The zebrafish slow-muscle-omitted gene product is required for Hedgehog signal transduction and the development of slow muscle identity

Generation and characterization of gsuα:EGFP transgenic zebrafish for evaluating endocrine-disrupting effects

The glycoprotein subunit α (gsuα) gene encodes the shared α subunit of the three pituitary heterodimeric glycoprotein hormones: follicle-stimulating hormone β (Fshβ), luteinizing hormone β (Lhβ) and thyroid stimulating hormone β (Tshβ). In our current study, we identified and characterized the promoter region of zebrafish gsuα and generated a stable gsuα:EGFP transgenic line, which recapitulated the endogenous gsuα expression in the early developing pituitary gland. A relatively conserved regulatory element set is presented in the promoter regions of zebrafish and three other known mammalian gsuα promoters. Our results also demonstrated that the expression patterns of the gsuα:EGFP transgene were all identical to those expression patterns of the endogenous gsuα expression in the pituitary tissue when our transgenic fish were treated with various endocrine chemicals, including forskolin (FSK), SP600125, trichostatin A (TSA), KClO4, dexamethasone (Dex), β-estradiol and progesterone. Thus, this gsuα:EGFP transgenic fish reporter line provides another valuable tool for investigating the lineage development of gsuα-expressing gonadotrophins and the coordinated regulation of various glycoprotein hormone subunit genes. These reporter fish can serve as a novel platform to perform screenings of endocrine-disrupting chemicals (EDCs) in vivo as well.

Comparative myogenesis in teleosts and mammals

Skeletal myogenesis has been and is currently under extensive study in both mammals and teleosts, with the latter providing a good model for skeletal myogenesis because of their flexible and conserved genome. Parallel investigations of muscle studies using both these models have strongly accelerated the advances in the field. However, when transferring the knowledge from one model to the other, it is important to take into account both their similarities and differences. The main difficulties in comparing mammals and teleosts arise from their different temporal development. Conserved aspects can be seen for muscle developmental origin and segmentation, and for the presence of multiple myogenic waves. Among the divergences, many fish have an indeterminate growth capacity throughout their entire life span, which is absent in mammals, thus implying different post-natal growth mechanisms. This review covers the current state of the art on myogenesis, with a focus on the most conserved and divergent aspects between mammals and teleosts.

Pax6 regulates the formation of the habenular nuclei by controlling the temporospatial expression of Shh in the diencephalon in vertebrates

Background

The habenula and the thalamus are two critical nodes in the forebrain circuitry and they connect the midbrain and the cerebral cortex in vertebrates. The habenula is derived from the epithalamus and rests dorsally to the thalamus. Both epithalamus and thalamus arise from a single diencephalon segment called prosomere (p)2. Shh is expressed in the ventral midline of the neural tube and in the mid-diencephalic organizer (MDO) at the zona limitans intrathalamica between thalamus and prethalamus. Acting as a morphogen, Shh plays an important role in regulating cell proliferation and survival in the diencephalon and thalamic patterning. The molecular regulation of the MDO Shh expression and the potential role of Shh in development of the habenula remain largely unclear.

Results

We show that deleting paired-box and homeobox-containing gene Pax6 results in precocious and expanded expression of Shh in the prospective MDO in fish and mice, whereas gain-of-function of pax6 inhibits MDO shh expression in fish. Using gene expression and genetic fate mapping, we have characterized the expression of molecular markers that demarcate the progenitors and precursors of habenular neurons. We show that the thalamic domain is shifted dorsally and the epithalamus is missing in the alar plate of p2 in the Pax6 mutant mouse. Conversely, the epithalamus is expanded ventrally at the expense of the thalamus in mouse embryos with reduced Shh activity. Significantly, attenuating Shh signaling largely rescues the patterning of p2 and restores the epithalamus in Pax6 mouse mutants, suggesting that Shh acts downstream of Pax6 in controlling the formation of the habenula. Similar to that found in the mouse, we show that pax6 controls the formation of the epithalamus mostly via the regulation of MDO shh expression in zebrafish.

Conclusions

Our findings demonstrate that Pax6 has an evolutionarily conserved function in establishing the temporospatial expression of Shh in the MDO in vertebrates. Furthermore, Shh mediates Pax6 function in regulating the partition of the p2 domain into the epithalamus and thalamus.

Control of muscle fibre-type diversity during embryonic development: the zebrafish paradigm

Vertebrate skeletal muscle is composed of distinct types of fibre that are functionally adapted through differences in their physiological and metabolic properties. An understanding of the molecular basis of fibre-type specification is of relevance to human health and fitness. The zebrafish provides an attractive model for investigating fibre type specification; not only are their rapidly developing embryos optically transparent, but in contrast to amniotes, the embryonic myotome shows a discrete temporal and spatial separation of fibre type ontogeny that simplifies its analysis. Here we review the current state of understanding of muscle fibre type specification and differentiation during embryonic development of the zebrafish, with a particular focus on the roles of the Prdm1a and Sox6 transcription factors, and consider the relevance of these findings to higher vertebrate muscle biology.

Heat-shock-mediated conditional regulation of hedgehog/gli signaling in zebrafish

BACKGROUND

Hedgehog (Hh) signaling is required for embryogenesis and continues to play key roles postembryonically in many tissues, influencing growth, stem cell proliferation, and tumorigenesis. Systems for conditional regulation of Hh signaling facilitate the study of these postembryonic Hh functions.

RESULTS

We used the hsp70l promoter to generated three heat-shock-inducible transgenic lines that activate Hh signaling and one line that represses Hh signaling. Heat-shock activation of these transgenes appropriately recapitulates early embryonic loss or gain of Hh function phenotypes. Hh signaling remains activated 24 hr after heat shock in the Tg(hsp70l:shha-EGFP) and Tg(hsp70l:dnPKA-BGFP) lines, while a single heat shock of the Tg(hsp70l:gli1-EGFP) or Tg(hsp70l:gli2aDR-EGFP) lines results in a 6- to 12-hr pulse of Hh signal activation or inactivation, respectively. Using both in situ hybridization and quantitative polymerase chain reaction, we show that these lines can be used to manipulate Hh signaling through larval and juvenile stages. A ptch2 promoter element was used to generate new reporter lines that allow clear visualization of Hh responding cells throughout the life cycle, including graded Hh responses in the embryonic central nervous system.

CONCLUSIONS

These zebrafish transgenic lines provide important new experimental tools to study the embryonic and postembryonic roles of Hh signaling.

