Right-elevated expression of charon is regulated by fluid flow in medaka Kupffer's vesicle

'Three signals - three body axes' as patterning principle in bilaterians

In vertebrates, the three orthogonal body axes, anteroposterior (AP), dorsoventral (DV) and left-right (LR) are determined at gastrula and neurula stages by the Spemann-Mangold organizer and its equivalents. A common feature of AP and DV axis formation is that an evolutionary conserved interplay between growth factors (Wnt, BMP) and their extracellular antagonists (e.g. Dkk1, Chordin) creates signaling gradients for axial patterning. Recent work showed that LR patterning in Xenopus follows the same principle, with R-spondin 2 (Rspo2) as an extracellular FGF antagonist, which creates a signaling gradient that determines the LR vector. That a triad of anti-FGF, anti-BMP, and anti-Wnt governs LR, DV, and AP axis formation reveals a unifying principle in animal development. We discuss how cross-talk between these three signals confers integrated AP-DV-LR body axis patterning underlying developmental robustness, size scaling, and harmonious regulation. We propose that Urbilateria featured three orthogonal body axes that were governed by a Cartesian coordinate system of orthogonal Wnt/AP, BMP/DV, and FGF/LR signaling gradients.

R-Spondin 2 governs Xenopus left-right body axis formation by establishing an FGF signaling gradient

Establishment of the left-right (LR, sinistral, dextral) body axis in many vertebrate embryos relies on cilia-driven leftward fluid flow within an LR organizer (LRO). A cardinal question is how leftward flow triggers symmetry breakage. The chemosensation model posits that ciliary flow enriches a signaling molecule on the left side of the LRO that promotes sinistral cell fate. However, the nature of this sinistralizing signal has remained elusive. In the Xenopus LRO, we identified the stem cell growth factor R-Spondin 2 (Rspo2) as a symmetrically expressed, sinistralizing signal. As predicted for a flow-mediated signal, Rspo2 operates downstream of leftward flow but upstream of the asymmetrically expressed gene dand5. Unexpectedly, in LR patterning, Rspo2 acts as an FGF receptor antagonist: Rspo2 via its TSP1 domain binds Fgfr4 and promotes its membrane clearance by Znrf3-mediated endocytosis. Concordantly, we find that at flow-stage, FGF signaling is dextralizing and forms a gradient across the LRO, high on the dextral- and low on the sinistral side. Rspo2 gain- and loss-of function equalize this FGF signaling gradient and sinistralize and dextralize development, respectively. We propose that leftward flow of Rspo2 produces an FGF signaling gradient that governs LR-symmetry breakage.

Mechanically sensitive HSF1 is a key regulator of left-right symmetry breaking in zebrafish embryos

The left-right symmetry breaking of vertebrate embryos requires nodal flow. However, the molecular mechanisms that mediate the asymmetric gene expression regulation under nodal flow remain elusive. Here, we report that heat shock factor 1 (HSF1) is asymmetrically activated in the Kupffer's vesicle of zebrafish embryos in the presence of nodal flow. Deficiency in HSF1 expression caused a significant situs inversus and disrupted gene expression asymmetry of nodal signaling proteins in zebrafish embryos. Further studies demonstrated that HSF1 is a mechanosensitive protein. The mechanical sensation ability of HSF1 is conserved in a variety of mechanical stimuli in different cell types. Moreover, cilia and Ca2+-Akt signaling axis are essential for the activation of HSF1 under mechanical stress in vitro and in vivo. Considering the conserved expression of HSF1 in organisms, these findings unveil a fundamental mechanism of gene expression regulation by mechanical clues during embryonic development and other physiological and pathological transformations.

Exome-wide assessment of isolated biliary atresia: A report from the National Birth Defects Prevention Study using child-parent trios and a case-control design to identify novel rare variants

The etiology of biliary atresia (BA) is unknown, but recent studies suggest a role for rare protein-altering variants (PAVs). Exome sequencing data from the National Birth Defects Prevention Study on 54 child-parent trios, one child-mother duo, and 1513 parents of children with other birth defects were analyzed. Most (91%) cases were isolated BA. We performed (1) a trio-based analysis to identify rare de novo, homozygous, and compound heterozygous PAVs and (2) a case-control analysis using a sequence kernel-based association test to identify genes enriched with rare PAVs. While we replicated previous findings on PKD1L1, our results do not suggest that recurrent de novo PAVs play important roles in BA susceptibility. In fact, our finding in NOTCH2, a disease gene associated with Alagille syndrome, highlights the difficulty in BA diagnosis. Notably, IFRD2 has been implicated in other gastrointestinal conditions and warrants additional study. Overall, our findings strengthen the hypothesis that the etiology of BA is complex.

