback loop.

A cardiac myosin light chain kinase regulates sarcomere assembly in the vertebrate heart

Marked sarcomere disorganization is a well-documented characteristic of cardiomyocytes in the failing human myocardium. Myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC2v), which is involved in the development of human cardiomyopathy, is an important structural protein that affects physiologic cardiac sarcomere formation and heart development. Integrated cDNA expression analysis of failing human myocardia uncovered a novel protein kinase, cardiac-specific myosin light chain kinase (cardiac-MLCK), which acts on MLC2v. Expression levels of cardiac-MLCK were well correlated with the pulmonary arterial pressure of patients with heart failure. In cultured cardiomyocytes, knockdown of cardiac-MLCK by specific siRNAs decreased MLC2v phosphorylation and impaired epinephrine-induced activation of sarcomere reassembly. To further clarify the physiologic roles of cardiac-MLCK in vivo, we cloned the zebrafish ortholog z-cardiac-MLCK. Knockdown of z-cardiac-MLCK expression using morpholino antisense oligonucleotides resulted in dilated cardiac ventricles and immature sarcomere structures. These results suggest a significant role for cardiac-MLCK in cardiogenesis.