Autonomous and nonautonomous roles of Hedgehog signaling in regulating limb muscle formation

Muscle progenitor cells migrate from the lateral somites into the developing vertebrate limb, where they undergo patterning and differentiation in response to local signals. Sonic hedgehog (Shh) is a secreted molecule made in the posterior limb bud that affects patterning and development of multiple tissues, including skeletal muscles. However, the cell-autonomous and non-cell-autonomous functions of Shh during limb muscle formation have remained unclear. We found that Shh affects the pattern of limb musculature non-cell-autonomously, acting through adjacent nonmuscle mesenchyme. However, Shh plays a cell-autonomous role in maintaining cell survival in the dermomyotome and initiating early activation of the myogenic program in the ventral limb. At later stages, Shh promotes slow muscle differentiation cell-autonomously. In addition, Shh signaling is required cell-autonomously to regulate directional muscle cell migration in the distal limb. We identify neuroepithelial cell transforming gene 1 (Net1) as a downstream target and effector of Shh signaling in that context.

Morphogenesis and cell fate determination within the adaxial cell equivalence group of the zebrafish myotome

One of the central questions of developmental biology is how cells of equivalent potential—an equivalence group—come to adopt specific cellular fates. In this study we have used a combination of live imaging, single cell lineage analyses, and perturbation of specific signaling pathways to dissect the specification of the adaxial cells of the zebrafish embryo. We show that the adaxial cells are myogenic precursors that form a cell fate equivalence group of approximately 20 cells that consequently give rise to two distinct sub-types of muscle fibers: the superficial slow muscle fibers (SSFs) and muscle pioneer cells (MPs), distinguished by specific gene expression and cell behaviors. Using a combination of live imaging, retrospective and indicative fate mapping, and genetic studies, we show that MP and SSF precursors segregate at the beginning of segmentation and that they arise from distinct regions along the anterior-posterior (AP) and dorsal-ventral (DV) axes of the adaxial cell compartment. FGF signaling restricts MP cell fate in the anterior-most adaxial cells in each somite, while BMP signaling restricts this fate to the middle of the DV axis. Thus our results reveal that the synergistic actions of HH, FGF, and BMP signaling independently create a three-dimensional (3D) signaling milieu that coordinates cell fate within the adaxial cell equivalence group.

How specific genes and signals act on initially identical cells to generate the different tissues of the body remains one of the central questions of developmental genetics. Zebrafish are a useful model system to tackle this question as the optically clear embryo allows direct imaging of forming tissues, tracking individual cells in a myriad of different genetic contexts. The zebrafish myotome, the compartment of the embryo that gives rise to skeletal muscle, is subdivided into a number of specific cell types—one of which, the adaxial cells, gives rise exclusively to muscle of the “slow twitch” class. The adaxial cells give rise to two types of slow muscle cell types, muscle pioneer cells and non-muscle pioneer slow cells, distinguished by gene expression and different cellular behaviours. In this study we use lineage tracing live imaging and the manipulation of distinct genetic pathways to demonstrate that the adaxial cells form a cell fate “equivalence group” that is specified using separate signaling pathways that operating in distinct dimensions.

Stac3 is required for myotube formation and myogenic differentiation in vertebrate skeletal muscle

Stac3 was identified as a nutritionally regulated gene from an Atlantic salmon subtractive hybridization library with highest expression in skeletal muscle. Salmon Stac3 mRNA was highly correlated with myogenin and myoD1a expression during differentiation of a salmon primary myogenic culture and was regulated by amino acid availability. In zebrafish embryos, stac3 was initially expressed in myotomal adaxial cells and in fast muscle fibers post-segmentation. Morpholino knockdown resulted in defects in myofibrillar protein assembly, particularly in slow muscle fibers, and decreased levels of the hedgehog receptor patched. The function of Stac3 was further characterized in vitro using the mammalian C2C12 myogenic cell line. Stac3 mRNA expression increased during the differentiation of the C2C12 myogenic cell line. Knockdown of Stac3 by RNAi inhibited myotube formation, and microarray analysis revealed that transcripts involved in cell cycle, focal adhesion, cytoskeleton, and the pro-myogenic factors Igfbp-5 and Igf2 were down-regulated. RNAi-treated cells had suppressed Akt signaling and exogenous insulin-like growth factor (Igf) 2 was unable to rescue the phenotype, however, Igf/Akt signaling was not blocked. Overexpression of Stac3, which results in increased levels of Igfbp-5 mRNA, did not lead to increased differentiation. In synchronized cells, Stac3 mRNA was most abundant during the G(1) phase of the cell cycle. RNAi-treated cells were smaller, had higher proliferation rates and a decreased proportion of cells in G(1) phase when compared with controls, suggesting a role in the G(1) phase checkpoint. These results identify Stac3 as a new gene required for myogenic differentiation and myofibrillar protein assembly in vertebrates.

Small molecule inhibitors of the hedgehog signaling pathway for the treatment of cancer

Over the past decade, the Hedgehog signaling pathway has attracted considerable interest because the pathway plays important roles in the tumorigenesis of several types of cancer as well as developmental processes. It has also been observed that Hedgehog signaling regulates the proliferation and self-renewal of cancer stem cells. A great number of Hedgehog pathway inhibitors have been discovered through small molecule screens and subsequent medicinal chemistry efforts. Among the inhibitors, several Smo antagonists have reached the clinical trial phase. It has been proved that the inhibition of Hedgehog signaling with Smo antagonists is beneficial to cancer patients with basal cell carcinoma and medulloblastoma. In this review, we provide an overview of Hedgehog pathway inhibitors with focusing on the preclinical and/or clinical efficacy and molecular mechanisms of these inhibitors.

Fss/Tbx6 is required for central dermomyotome cell fate in zebrafish

The dermomyotome is a pool of progenitor cells on the surface of the myotome. In zebrafish, dermomyotome precursors (anterior border cells, ABCs) can be first identified in the anterior portion of recently formed somites. They must be prevented from undergoing terminal differentiation during segmentation, even while mesodermal cells around them respond to signaling cues and differentiate.

T-box containing transcription factors regulate many aspects of mesoderm fate including segmentation and somite patterning. The fused somites (fss) gene is the zebrafish ortholog of tbx6. We demonstrate that in addition to its requirement for segmentation, fss/tbx6 is also required for the specification of ABCs and subsequently the central dermomyotome. The absence of Tbx6-dependent central dermomyotome cells in fss/tbx6 mutants is spatially coincident with a patterning defect in the myotome.