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

Origin, form and function of extraembryonic structures in teleost fishes

Teleost eggs have evolved a highly derived early developmental pattern within vertebrates as a result of the meroblastic cleavage pattern, giving rise to a polar stratified architecture containing a large acellular yolk and a small cellular blastoderm on top. Besides the acellular yolk, the teleost-specific yolk syncytial layer (YSL) and the superficial epithelial enveloping layer are recognized as extraembryonic structures that play critical roles throughout embryonic development. They provide enriched microenvironments in which molecular feedback loops, cellular interactions and mechanical signals emerge to sculpt, among other things, embryonic patterning along the dorsoventral and left-right axes, mesendodermal specification and the execution of morphogenetic movements in the early embryo and during organogenesis. An emerging concept points to a critical role of extraembryonic structures in reinforcing early genetic and morphogenetic programmes in reciprocal coordination with the embryonic blastoderm, providing the necessary boundary conditions for development to proceed. In addition, the role of the enveloping cell layer in providing mechanical, osmotic and immunological protection during early stages of development, and the autonomous nutritional support provided by the yolk and YSL, have probably been key aspects that have enabled the massive radiation of teleosts to colonize every ecological niche on the Earth. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Maternal transfer and biodistribution of citrate and luminogens coated silver nanoparticles in medaka fish

Given the wide applications of silver nanoparticles (AgNPs), it is necessary to evaluate their potentially adverse long-term effects. In this study, we performed a 100-day exposure of medaka fish to citrate and luminogens coated AgNPs and investigated the maternal transfer potentials and biodistribution of AgNPs. Following long-term AgNPs exposure, AgNPs were mainly distributed in the liver, followed by gills, intestine, and brain, but were also detected in the ovary and strongly colocalized with the dissolved Ag⁺. The quantified transfer efficiency of different Ag species was 1.56-5.07%. Long-term exposure of medaka to small size of AgNPs (20 nm) reduced the hatching rate attributable to the accumulation of AgNPs and their dissolved Ag⁺. The maternally transferred AgNPs were mainly concentrated in the Kupffer's vesicle of embryos, while their dissolved Ag⁺ was almost homogeneously distributed in the embryos. In contrast, the newly accumulated AgNPs were mainly absorbed at the chorion of embryos. During initial larval development, the maternally transferred AgNPs and their dissolved Ag⁺ were consistently concentrated in intestine. Significant dissolution of maternally transferred AgNPs occurred during larval development. Our results showed that long-term exposure to AgNPs caused distinct biodistribution in the next generation of medaka, and may have implications for assessing their potential adverse effects.

dmrt2 and myf5 Link Early Somitogenesis to Left-Right Axis Determination in Xenopus laevis

The vertebrate left-right axis is specified during neurulation by events occurring in a transient ciliated epithelium termed left-right organizer (LRO), which is made up of two distinct cell types. In the axial midline, central LRO (cLRO) cells project motile monocilia and generate a leftward fluid flow, which represents the mechanism of symmetry breakage. This directional fluid flow is perceived by laterally positioned sensory LRO (sLRO) cells, which harbor non-motile cilia. In sLRO cells on the left side, flow-induced signaling triggers post-transcriptional repression of the multi-pathway antagonist dand5. Subsequently, the co-expressed Tgf-β growth factor Nodal1 is released from Dand5-mediated repression to induce left-sided gene expression. Interestingly, Xenopus sLRO cells have somitic fate, suggesting a connection between LR determination and somitogenesis. Here, we show that doublesex and mab3-related transcription factor 2 (Dmrt2), known to be involved in vertebrate somitogenesis, is required for LRO ciliogenesis and sLRO specification. In dmrt2 morphants, misexpression of the myogenic transcription factors tbx6 and myf5 at early gastrula stages preceded the misspecification of sLRO cells at neurula stages. myf5 morphant tadpoles also showed LR defects due to a failure of sLRO development. The gain of myf5 function reintroduced sLRO cells in dmrt2 morphants, demonstrating that paraxial patterning and somitogenesis are functionally linked to LR axis formation in Xenopus.

Reemployment of Kupffer's vesicle cells into axial and paraxial mesoderm via transdifferentiation

Kupffer's vesicle (KV) in the teleost embryo is a fluid-filled vesicle surrounded by a layer of epithelial cells with rotating primary cilia. KV transiently acts as the left-right organizer and degenerates after the establishment of left-right asymmetric gene expression. Previous labelling experiments in zebrafish embryos indicated that descendants of KV-epithelial cells are incorporated into mesodermal tissues after the collapse of KV. However, the overall picture of their differentiation potency had been unclear due to the lack of suitable genetic tools and molecular analyses. In the present study, we established a novel zebrafish transgenic line with a promoter of dand5, in which all KV-epithelial cells and their descendants are specifically labelled until the larval stage. We found that KV-epithelial cells undergo epithelial-mesenchymal transition upon KV collapse and infiltrate into adjacent mesodermal progenitors, the presomitic mesoderm and chordoneural hinge. Once incorporated, the descendants of KV-epithelial cells expressed distinct mesodermal differentiation markers and contributed to the mature populations such as the axial muscles and notochordal sheath through normal developmental process. These results indicate that differentiated KV-epithelial cells possess unique plasticity in that they are reemployed into mesodermal lineages through transdifferentiation after they complete their initial role in KV.

Bicc1 and Dicer regulate left-right patterning through post-transcriptional control of the Nodal inhibitor Dand5

Rotating cilia at the vertebrate left-right organizer (LRO) generate an asymmetric leftward flow, which is sensed by cells at the left LRO margin. Ciliary activity of the calcium channel Pkd2 is crucial for flow sensing. How this flow signal is further processed and relayed to the laterality-determining Nodal cascade in the left lateral plate mesoderm (LPM) is largely unknown. We previously showed that flow down-regulates mRNA expression of the Nodal inhibitor Dand5 in left sensory cells. De-repression of the co-expressed Nodal, complexed with the TGFß growth factor Gdf3, drives LPM Nodal cascade induction. Here, we show that post-transcriptional repression of dand5 is a central process in symmetry breaking of Xenopus, zebrafish and mouse. The RNA binding protein Bicc1 was identified as a post-transcriptional regulator of dand5 and gdf3 via their 3'-UTRs. Two distinct Bicc1 functions on dand5 mRNA were observed at pre- and post-flow stages, affecting mRNA stability or flow induced translational inhibition, respectively. To repress dand5, Bicc1 co-operates with Dicer1, placing both proteins in the process of flow sensing. Intriguingly, Bicc1 mediated translational repression of a dand5 3'-UTR mRNA reporter was responsive to pkd2, suggesting that a flow induced Pkd2 signal triggers Bicc1 mediated dand5 inhibition during symmetry breakage.

Pkd2 Affects Cilia Length and Impacts LR Flow Dynamics and Dand5

The left-right (LR) field recognizes the importance of the mechanism involving the calcium permeable channel Polycystin-2. However, whether the early LR symmetry breaking mechanism is exclusively via Polycystin-2 has not been tested. For that purpose, we need to be able to isolate the effects of decreasing the levels of Pkd2 protein from any eventual effects on flow dynamics. Here we demonstrate that curly-up (cup) homozygous mutants have abnormal flow dynamics. In addition, we performed one cell stage Pkd2 knockdowns and LR organizer specific Pkd2 knockdowns and observed that both techniques resulted in shorter cilia length and abnormal flow dynamics. We conclude that Pkd2 reduction leads to LR defects that cannot be assigned exclusively to its putative role in mediating mechanosensation because indirectly, by modifying cell shape or decreasing cilia length, Pkd2 deficit affects LR flow dynamics.

On being the right shape: Roles for motile cilia and cerebrospinal fluid flow in body and spine morphology

How developing and growing organisms attain their proper shape is a central problem of developmental biology. In this review, we investigate this question with respect to how the body axis and spine form in their characteristic linear head-to-tail fashion in vertebrates. Recent work in the zebrafish has implicated motile cilia and cerebrospinal fluid flow in axial morphogenesis and spinal straightness. We begin by introducing motile cilia, the fluid flows they generate and their roles in zebrafish development and growth. We then describe how cilia control body and spine shape through sensory cells in the spinal canal, a thread-like extracellular structure called the Reissner fiber, and expression of neuropeptide signals. Last, we discuss zebrafish mutants in which spinal straightness breaks down and three-dimensional curves form. These curves resemble the common but little-understood human disease Idiopathic Scoliosis. Zebrafish research is therefore poised to make progress in our understanding of this condition and, more generally, how body and spine shape is acquired and maintained through development and growth.

Epb41l5 interacts with Iqcb1 and regulates ciliary function in zebrafish embryos

Erythrocyte protein band 4.1 like 5 (EPB41L5) is an adaptor protein beneath the plasma membrane that functions to control epithelial morphogenesis. Here we report a previously uncharacterized role of EPB41L5 in controlling ciliary function. We found that EPB41L5 forms a complex with IQCB1 (previously known as NPHP5), a ciliopathy protein. Overexpression of EPB41L5 reduced IQCB1 localization at the ciliary base in cultured mammalian epithelial cells. Conversely, epb41l5 knockdown increased IQCB1 localization at the ciliary base. epb41l5-deficient zebrafish embryos or embryos expressing C-terminally modified forms of Epb41l5 developed cilia with reduced motility and exhibited left-right patterning defects, an outcome of abnormal ciliary function. We observed genetic synergy between epb41l5 and iqcb1. Moreover, EPB41L5 decreased IQCB1 interaction with CEP290, another ciliopathy protein and a component of the ciliary base and centrosome. Together, these observations suggest that EPB41L5 regulates the composition of the ciliary base and centrosome through IQCB1 and CEP290.

The maternal coordinate system: Molecular-genetics of embryonic axis formation and patterning in the zebrafish

Axis specification of the zebrafish embryo begins during oogenesis and relies on proper formation of well-defined cytoplasmic domains within the oocyte. Upon fertilization, maternally-regulated cytoplasmic flow and repositioning of dorsal determinants establish the coordinate system that will build the structure and developmental body plan of the embryo. Failure of specific genes that regulate the embryonic coordinate system leads to catastrophic loss of body structures. Here, we review the genetic principles of axis formation and discuss how maternal factors orchestrate axis patterning during zebrafish early embryogenesis. We focus on the molecular identity and functional contribution of genes controlling critical aspects of oogenesis, egg activation, blastula, and gastrula stages. We examine how polarized cytoplasmic domains form in the oocyte, which set off downstream events such as animal-vegetal polarity and germ line development. After gametes interact and form the zygote, cytoplasmic segregation drives the animal-directed reorganization of maternal determinants through calcium- and cell cycle-dependent signals. We also summarize how maternal genes control dorsoventral, anterior-posterior, mesendodermal, and left-right cell fate specification and how signaling pathways pattern these axes and tissues during early development to instruct the three-dimensional body plan. Advances in reverse genetics and phenotyping approaches in the zebrafish model are revealing positional patterning signatures at the single-cell level, thus enhancing our understanding of genotype-phenotype interactions in axis formation. Our emphasis is on the genetic interrogation of novel and specific maternal regulatory mechanisms of axis specification in the zebrafish.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.