Using transgenic fish with a heat-shock inducible tbx6 gene in the fss/tbx6 mutant background, we further demonstrate that ubiquitous fss/tbx6 expression has spatially distinct effects on recovery of the dermomyotome and segment boundaries, suggesting that the mechanism of Fss/Tbx6 action is distinct with respect to dermomyotome development and segmentation.

We propose that Fss/Tbx6 is required for preventing myogenic differentiation of central dermomyotome precursors before and after segmentation and that central dermomyotome cells represent a genetically and functionally distinct subpopulation within the zebrafish dermomyotome.

Hh signaling regulates patterning and morphogenesis of the pharyngeal arch-derived skeleton

The proper function of the craniofacial skeleton requires the proper shaping of many individual skeletal elements. Neural crest cells generate much of the craniofacial skeleton and morphogenesis of skeletal elements occurs in transient, reiterated structures termed pharyngeal arches. The shape of individual elements depends upon intrinsic patterning within the neural crest as well as extrinsic signals to the neural crest from adjacent tissues within the arches. Hedgehog (Hh) signaling is known to play roles in craniofacial development, yet its involvement in intrinsic and extrinsic patterning of the craniofacial skeleton is still not well understood. Here, we show that morphogenetic movements of the pharyngeal arches and patterning of the neural crest require Hh signaling. Loss of Hh signaling, in smoothened (smo) mutants, disrupts the expression of some Dlx genes as well as other markers of dorsal/ventral patterning of the neural crest. Transplantation of wild-type neural crest cells into smo mutants rescues this defect, demonstrating that the neural crest requires reception of Hh signals for proper patterning. Despite the rescue, morphogenesis of the facial skeleton is not fully recovered. Through transplant analyses, we find two additional requirements for Hh signaling. The endoderm requires the reception of Hh signals for proper morphogenetic movements of the pharyngeal arches and the neural crest require the reception of Hh signaling for the activity of a reverse signal that maintains sonic hedgehog expression in the endoderm. Collectively, these results demonstrate that Hh signaling is essential to establish intrinsic and extrinsic patterning information for the craniofacial skeleton.

Scube activity is necessary for Hedgehog signal transduction in vivo

The Hedgehog (HH) signaling pathway is a central regulator of embryonic development, controlling the pattern and proliferation of a wide variety of organs. Previous studies have implicated the secreted protein, Scube2, in HH signal transduction in the zebrafish embryo (Hollway et al., 2006; Kawakami et al., 2005; Woods and Talbot, 2005) although the nature of the molecular function of Scube2 in this process has remained undefined. This analysis has been compounded by the fact that removal of Scube2 activity in the zebrafish embryo leads to only subtle defects in HH signal transduction in vivo (Barresi et al., 2000; Hollway et al., 2006; Ochi and Westerfield, 2007; van Eeden et al., 1996; Wolff et al., 2003). Here we present the discovery of two additional scube genes in zebrafish, scube1 and scube3, and demonstrate their roles in facilitating HH signal transduction. Knocking down the function of all three scube genes simultaneously phenocopies a complete loss of HH signal transduction in the embryo, revealing that Scube signaling is essential for HH signal transduction in vivo. We further define the molecular role of scube2 in HH signaling.

Macondo crude oil from the Deepwater Horizon oil spill disrupts specific developmental processes during zebrafish embryogenesis

Background

The Deepwater Horizon disaster was the largest marine oil spill in history, and total vertical exposure of oil to the water column suggests it could impact an enormous diversity of ecosystems. The most vulnerable organisms are those encountering these pollutants during their early life stages. Water-soluble components of crude oil and specific polycyclic aromatic hydrocarbons have been shown to cause defects in cardiovascular and craniofacial development in a variety of teleost species, but the developmental origins of these defects have yet to be determined. We have adopted zebrafish, Danio rerio, as a model to test whether water accumulated fractions (WAF) of the Deepwater Horizon oil could impact specific embryonic developmental processes. While not a native species to the Gulf waters, the developmental biology of zebrafish has been well characterized and makes it a powerful model system to reveal the cellular and molecular mechanisms behind Macondo crude toxicity.

Results

WAF of Macondo crude oil sampled during the oil spill was used to treat zebrafish throughout embryonic and larval development. Our results indicate that the Macondo crude oil causes a variety of significant defects in zebrafish embryogenesis, but these defects have specific developmental origins. WAF treatments caused defects in craniofacial development and circulatory function similar to previous reports, but we extend these results to show they are likely derived from an earlier defect in neural crest cell development. Moreover, we demonstrate that exposure to WAFs causes a variety of novel deformations in specific developmental processes, including programmed cell death, locomotor behavior, sensory and motor axon pathfinding, somitogenesis and muscle patterning. Interestingly, the severity of cell death and muscle phenotypes decreased over several months of repeated analysis, which was correlated with a rapid drop-off in the aromatic and alkane hydrocarbon components of the oil.

Conclusions

Whether these teratogenic effects are unique to the oil from the Deepwater Horizon oil spill or generalizable for most crude oil types remains to be determined. This work establishes a model for further investigation into the molecular mechanisms behind crude oil mediated deformations. In addition, due to the high conservation of genetic and cellular processes between zebrafish and other vertebrates, our work also provides a platform for more focused assessment of the impact that the Deepwater Horizon oil spill has had on the early life stages of native fish species in the Gulf of Mexico and the Atlantic Ocean.

Application of complementary luminescent and fluorescent imaging techniques to visualize nuclear and cytoplasmic Ca²⁺ signalling during the in vivo differentiation of slow muscle cells in zebrafish embryos under normal and dystrophic conditions

1. Evidence is accumulating for a role for Ca²⁺ signalling in the differentiation and development of embryonic skeletal muscle. 2. Imaging of intact, normally developing transgenic zebrafish that express the protein component of the Ca²⁺-sensitive complex aequorin, specifically in skeletal muscle, show that two distinct periods of spontaneous synchronised Ca²⁺ transients occur in the trunk: one at approximately 17.5-19.5 h post-fertilization (h.p.f.; termed signalling period SP1) and the other after approximately 23 h.p.f. (termed SP2). These periods of intense Ca²⁺ signalling activity are separated by a quiet period. 3. Higher-resolution confocal imaging of embryos loaded with the fluorescent Ca²⁺ reporter calcium green-1 dextran shows that the Ca²⁺ signals are generated almost exclusively in the slow muscle cells, the first muscle cells to differentiate, with distinct nuclear and cytoplasmic components. 4. Here, we show that coincidental with the SP1 Ca²⁺ signals, dystrophin becomes localized to the vertical myoseptae of the myotome. Introduction of a dmd morpholino (dmd-MO) resulted in no dystrophin being expressed in the vertical myoseptae, as well as a disruption of myotome morphology and sarcomere organization. In addition, the Ca²⁺ signalling signatures of dmd-MO-injected embryos or homozygous sapje mutant embryos were abnormal such that the frequency, amplitude and timing of the Ca²⁺ signals were altered compared with controls. 5. Our new data suggest that, in addition to a structural role, dystrophin may function in the regulation of [Ca²⁺](i) during the early stages of slow muscle cell differentiation when the Ca²⁺ signals generated in these cells coincide with the first spontaneous contractions of the trunk.