Left-right asymmetric heart jogging increases the robustness of dextral heart looping in zebrafish

Building a left-right (L-R) asymmetric organ requires asymmetric information. This comes from various sources, including asymmetries in embryo-scale genetic cascades (including the left-sided Nodal cascade), organ-intrinsic mechanical forces, and cell-level chirality, but the relative influence of these sources and how they collaborate to drive asymmetric morphogenesis is not understood. During zebrafish heart development, the linear heart tube extends to the left of the midline in a process known as jogging. The jogged heart then undergoes dextral (i.e. rightward) looping to correctly position the heart chambers relative to one another. Left lateralized jogging is governed by the left-sided expression of Nodal in mesoderm tissue, while looping laterality is mainly controlled by heart-intrinsic cell-level asymmetries in the actomyosin cytoskeleton. The purpose of lateralized jogging is not known. Moreover, after jogging, the heart tube returns to an almost midline position and so it is not clear whether or how jogging may impact the dextral loop. Here, we characterize a novel loss-of-function mutant in the zebrafish Nodal homolog southpaw (spaw) that appears to be a true null. We then assess the relationship between jogging and looping laterality in embryos lacking asymmetric Spaw signals. We found that the probability of a dextral loop occurring, does not depend on asymmetric Spaw signals per se, but does depend on the laterality of jogging. Thus, we conclude that the role of leftward jogging is to spatially position the heart tube in a manner that promotes robust dextral looping. When jogging laterality is abnormal, the robustness of dextral looping decreases. This establishes a cooperation between embryo-scale Nodal-dependent L-R asymmetries and organ-intrinsic cellular chirality in the control of asymmetric heart morphogenesis and shows that the transient laterality of the early heart tube has consequences for later heart morphogenetic events.

New insights into ion channel-dependent signalling during left-right patterning

The left-right organizer (LRO) in the mouse consists of pit cells within the depression, located at the end of the developing notochord, also known as the embryonic node and crown cells lining the outer periphery of the node. Cilia on pit cells are posteriorly tilted, rotate clockwise and generate leftward fluid flow. Primary cilia on crown cells are required to interpret the directionality of fluid movement and initiate flow-dependent gene transcription. Crown cells express PC1-L1 and PC2, which may form a heteromeric polycystin channel complex on primary cilia. It is still only poorly understood how fluid flow activates the ciliary polycystin complex. Besides polycystin channels voltage gated channels like HCN4 and KCNQ1 have been implicated in establishing asymmetry. How this electrical network of ion channels initiates left-sided signalling cascades and differential gene expression is currently only poorly defined.

Genetic factors contributing to human primary ciliary dyskinesia and male infertility

Primary ciliary dyskinesia (PCD) is an autosomal-recessive disorder resulting from the loss of normal ciliary function. Symptoms include neonatal respiratory distress, chronic sinusitis, bronchiectasis, situs inversus, and infertility. However, only 15 PCD-associated genes have been identified to cause male infertility to date. Owing to the genetic heterogeneity of PCD, comprehensive molecular genetic testing is not considered the standard of care. Here, we provide an update of the progress on the identification of genetic factors related to PCD associated with male infertility, summarizing the underlying molecular mechanisms, and discuss the clinical implications of these findings. Further research in this field will impact the diagnostic strategy for male infertility, enabling clinicians to provide patients with informed genetic counseling, and help to adopt the best course of treatment for developing directly targeted personalized medicine.

The Role of Cerl2 in the Establishment of Left-Right Asymmetries during Axis Formation and Heart Development

The formation of the asymmetric left-right (LR) body axis is one of the fundamental aspects of vertebrate embryonic development, and one still raising passionate discussions among scientists. Although the conserved role of nodal is unquestionable in this process, several of the details around this signaling cascade are still unanswered. To further understand this mechanism, we have been studying Cerberus-like 2 (Cerl2), an inhibitor of Nodal, and its role in the generation of asymmetries in the early vertebrate embryo. The absence of Cerl2 results in a wide spectrum of malformations commonly known as heterotaxia, which comprises defects in either global organ position (e.g., situs inversus totalis), reversed orientation of at least one organ (e.g., situs ambiguus), and mirror images of usually asymmetric paired organs (e.g., left or right isomerisms of the lungs). Moreover, these laterality defects are frequently associated with congenital heart diseases (e.g., transposition of the great arteries, or atrioventricular septal defects). Here, reviewing the knowledge on the establishment of LR asymmetry in mouse embryos, the emerging conclusion is that as necessary as is the activation of the Nodal signaling cascade, the tight control that Cerl2-mediates on Nodal signaling is equally important, and that generates a further regionalized LR genetic program in the proper time and space.