Muscle development and differentiation in the urodele Ambystoma mexicanum

Muscle differentiation has been widely described in zebrafish and Xenopus, but nothing is known about this process in amphibian urodeles. Both anatomical features and locomotor activity in urodeles are known to show intermediate features between fish and anurans. Therefore, a better understanding of myogenesis in urodeles could be useful to clarify the evolutionary changes that led to the formation of skeletal muscle in the trunk of land vertebrates. We report here a detailed morphological and molecular investigation on several embryonic stages of Ambystoma mexicanum and show that the first differentiating muscle fibers are the slow ones, originating from a myoblast population initially localized close to the notochord that forms a superficial layer on the somitic surface afterwards. Subsequently, fast fibers differentiation ensues. We also identified and cloned A. mexicanum Myf5 as a muscle-specific transcriptional factor likely involved in urodele muscle differentiation.

The role of glypicans in Wnt inhibitory factor-1 activity and the structural basis of Wif1's effects on Wnt and Hedgehog signaling

Proper assignment of cellular fates relies on correct interpretation of Wnt and Hedgehog (Hh) signals. Members of the Wnt Inhibitory Factor-1 (WIF1) family are secreted modulators of these extracellular signaling pathways. Vertebrate WIF1 binds Wnts and inhibits their signaling, but its Drosophila melanogaster ortholog Shifted (Shf) binds Hh and extends the range of Hh activity in the developing D. melanogaster wing. Shf activity is thought to depend on reinforcing interactions between Hh and glypican HSPGs. Using zebrafish embryos and the heterologous system provided by D. melanogaster wing, we report on the contribution of glypican HSPGs to the Wnt-inhibiting activity of zebrafish Wif1 and on the protein domains responsible for the differences in Wif1 and Shf specificity. We show that Wif1 strengthens interactions between Wnt and glypicans, modulating the biphasic action of glypicans towards Wnt inhibition; conversely, glypicans and the glypican-binding “EGF-like” domains of Wif1 are required for Wif1's full Wnt-inhibiting activity. Chimeric constructs between Wif1 and Shf were used to investigate their specificities for Wnt and Hh signaling. Full Wnt inhibition required the “WIF” domain of Wif1, and the HSPG-binding EGF-like domains of either Wif1 or Shf. Full promotion of Hh signaling requires both the EGF-like domains of Shf and the WIF domains of either Wif1 or Shf. That the Wif1 WIF domain can increase the Hh promoting activity of Shf's EGF domains suggests it is capable of interacting with Hh. In fact, full-length Wif1 affected distribution and signaling of Hh in D. melanogaster, albeit weakly, suggesting a possible role for Wif1 as a modulator of vertebrate Hh signaling.

In developing organisms, cells choose between alternative fates in order to make appropriately patterned tissues, and misregulation of those choices can underlie both developmental defects and cancers. Cells often make these decisions because of signals received from neighboring cells, such as those mediated by the secreted signaling proteins of the Wnt and Hedgehog (Hh) families. While signaling can be regulated by the levels of signaling or receptor proteins expressed by cells, another level of control is exerted by proteins that bind signaling proteins outside of cells and either inhibit or promote the signaling process. In the fruitfly Drosophilamelanogaster, the secreted Shifted protein has been shown to bind Hh and to increase Hh signaling, likely by reinforcing interactions between Hh and cell surface proteins of the glypican family. We provide evidence that the vertebrate homolog of Shifted, Wnt Inhibitory Factor-1 (Wif1), inhibits Wnt activity by a similar mechanism, reinforcing interactions between Wnts and glypicans in a manner that sequesters Wnts from their receptors. We also examine the structural basis for the specificities of Wif1 and Shifted for Wnt and Hh signaling, respectively, and provide evidence that Wif1, although a potent inhibitor of Wnt activity, influences D. melanogaster Hh signaling.

The genetics of ocular disorders: insights from the zebrafish

Proper formation of the vertebrate eye requires a precisely coordinated sequence of morphogenetic events that integrate the developmental contributions of the skin ectoderm, neuroectoderm, and head mesenchyme. Disruptions in this process result in ocular malformations or retinal degeneration and can cause significant visual impairment. The zebrafish is an excellent vertebrate model for the study of eye development and disease due to the transparency of the embryo, its ex utero development, and its amenability to forward genetic screens. This review will present an overview of the genetic methodologies utilized in the zebrafish, a description of several zebrafish models of congenital ocular diseases, and a discussion of the utility of the zebrafish for assessing the pathogenicity of candidate disease alleles.

Immunocytochemistry to study myogenesis in zebrafish

During myogenesis, cells gradually transition from mesodermal precursors to myoblasts, myocytes, and then to muscle fibers. The molecular characterization of this process requires the ability to identify each of these cell types and the factors that regulate the transitions between them. The most versatile technique for assaying cell identities in situ is immunocytochemistry, because multiple independent molecular markers of differentiation can be assayed simultaneously. The zebrafish has developed into a popular model for the study of myogenesis, and immunocytochemical techniques have been critical. We have adapted existing protocols to optimize immunocytochemistry in zebrafish, and have tested many antibodies developed against mouse, chick, and frog muscle antigens for their cross-reactivity in zebrafish. Here, we present protocols for whole mount immunocytochemistry on both formaldehyde and Carnoy's fixed embryos as well as on sectioned zebrafish tissue. We include a table of antibodies useful for experiments on the molecular biology of myogenesis in zebrafish.

Calcium imaging in the zebrafish

The zebrafish (Danio rerio) has emerged as a new model system during the last three decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging. While being the vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function. Since the mid 1990s, the embryonic development and neuronal function of the larval, and later, adult zebrafish have been studied using calcium imaging methods. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Dextran indicators have been used to label cells in the developing embryo from dye injection into the one-cell stage. Dextrans have also been useful for retrograde labeling of spinal cord neurons and cells in the olfactory system. Acetoxymethyl (AM) esters permit labeling of larger areas of tissue such as the tectum, a region responsible for visual processing. Genetically encoded calcium indicators have been expressed in various tissues by the use of cell-specific promoters. These studies have contributed greatly to our understanding of basic biological principles during development and adulthood, and of the function of disease-related genes in a vertebrate system.