Antagonistic interactions in the zebrafish midline prior to the emergence of asymmetric gene expression are important for left-right patterning

Left-right (L-R) asymmetry of the internal organs of vertebrates is presaged by domains of asymmetric gene expression in the lateral plate mesoderm (LPM) during somitogenesis. Ciliated L-R coordinators (LRCs) are critical for biasing the initiation of asymmetrically expressed genes, such as nodal and pitx2, to the left LPM. Other midline structures, including the notochord and floorplate, are then required to maintain these asymmetries. Here we report an unexpected role for the zebrafish EGF-CFC gene one-eyed pinhead (oep) in the midline to promote pitx2 expression in the LPM. Late zygotic oep (LZoep) mutants have strongly reduced or absent pitx2 expression in the LPM, but this expression can be rescued to strong levels by restoring oep in midline structures only. Furthermore, removing midline structures from LZoep embryos can rescue pitx2 expression in the LPM, suggesting the midline is a source of an LPM pitx2 repressor that is itself inhibited by oep Reducing lefty1 activity in LZoep embryos mimics removal of the midline, implicating lefty1 in the midline-derived repression. Together, this suggests a model where Oep in the midline functions to overcome a midline-derived repressor, involving lefty1, to allow for the expression of left side-specific genes in the LPM.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Gdf3 is required for robust Nodal signaling during germ layer formation and left-right patterning

Vertebrate embryonic patterning depends on signaling from Nodal, a TGFβ superfamily member. There are three Nodal orthologs in zebrafish; southpaw directs left-right asymmetries, while squint and cyclops function earlier to pattern mesendoderm. TGFβ member Vg1 is implicated in mesoderm formation but the role of the zebrafish ortholog, Growth differentiation factor 3 (Gdf3), has not been fully explored. We show that zygotic expression of gdf3 is dispensable for embryonic development, while maternally deposited gdf3 is required for mesendoderm formation and dorsal-ventral patterning. We further show that Gdf3 can affect left-right patterning at multiple stages, including proper development of regional cell morphology in Kupffer’s vesicle and the establishment of southpaw expression in the lateral plate mesoderm. Collectively, our data indicate that gdf3 is critical for robust Nodal signaling at multiple stages in zebrafish embryonic development.

Hedgehog participates in the establishment of left-right asymmetry during amphioxus development by controlling Cerberus expression

Correct patterning of left-right (LR) asymmetry is essential during the embryonic development of bilaterians. Hedgehog (Hh) signaling is known to play a role in LR asymmetry development of mouse, chicken and sea urchin embryos by regulating Nodal expression. In this study, we report a novel regulatory mechanism for Hh in LR asymmetry development of amphioxus embryos. Our results revealed that Hh^-/- embryos abolish Cerberus (Cer) transcription, with bilaterally symmetric expression of Nodal, Lefty and Pitx In consequence, Hh^-/- mutants duplicated left-side structures and lost right-side characters, displaying an abnormal bilaterally symmetric body plan. These LR defects in morphology and gene expression could be rescued by Hh mRNA injection. Our results indicate that Hh participates in amphioxus LR patterning by controlling Cer gene expression. Curiously, however, upregulation of Hh signaling failed to alter the Cer expression pattern or LR morphology in amphioxus embryos, indicating that Hh might not provide an asymmetric cue for Cer expression. In addition, Hh is required for mouth opening in amphioxus, hinting at a homologous relationship between amphioxus and vertebrate mouth development.

Loss of zinc finger MYND-type containing 10 (zmynd10) affects cilia integrity and axonemal localization of dynein arms, resulting in ciliary dysmotility, polycystic kidney and scoliosis in medaka (Oryzias latipes)

Cilia and flagella are hair-like organelles that project from the cell surface and play important roles in motility and sensory perception. Motility defects in cilia and flagella lead to primary ciliary dyskinesia (PCD), a rare human disease. Recently zinc finger MYND-type containing 10 (ZMYND10) was identified in humans as a PCD-associated gene. In this study, we use medaka fish as a model to characterize the precise functions of zmynd10. In medaka, zmynd10 is exclusively expressed in cells with motile cilia. Embryos with zmynd10 Morpholino knockdown exhibited a left-right (LR) defect associated with loss of motility in Kupffer's vesicle (KV) cilia. This immotility was caused by loss of the outer dynein arms, which is a characteristic ultrastructural phenotype in PCD. In addition, KV cilia in zmynd10 knockdown embryos had a swollen and wavy morphology. Together, these results suggest that zmynd10 is a multi-functional protein that has independent roles in axonemal localization of dynein arms and in formation and/or maintenance of cilia. The C-terminal region of zmynd10 has a MYND-type zinc finger domain (zf-MYND) that is important for its function. Our rescue experiment showed that the zmynd10-ΔC truncated protein, which lacks zf-MYND, was still partially functional, suggesting that zmynd10 has another functional domain besides zf-MYND. To analyze the later stages of development, we generated a zmynd10 knockout mutant using transcription activator-like effector nuclease (TALEN) technology. Adult mutants exhibited sperm dysmotility, scoliosis and progressive polycystic kidney.

Left-Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

Vertebrates exhibit striking left-right (L-R) asymmetries in the structure and position of the internal organs. Symmetry is broken by motile cilia-generated asymmetric fluid flow, resulting in a signaling cascade - the Nodal-Pitx2 pathway - being robustly established within mesodermal tissue on the left side only. This pathway impinges upon various organ primordia to instruct their side-specific development. Recently, progress has been made in understanding both the breaking of embryonic L-R symmetry and how the Nodal-Pitx2 pathway controls lateralized cell differentiation, migration, and other aspects of cell behavior, as well as tissue-level mechanisms, that drive asymmetries in organ formation. Proper execution of asymmetric organogenesis is critical to health, making furthering our understanding of L-R development an important concern.

Cilia are required for asymmetric nodal induction in the sea urchin embryo

Background

Left-right (LR) organ asymmetries are a common feature of metazoan animals. In many cases, laterality is established by a conserved asymmetric Nodal signaling cascade during embryogenesis. In most vertebrates, asymmetric nodal induction results from a cilia-driven leftward fluid flow at the left-right organizer (LRO), a ciliated epithelium present during gastrula/neurula stages. Conservation of LRO and flow beyond the vertebrates has not been reported yet.

Results

Here we study sea urchin embryos, which use nodal to establish larval LR asymmetry as well. Cilia were found in the archenteron of embryos undergoing gastrulation. Expression of foxj1 and dnah9 suggested that archenteron cilia were motile. Cilia were polarized to the posterior pole of cells, a prerequisite of directed flow. High-speed videography revealed rotating cilia in the archenteron slightly before asymmetric nodal induction. Removal of cilia through brief high salt treatments resulted in aberrant patterns of nodal expression. Our data demonstrate that cilia - like in vertebrates - are required for asymmetric nodal induction in sea urchin embryos.

Conclusions

Based on these results we argue that the anterior archenteron represents a bona fide LRO and propose that cilia-based symmetry breakage is a synapomorphy of the deuterostomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0128-7) contains supplementary material, which is available to authorized users.

Left-right asymmetry: cilia and calcium revisited

Leftward flow generated by motile cilia is known to underlie left-right asymmetry in vertebrate embryos. A new study now links intraciliary calcium oscillations to cilia motility and the downstream nodal signaling cascade that drives left-sided development.

Kupffer's vesicle size threshold for robust left-right patterning of the zebrafish embryo

BACKGROUND

Motile cilia in the "organ of asymmetry" create directional fluid flows that are vital for left-right (LR) asymmetric patterning of vertebrate embryos. Organ function often depends on tightly regulated organ size control, but the role of organ of asymmetry size in LR patterning has remained unknown. Observations of the organ of asymmetry in the zebrafish, called Kupffer's vesicle (KV), have suggested significant variations in KV size in wild-type embryos, raising questions about the impact of KV organ size on LR patterning.

RESULTS

To understand the relationship between organ of asymmetry size and its function, we characterized variations in KV at several developmental stages and in several different zebrafish strains. We found that the number of KV cilia and the size of the KV lumen were highly variable, whereas the length of KV cilia showed less variation. These variabilities were similar among different genetic backgrounds. By specifically modulating KV size and analyzing individual embryos, we identified a size threshold that is necessary for KV function.

CONCLUSIONS

Together these results indicate the KV organ of asymmetry size is not tightly controlled during development, but rather must only exceed a threshold to direct robust LR patterning of the zebrafish embryo.

Both ciliary and non-ciliary functions of Foxj1a confer Wnt/β-catenin signaling in zebrafish left-right patterning

The Wnt/β-catenin pathway is implicated in left-right (LR) axis determination; however, the underlying mechanism remains elusive. Prompted by our recent discovery that Wnt signaling regulates ciliogenesis in the zebrafish Kupffer's vesicle (KV) via Foxj1a, a ciliogenic transcription factor, we decided to elucidate functions of Foxj1a in Wnt-regulated LR pattern formation. We showed that targeted injection of wnt8a mRNA into a single cell at the 128-cell stage is sufficient to induce ectopic foxj1a expression and ectopic cilia. By interrogating the transcription circuit of foxj1a regulation, we found that both Lef1 and Tcf7 bind to a consensus element in the foxj1a promoter region. Depletion of Lef1 and Tcf7 inhibits foxj1a transcription in the dorsal forerunner cells, downregulates cilia length and number in KV, and randomizes LR asymmetry. Targeted overexpression of a constitutively active form of Lef1 also induced an ectopic protrusion that contains ectopic transcripts for sox17, foxj1a, and charon, and ectopic monocilia. Further genetic studies using this ectopic expression platform revealed two distinct functions of Foxj1a; mediating Wnt-governed monocilia length elongation as well as charon transcription. The novel Foxj1a-charon regulation is conserved in KV, and importantly, it is independent of the canonical role of Foxj1a in the biosynthesis of motile cilia. Together with the known function of motile cilia movement in generating asymmetric expression of charon, our data put forward a hypothesis that Foxj1a confers both ciliary and non-ciliary functions of Wnt signaling, which converge on charon to regulate LR pattern formation.

Summary: Using a targeted overexpression platform, we showed that Wnt activation induces ectopic foxj1a expression and ectopic cilia formation, and revealed two distinct roles of Foxj1a in conferring Wnt-governed left-right patterning.

The medaka dhc2 mutant reveals conserved and distinct mechanisms of Hedgehog signaling in teleosts

Background

Primary cilia are essential for Hedgehog (Hh) signal transduction in vertebrates. Although the core components of the Hh pathway are highly conserved, the dependency on cilia in Hh signaling is considered to be lower in fish than in mice, suggesting the presence of species-specific mechanisms for Hh signal transduction.