Transcriptional response of zebrafish embryos exposed to neurotoxic compounds reveals a muscle activity dependent hspb11 expression

Acetylcholinesterase (AChE) inhibitors are widely used as pesticides and drugs. Their primary effect is the overstimulation of cholinergic receptors which results in an improper muscular function. During vertebrate embryonic development nerve activity and intracellular downstream events are critical for the regulation of muscle fiber formation. Whether AChE inhibitors and related neurotoxic compounds also provoke specific changes in gene transcription patterns during vertebrate development that allow them to establish a mechanistic link useful for identification of developmental toxicity pathways has, however, yet not been investigated. Therefore we examined the transcriptomic response of a known AChE inhibitor, the organophosphate azinphos-methyl (APM), in zebrafish embryos and compared the response with two non-AChE inhibiting unspecific control compounds, 1,4-dimethoxybenzene (DMB) and 2,4-dinitrophenol (DNP). A highly specific cluster of APM induced gene transcripts was identified and a subset of strongly regulated genes was analyzed in more detail. The small heat shock protein hspb11 was found to be the most sensitive induced gene in response to AChE inhibitors. Comparison of expression in wildtype, ache and sop(fixe) mutant embryos revealed that hspb11 expression was dependent on the nicotinic acetylcholine receptor (nAChR) activity. Furthermore, modulators of intracellular calcium levels within the whole embryo led to a transcriptional up-regulation of hspb11 which suggests that elevated intracellular calcium levels may regulate the expression of this gene. During early zebrafish development, hspb11 was specifically expressed in muscle pioneer cells and Hspb11 morpholino-knockdown resulted in effects on slow muscle myosin organization. Our findings imply that a comparative toxicogenomic approach and functional analysis can lead to the identification of molecular mechanisms and specific marker genes for potential neurotoxic compounds.

Alkali-like myosin light chain-1 (myl1) is an early marker for differentiating fast muscle cells in zebrafish

During myogenesis, muscle precursors become divided into either fast- or slow-twitch fibres, which in the zebrafish occupy distinct domains in the embryo. Genes encoding sarcomeric proteins specific for fast or slow fibres are frequently used as lineage markers. In an attempt to identify and evaluate early definitive markers for cells in the fast-twitch pathway, we analysed genes encoding proteins contributing to the fast sarcomeric structures. The previously uncharacterized zebrafish alkali-like myosin light chain gene (myl1) was found to be expressed exclusively in cells in the fast-twitch pathway initiated at an early stage of fast fibre differentiation. Myl1 was expressed earlier, and in a more fibre type restricted manner, than any of the previously described and frequently used fast myosin light and heavy chain and troponin muscle markers mylz2, mylz3, tnni2, tnnt3a, fMyHC1.3. In summary, this study introduces a novel marker for early differentiating fast muscle cells.

Perturbation of zebrafish swimbladder development by enhancing Wnt signaling in Wif1 morphants

Wnt signaling plays critical roles in development of both tetrapod lung and fish swimbladder, which are the two evolutionary homologous organs. Our previous data reveal that down-regulation of Wnt signaling leads to defective swimbladder development. However, the effects of up-regulation of Wnt signaling on swimbladder development remain unclear. By knockdown of the Wnt protein inhibitory gene wif1, we demonstrated that up-regulation of Wnt signaling also resulted in perturbed development of the swimbladder. Specifically, the growth of epithelium and mesenchyme was greatly inhibited, the smooth muscle differentiation was abolished, and the organization of mesothelium was disturbed. Furthermore, our data reveal that it is the reduced cell proliferation, but not enhanced apoptosis, that contributes to the disturbance of swimbladder development in wif1 morphants. Blocking Wnt signaling by the Wnt antagonist IWR-1 did not affect wif1 expression in the swimbladder, but complete suppression of Hedgehog signaling in smo-/- mutants abolished wif expression, consistent with our earlier report of a negative feedback regulation of Wnt signaling in the swimbladder by the Hedgehog signaling. Our works established the importance of proper level of Wnt signaling for normal development of swimbladder in zebrafish.

Growth Arrest Specific 8 (Gas8) and G protein-coupled receptor kinase 2 (GRK2) cooperate in the control of Smoothened signaling

The G protein-coupled receptor (GPCR)-like molecule Smoothened (Smo) undergoes dynamic intracellular trafficking modulated by the microtubule associated kinase GRK2 and recruitment of β-arrestin. Of this trafficking, especially the translocation of Smo into primary cilia and back to the cytoplasm is essential for the activation of Hedgehog (Hh) signaling in vertebrates. The complete mechanism of this bidirectional transport, however, is not completely understood. Here we demonstrate that Growth Arrest Specific 8 (Gas8), a microtubule associated subunit of the Dynein Regulatory Complex (DRC), interacts with Smo to modulate this process. Gas8 knockdown in ciliated cells reduces Smo signaling activity and ciliary localization whereas overexpression stimulates Smo activity in a GRK2-dependent manner. The C terminus of Gas8 is important for both Gas8 interaction with Smo and facilitating Smo signaling. In zebrafish, knocking down Gas8 results in attenuated Hh transcriptional responses and impaired early muscle development. These effects can be reversed by the co-injection of Gas8 mRNA or by constitutive activation of the downstream Gli transcription factors. Furthermore, Gas8 and GRK2 display a synergistic effect on zebrafish early muscle development and some effects of GRK2 knockdown can be rescued by Gas8 mRNA. Interestingly, Gas8 does not interfere with cilia assembly, as the primary cilia architecture is unchanged upon Gas8 knock down or heterologous expression. This is in contrast to cells stably expressing both GRK2 and Smo, in which cilia are significantly elongated. These results identify Gas8 as a positive regulator of Hh signaling that cooperates with GRK2 to control Smo signaling.

Wnt signaling is required for early development of zebrafish swimbladder

Background

Wnt signaling plays critical roles in mammalian lung development. However, Wnt signaling in the development of the zebrafish swimbladder, which is considered as a counterpart of mammalian lungs, have not been explored. To investigate the potential conservation of signaling events in early development of the lung and swimbladder, we wish to address the question whether Wnt signaling plays a role in swimbladder development.