Results

To precisely understand the role of cilia in Hh signaling in fish and explore the evolution of Hh signaling, we have generated a maternal-zygotic medaka (Oryzias latipes) mutant that lacks cytoplasmic dynein heavy chain 2 (dhc2; MZdhc2), a component required for retrograde intraflagellar transport. We found that MZdhc2 exhibited the shortened cilia and partial defects in Hh signaling, although the Hh defects were milder than zebrafish mutants which completely lack cilia. This result suggests that Hh activity in fish depends on the length of cilium. However, the activity of Hh signaling in MZdhc2 appeared to be higher than that in mouse Dnchc2 mutants, suggesting a lower requirement for cilia in Hh signaling in fish. We confirmed that Ptch1 receptor is exclusively localized on the cilium in fish as in mammals. Subsequent analyses revealed that Fused, an essential mediator for Hh signaling in Drosophila and fish but not in mammals, augments the activity of Hh signaling in fish as a transcriptional target of Hh signaling.

Conclusions

Ciliary requirement for Hh signaling in fish is lower than that in mammals, possibly due to fused-mediated positive feedback in Hh signaling. The finding of this fish-specific augmentation provides a novel insight into the evolution of Hh signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0057-x) contains supplementary material, which is available to authorized users.

Unexpected link between polyketide synthase and calcium carbonate biomineralization

INTRODUCTION

Calcium carbonate biominerals participate in diverse physiological functions. Despite intensive studies, little is known about how mineralization is initiated in organisms.

RESULTS

We analyzed the medaka spontaneous mutant, ha, defective in otolith (calcareous ear stone) formation. ha lacks a trigger for otolith mineralization, and the causative gene was found to encode polyketide synthase (pks), a multifunctional enzyme mainly found in bacteria, fungi, and plant. Subsequent experiments demonstrate that the products of medaka PKS, most likely polyketides or their derivatives, act as nucleation facilitators in otolith mineralization. The generality of this novel PKS function is supported by the essential role of echinoderm PKS in calcareous skeleton formation together with the presence of PKSs in a much wider range of animals from coral to vertebrates.

CONCLUSION

The present study first links PKS to biomineralization and provides a genetic cue for biogeochemistry of carbon and calcium cycles.

Symmetry breakage in the vertebrate embryo: when does it happen and how does it work?

Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.

Left-right organizer flow dynamics: how much cilia activity reliably yields laterality?

Internal organs are asymmetrically positioned inside the body. Embryonic motile cilia play an essential role in this process by generating a directional fluid flow inside the vertebrate left-right organizer. Detailed characterization of how fluid flow dynamics modulates laterality is lacking. We used zebrafish genetics to experimentally generate a range of flow dynamics. By following the development of each embryo, we show that fluid flow in the left-right organizer is asymmetric and provides a good predictor of organ laterality. This was tested in mosaic organizers composed of motile and immotile cilia generated by dnah7 knockdowns. In parallel, we used simulations of fluid dynamics to analyze our experimental data. These revealed that fluid flow generated by 30 or more cilia predicts 90% situs solitus, similar to experimental observations. We conclude that cilia number, dorsal anterior motile cilia clustering, and left flow are critical to situs solitus via robust asymmetric charon expression.

The evolution and conservation of left-right patterning mechanisms

Morphological asymmetry is a common feature of animal body plans, from shell coiling in snails to organ placement in humans. The signaling protein Nodal is key for determining this laterality. Many vertebrates, including humans, use cilia for breaking symmetry during embryonic development: rotating cilia produce a leftward flow of extracellular fluids that induces the asymmetric expression of Nodal. By contrast, Nodal asymmetry can be induced flow-independently in invertebrates. Here, we ask when and why flow evolved. We propose that flow was present at the base of the deuterostomes and that it is required to maintain organ asymmetry in otherwise perfectly bilaterally symmetrical vertebrates.

Salient features of the ciliated organ of asymmetry

Many internal organs develop distinct left and right sides that are essential for their functions. In several vertebrate embryos, motile cilia generate an asymmetric fluid flow that plays an important role in establishing left-right (LR) signaling cascades. These ‘LR cilia’ are found in the ventral node and posterior notochordal plate in mammals, the gastrocoel roof plate in amphibians and Kupffer’s vesicle in teleost fish. I consider these transient ciliated structures as the ‘organ of asymmetry’ that directs LR patterning of the developing embryo. Variations in size and morphology of the organ of asymmetry in different vertebrate species have raised questions regarding the fundamental features that are required for LR determination. Here, I review current models for how LR asymmetry is established in vertebrates, discuss the cellular architecture of the ciliated organ of asymmetry and then propose key features of this organ that are critical for orienting the LR body axis.