Methodology/Principal Findings

For analysis of zebrafish swimbladder development, we first identified, by whole-mount in situ hybridization (WISH), has2 as a mesenchymal marker, sox2 as the earliest epithelial marker, as well as hprt1l and elovl1a as the earliest mesothelial markers. We also demonstrated that genes encoding Wnt signaling members Wnt5b, Fz2, Fz7b, Lef1, Tcf3 were expressed in different layers of swimbladder. Then we utilized the heat-shock inducible transgenic lines hs:Dkk1-GFP and hs:ΔTcf-GFP to temporarily block canonical Wnt signaling. Inhibition of canonical Wnt signaling at various time points disturbed precursor cells specification, organization, anterioposterior patterning, and smooth muscle differentiation in all three tissue layers of swimbladder. These observations were also confirmed by using a chemical inhibitor (IWR-1) of Wnt signaling. In addition, we found that Hedgehog (Hh) signaling was activated by canonical Wnt signaling and imposed a negative feedback on the latter.

Significance/Conclusion

We first provided a new set of gene markers for the three tissue layers of swimbladder in zebrafish and demonstrated the expression of several key genes of Wnt signaling pathway in developing swimbladder. Our functional analysis data indicated that Wnt/β-catenin signaling is required for swimbladder early development and we also provided evidence for the crosstalk between Wnt and Hh signaling in early swimbladder development.

Fine-tuning of Hh signaling by the RNA-binding protein Quaking to control muscle development

The development of the different muscles within the somite is a complex process that involves the Hedgehog (Hh) signaling pathway. To specify the proper number of muscle cells and organize them spatially and temporally, the Hh signaling pathway needs to be precisely regulated at different levels, but only a few factors external to the pathway have been described. Here, we report for the first time the role of the STAR family RNA-binding protein Quaking A (QkA) in somite muscle development. We show in zebrafish that the loss of QkA function affects fast muscle fiber maturation as well as Hh-induced muscle derivative specification and/or morphogenesis. Mosaic analysis reveals that fast fiber maturation depends on the activity of QkA in the environment of fast fiber progenitors. We further show that Hh signaling requires QkA activity for muscle development. By an in silico approach, we screened the 3'UTRs of known Hh signaling component mRNAs for the Quaking response element and found the transcription factor Gli2a, a known regulator of muscle fate development. Using destabilized GFP as a reporter, we show that the gli2a mRNA 3'UTR is a functional QkA target. Consistent with this notion, the loss of QkA function rescued slow muscle fibers in yot mutant embryos, which express a dominant-negative Gli2a isoform. Thus, our results reveal a new mechanism to ensure muscle cell fate diversity by fine-tuning of the Hh signaling pathway via RNA-binding proteins.

Core fucosylation is required for midline patterning during zebrafish development

Complex carbohydrates represent one of the most polymorphic classes of macromolecules, but their functions during embryonic development remain poorly defined. Herein, we show that knockdown of FucT8, the fucosyltransferase responsible for adding an α1,6 fucosyl residue to the core region of N-linked oligosaccharides, results in defective midline patterning during zebrafish development. Reduced FucT8 expression leads to mild cyclopia, small forebrains, U-shaped somites, among other midline patterning defects. One of the principal FucT8 substrates was identified as Apolipoprotein B (ApoB), the major scaffold protein that is responsible for assembly and secretion of lipoprotein particles in vertebrates. In Drosophila, lipoprotein particles are thought to facilitate cell signaling by serving as a transport vehicle for lipid-modified cell signaling proteins, such as hedgehog. In this regard, knockdown of ApoB expression in zebrafish embryos leads to similar midline patterning defects as those seen in FucT8 morphant embryos. Furthermore, preliminary studies suggest that ApoB facilitates Sonic hedgehog signaling during zebrafish development, analogous to the function of lipoprotein particles during hedgehog signaling in Drosophila.

Sonic hedgehog (Shh)-Gli signaling controls neural progenitor cell division in the developing tectum in zebrafish

Despite considerable progress, the mechanisms that control neural progenitor differentiation and behavior, as well as their functional integration into adult neural circuitry, are far from being understood. Given the complexity of the mammalian brain, non-mammalian models provide an excellent model to study neurogenesis, including both the cellular composition of the neurogenic microenvironment, and the factors required for precursor growth and maintenance. In particular, we chose to address the question of the control of progenitor proliferation by Sonic hedgehog (Shh) using the zebrafish dorsal mesencephalon, known as the optic tectum (OT), as a model system. Here we show that either inhibiting pharmacologically or eliminating hedgehog (Hh) signaling by using mutants that lack essential components of the Hh pathway reduces neural progenitor cell proliferation affecting neurogenesis in the OT. On the contrary, pharmacological gain-of-function experiments result in significant increase in proliferation. Importantly, Shh-dependent function controls neural progenitor cell behavior as sox2-positive cell populations were lost in the OT in the absence of Hh signaling, as evidenced in slow-muscle-omitted (smu) mutants and with timed cyclopamine inhibition. Expressions of essential components of the Hh pathway reveal for the first time a late dorsal expression in the embryonic OT. Our observations argue strongly for a role of Shh in neural progenitor biology in the OT and provide comparative data to our current understanding of progenitor/stem cell mechanisms that place Shh as a key niche factor in the dorsal brain.

Cdkn1c drives muscle differentiation through a positive feedback loop with Myod

Differentiation often requires conversion of analogue signals to a stable binary output through positive feedback. Hedgehog (Hh) signalling promotes myogenesis in the vertebrate somite, in part by raising the activity of muscle regulatory factors (MRFs) of the Myod family above a threshold. Hh is known to enhance MRF expression. Here we show that Hh is also essential at a second step that increases Myod protein activity, permitting it to promote Myogenin expression. Hh acts by inducing expression of cdkn1c (p57(Kip2)) in slow muscle precursor cells, but neither Hh nor Cdkn1c is required for their cell cycle exit. Cdkn1c co-operates with Myod to drive differentiation of several early zebrafish muscle fibre types. Myod in turn up-regulates cdkn1c, thereby providing a positive feedback loop that switches myogenic cells to terminal differentiation.

Hedgehog signaling and laminin play unique and synergistic roles in muscle development

Hedgehog (Hh) signaling and laminin-111, a basement membrane protein, are required for early muscle development. Hh signaling specifies different populations of muscle fibers and laminin-111 is critical for early muscle morphogenesis. However, additional requirements for Hh signaling and laminin during later phases of muscle development are not known. Furthermore, interactions between Hh signaling and laminin in this context are unknown. We used laminin gamma1 mutant zebrafish and cyclopamine to block Hh signal transduction separately and in combination to investigate their functions and interactions. We found that both Hh signaling and laminin are required for normal myosin chain expression. In addition, Hh signaling and laminin act synergistically during fast-twitch fiber elongation: fast muscle cells do not elongate in embryos deficient for both Hh signaling and laminin. Finally, we present evidence that suggests that Hh signaling is indirectly required via slow fiber specification for recovery of fast fiber elongation in laminin gamma1 mutant embryos.

Brother of cdo (umleitung) is cell-autonomously required for Hedgehog-mediated ventral CNS patterning in the zebrafish

The transmembrane protein Brother of Cdo (Boc) has been implicated in Shh-mediated commissural axon guidance, and can both positively and negatively regulate Hedgehog (Hh) target gene transcription, however, little is known about in vivo requirements for Boc during vertebrate embryogenesis. The zebrafish umleitung (uml(ty54)) mutant was identified by defects in retinotectal axon projections. Here, we show that the uml locus encodes Boc and that Boc function is cell-autonomously required for Hh-mediated neural patterning. Our phenotypic analysis suggests that Boc is required as a positive regulator of Hh signaling in the spinal cord, hypothalamus, pituitary, somites and upper jaw, but that Boc might negatively regulate Hh signals in the lower jaw. This study reveals a role for Boc in ventral CNS cells that receive high levels of Hh and uncovers previously unknown roles for Boc in vertebrate embryogenesis.

Inositol hexakisphosphate kinase-2 acts as an effector of the vertebrate Hedgehog pathway

Inositol phosphate (IP) kinases constitute an emerging class of cellular kinases linked to multiple cellular activities. Here, we report a previously uncharacterized cellular function in Hedgehog (Hh) signaling for the IP kinase designated inositol hexakisphosphate kinase-2 (IP6K2) that produces diphosphoryl inositol phosphates (PP-IPs). In zebrafish embryos, IP6K2 activity was required for normal development of craniofacial structures, somites, and neural crest cells. ip6k2 depletion in both zebrafish and mammalian cells also inhibited Hh target gene expression. Inhibiting IP(6) kinase activity using N(2)-(m-(trifluoromethy)lbenzyl) N(6)-(p-nitrobenzyl)purine (TNP) resulted in altered Hh signal transduction. In zebrafish, restoring IP6K2 levels with exogenous ip6k2 mRNA reversed the effects of IP6K2 depletion. Furthermore, overexpression of ip6k2 in mammalian cells enhanced the Hh pathway response, suggesting IP6K2 is a positive regulator of Hh signaling. Perturbations from IP6K2 depletion or TNP were reversed by overexpressing smoM2, gli1, or ip6k2. Moreover, the inhibitory effect of cyclopamine was reversed by overexpressing ip6k2. This identified roles for the inositol kinase pathway in early vertebrate development and tissue morphogenesis, and in Hh signaling. We propose that IP6K2 activity is required at the level or downstream of Smoothened but upstream of the transcription activator Gli1.

Early development of medaka Oryzias latipes muscles as revealed by transgenic approaches using embryonic and larval types of myosin heavy chain genes

We cloned three full-length cDNAs encoding myosin heavy chains (MYHs) previously found to be expressed in embryos or larvae of medaka Oryzias latipes. Based on cDNA sequence information, the three medaka MYH genes, mMYH(emb1), mMYH(L1) and mMYH(L2), were localized on the chromosomes. In vivo promoter assay using the gene encoding green or red fluorescent protein and linked to the 5'-flanking region of mMYH demonstrated that the transcripts of fast-type mMYH(emb1), first expressed in embryos but belonging to the adult type in phylogenetic analysis, were located in the horizontal myoseptum. On the other hand, embryonic fast-type mMYH(L1) and mMYH(L2) were expressed in the whole myotomes. Interestingly, cells expressing mMYH(emb1) were localized together with engrailed, and cyclopamine, which blocks hedgehog signaling, inhibited mMYH(emb1) expression as well as the formation of the horizontal myoseptum, suggesting that muscle pioneer cells express mMYH(emb1) as a key protein in the formation of the horizontal myoseptum.

Gli2a protein localization reveals a role for Iguana/DZIP1 in primary ciliogenesis and a dependence of Hedgehog signal transduction on primary cilia in the zebrafish

BACKGROUND

In mammalian cells, the integrity of the primary cilium is critical for proper regulation of the Hedgehog (Hh) signal transduction pathway. Whether or not this dependence on the primary cilium is a universal feature of vertebrate Hedgehog signalling has remained contentious due, in part, to the apparent divergence of the intracellular transduction pathway between mammals and teleost fish.

RESULTS

Here, using a functional Gli2-GFP fusion protein, we show that, as in mammals, the Gli2 transcription factor localizes to the primary cilia of cells in the zebrafish embryo and that this localization is modulated by the activity of the Hh pathway. Moreover, we show that the Igu/DZIP1protein, previously implicated in the modulation of Gli activity in zebrafish, also localizes to the primary cilium and is required for its proper formation.

CONCLUSION

Our findings demonstrate a conserved role of the primary cilium in mediating Hedgehog signalling activity across the vertebrate phylum and validate the use of the zebrafish as a representative model for the in vivo analysis of vertebrate Hedgehog signalling.

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.

Zebrafish ift57, ift88, and ift172 intraflagellar transport mutants disrupt cilia but do not affect hedgehog signaling

Cilia formation requires intraflagellar transport (IFT) proteins. Recent studies indicate that mammalian Hedgehog (Hh) signaling requires cilia. It is unclear, however, if the requirement for cilia and IFT proteins in Hh signaling represents a general rule for all vertebrates. Here we examine zebrafish ift57, ift88, and ift172 mutants and morphants for defects in Hh signaling. Although ift57 and ift88 mutants and morphants contained residual maternal protein, the cilia were disrupted. In contrast to previous genetic studies in mouse, mutations in zebrafish IFT genes did not affect the expression of Hh target genes in the neural tube and forebrain and had no quantitative effect on Hh target gene expression. Zebrafish IFT mutants also exhibited no dramatic changes in the craniofacial skeleton, somite formation, or motor neuron patterning. Thus, our data indicate the requirement for cilia in the Hh signal transduction pathway may not represent a universal mechanism in vertebrates.

Znrg, a novel gene expressed mainly in the developing notochord of zebrafish

The notochord, a defining characteristic of the chordate embryo is a critical midline structure required for axial skeletal formation in vertebrates, and acts as a signaling center throughout embryonic development. We utilized the digital differential display program of the National Center for Biotechnology Information, and identified a contig of expressed sequence tags (no. Dr. 83747) from the zebrafish ovary library in Genbank. Full-length cDNA of the identified gene was cloned by 5'- and 3'- RACE, and the resulting sequence was confirmed by polymerase chain reaction and sequencing. The cDNA clone contains 2,505 base pairs and encodes a novel protein of 707 amino acids that shares no significant homology with any known proteins. This gene was expressed in mature oocytes and at the one-cell stage, and persisted until the 5th day of development, as determined by RT-PCR. Transcripts were detected by whole-mount RNA in situ hybridization from the two-cell stage to 72 h of embryonic development. This gene was uniformly distributed from the cleavage stage up to the blastula stage. During early gastrulation, it was present in the dorsal region, and became restricted to the notochord and pectoral fin at 48 and 72 h of embryonic development. Based on its abundance in the notochord, we hypothesized that the novel gene may play an important role in notochord development in zebrafish; we named this gene, zebrafish notochord-related gene, or znrg.

Deciphering the role of Shh signaling in axial defects produced by ethanol exposure

BACKGROUND

The phenotype of embryos exposed to ethanol is complex and likely due to multiple alterations in developmental pathways. We have previously demonstrated that Sonic hedgehog signaling (Shh-s) was reduced in both chicken and zebrafish embryos when exposed to ethanol.

METHODS

There are many tissues affected by embryonic ethanol exposure, and in this article we explore the development of axial tissues, using zebrafish embryos. We then compare these effects to the phenotypes produced by exposure to two drugs that also inhibit Shh-s: cyclopamine and forskolin.

RESULTS

We found alterations in the development of the notochord and somites produced by all three compounds, although only ethanol produced developmental delay of epiboly. Upon observation of early developing embryos, muscle pioneer cells were completely lost in cyclopamine-treated embryos, and reduced, but less so, in embryos treated with forskolin and ethanol. Ethanol treatment produced a dose-dependent reduction in total body length that may be linked to epiboly delay seen earlier during development. Despite the differences between cyclopamine and forskolin, we found that shh mRNA injection rescued the short body length, the alteration in somite shape, and the cyclopia produced by ethanol exposure.

CONCLUSIONS

Taken together, each teratogen produced a unique set of phenotypic changes in the body axis, suggesting that each compound affects Shh-s and also produces a distinctive set of molecular alterations. However, addition of exogenous Shh to ethanol treated zebrafish prevented many of the gross physical phenotypes, suggesting that the suppression of Shh-s is one of the major effects of ethanol exposure.

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.

Zic-associated holoprosencephaly: zebrafish Zic1 controls midline formation and forebrain patterning by regulating Nodal, Hedgehog, and retinoic acid signaling

Holoprosencephaly (HPE) is the most frequently observed human embryonic forebrain defect. Recent evidence indicates that the two major forms of HPE, classic HPE and midline interhemispheric (MIH) HPE, are elicited by two different mechanisms. The only gene known to be associated with both forms of HPE is Zic2. We used the zebrafish Danio rerio as a model system to study Zic knockdown during midline formation by looking at the close homolog Zic1, which is expressed in an overlapping fashion with Zic2. Zic1 knockdown in zebrafish leads to a strong midline defect including partial cyclopia due to attenuated Nodal and Hedgehog signaling in the anterior ventral diencephalon. Strikingly, we were able to show that Zic1 is also required for maintaining early forebrain expression of the retinoic acid (RA)-degrading enzyme cyp26a1. Zic1 LOF leads to increased RA levels in the forebrain, subsequent ventralization of the optic vesicle and down-regulation of genes involved in dorsal BMP signaling. Repression of BMP signaling in dorsal forebrain has been implicated in causing MIH HPE. This work provides a mechanistical explanation at the molecular level of why Zic factors are associated with both major forms of HPE.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish

BACKGROUND

It has been shown that mutations in at least four myotubularin family genes (MTM1, MTMR1, 2 and 13) are causative for human neuromuscular disorders. However, the pathway and regulative mechanism remain unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we reported a new role for Mtmr8 in neuromuscular development of zebrafish. Firstly, we cloned and characterized zebrafish Mtmr8, and revealed the expression pattern predominantly in the eye field and somites during early somitogenesis. Using morpholino knockdown, then, we observed that loss-of-function of Mtmr8 led to defects in somitogenesis. Subsequently, the possible underlying mechanism and signal pathway were examined. We first checked the Akt phosphorylation, and observed an increase of Akt phosphorylation in the morphant embryos. Furthermore, we studied the PH/G domain function within Mtmr8. Although the PH/G domain deletion by itself did not result in embryonic defect, addition of PI3K inhibitor LY294002 did give a defective phenotype in the PH/G deletion morphants, indicating that the PH/G domain was essential for Mtmr8's function. Moreover, we investigated the cooperation of Mtmr8 with PI3K in actin filament modeling and muscle development, and found that both Mtmr8-MO1 and Mtmr8-MO2+LY294002 led to the disorganization of the actin cytoskeleton. In addition, we revealed a possible participation of Mtmr8 in the Hedgehog pathway, and cell transplantation experiments showed that Mtmr8 worked in a non-cell autonomous manner in actin modeling.

CONCLUSION/SIGNIFICANCE

The above data indicate that a conserved functional cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish, and reveal a possible participation of Mtmr8 in the Hedgehog pathway. Therefore, this work provides a new clue to study the physiological function of MTM family members.

Small molecule modifier screen for kit-dependent functions in zebrafish embryonic melanocytes

Zebrafish is gaining popularity as a vertebrate model for screening small molecules that affect specific phenotypes or genetic pathways. In this study, we present a targeted drug screen to identify drug modifiers of the melanocyte migration defect of a temperature-sensitive allele of the Kit receptor tyrosine kinase, kit(ts). We first test two candidate drugs, the phosphatidylinositol-3-kinase kinase inhibitor (LY294002) and the Erk/MAP kinase inhibitor (PD98059), for their effect on melanocyte migration and survival. We find that LY294002 enhances the migration defect of kit(ts), implicating the phosphatidylinositol-3-kinase kinase pathway in promoting kit-dependent melanocyte migration, but not survival. We then used the kit(ts)-sensitized genetic background to screen a panel of 1280 pharmacologically active drugs to identify drug enhancers and suppressors of the kit(ts) melanocyte migration defect. We identified three drug enhancers of migration, two of which, Papaverine and Isoliquiritigenin, specifically enhance the kit(ts) migration defect, while 8-DPAT affected both melanocyte migration and survival. These drugs now provide additional experimental tools for investigating the mechanisms of kit-promoted melanocyte migration and survival in the zebrafish embryo.