Wnt11b is involved in cilia-mediated symmetry breakage during Xenopus left-right development

Breakage of bilateral symmetry in amphibian embryos depends on the development of a ciliated epithelium at the gastrocoel roof during early neurulation. Motile cilia at the gastrocoel roof plate (GRP) give rise to leftward flow of extracellular fluids. Flow is required for asymmetric gene expression and organ morphogenesis. Wnt signaling has previously been involved in two steps, Wnt/ß-catenin mediated induction of Foxj1, a regulator of motile cilia, and Wnt/planar cell polarity (PCP) dependent cilia polarization to the posterior pole of cells. We have studied Wnt11b in the context of laterality determination, as this ligand was reported to activate canonical and non-canonical Wnt signaling. Wnt11b was found to be expressed in the so-called superficial mesoderm (SM), from which the GRP derives. Surprisingly, Foxj1 was only marginally affected in loss-of-function experiments, indicating that another ligand acts in this early step of laterality specification. Wnt11b was required, however, for polarization of GRP cilia and GRP morphogenesis, in line with the known function of Wnt/PCP in cilia-driven leftward flow. In addition Xnr1 and Coco expression in the lateral-most GRP cells, which sense flow and generate the first asymmetric signal, was attenuated in morphants, involving Wnt signaling in yet another process related to symmetry breakage in Xenopus.

Differential roles for 3-OSTs in the regulation of cilia length and motility

As cells integrate molecular signals from their environment, cell surface receptors require modified proteoglycans for the robust activation of signaling pathways. Heparan sulfate proteoglycans (HSPGs) have long unbranched chains of repetitive disaccharide units that can be sulfated at specific positions by heparan sulfate O-sulfotransferase (OST) families. Here, we show that two members of the 3-OST family are required in distinct signaling pathways to control left-right (LR) patterning through control of Kupffer's vesicle (KV) cilia length and motility. 3-OST-5 functions in the fibroblast growth factor pathway to control cilia length via the ciliogenic transcription factors FoxJ1a and Rfx2. By contrast, a second 3-OST family member, 3-OST-6, does not regulate cilia length, but regulates cilia motility via kinesin motor molecule (Kif3b) expression and cilia arm dynein assembly. Thus, two 3-OST family members cell-autonomously control LR patterning through distinct pathways that regulate KV fluid flow. We propose that individual 3-OST isozymes create distinct modified domains or 'glycocodes' on cell surface proteoglycans, which in turn regulate the response to diverse cell signaling pathways.

The dynamic right-to-left translocation of Cerl2 is involved in the regulation and termination of Nodal activity in the mouse node

The determination of left-right body asymmetry in mouse embryos depends on the interplay of molecules in a highly sensitive structure, the node. Here, we show that the localization of Cerl2 protein does not correlate to its mRNA expression pattern, from 3-somite stage onwards. Instead, Cerl2 protein displays a nodal flow-dependent dynamic behavior that controls the activity of Nodal in the node, and the transmission of the laterality information to the left lateral plate mesoderm (LPM). Our results indicate that Cerl2 initially localizes and prevents the activation of Nodal genetic circuitry on the right side of the embryo, and later its right-to-left translocation shutdowns Nodal activity in the node. The consequent prolonged Nodal activity in the node by the absence of Cerl2 affects local Nodal expression and prolongs its expression in the LPM. Simultaneous genetic removal of both Nodal node inhibitors, Cerl2 and Lefty1, sustains even longer and bilateral this LPM expression.

Left-right patterning: conserved and divergent mechanisms

The left-right (LR) asymmetry of visceral organs is fundamental to their function and position within the body. Over the past decade or so, the molecular mechanisms underlying the establishment of such LR asymmetry have been revealed in many vertebrate and invertebrate model organisms. These studies have identified a gene network that contributes to this process and is highly conserved from sea urchin to mouse. By contrast, some specific steps of the process, such as the symmetry-breaking event and situs-specific organogenesis, appear to have diverged during evolution. Here, we summarize the common and divergent mechanisms by which LR asymmetry is established in vertebrates.

Connexin26-mediated transfer of laterality cues in Xenopus

A cilia-driven leftward flow of extracellular fluid breaks bilateral symmetry in the dorsal midline of the neurula stage vertebrate embryo. The left-specific Nodal signaling cascade in the lateral plate mesoderm (LPM) is key to asymmetric morphogenesis and placement of organs during subsequent development. The nature of the initial asymmetric cue(s) as well as the transfer of information from the midline to the left side has remained elusive. Gap junctional communication has been previously involved in Xenopus left-right (LR) development, however a function at cleavage stages was inferred from inhibitor experiments. Here we show by heptanol-mediated block of connexin function that flow stages during neurulation represent the critical time window. Flow in Xenopus occurs at the gastrocoel roof plate (GRP), a ciliated sheath of cells of mesodermal fate transiently positioned within the dorsal epithelial lining of the forming archenteron. We reasoned that endodermal cells immediately adjacent to the GRP are important for transfer of asymmetry. A systematic screen identified two connexin genes, Cx26 and Cx32, which were co-expressed in these lateral endodermal cells. Gain- and loss-of-function experiments pinpointed Cx26 as the critical connexin for LR development, while Cx32 had no effect on laterality. Importantly, GRP morphology, ciliation and flow were not affected in Cx26 morphants. Our results demonstrate a decisive role of Cx26 in the transfer of laterality cues from the GRP to the left LPM, providing a novel access to the identification of the initial asymmetric signal generated by flow.

Linking early determinants and cilia-driven leftward flow in left-right axis specification of Xenopus laevis: a theoretical approach

In vertebrates, laterality - the asymmetric placement of the viscera including organs of the gastrointestinal system, heart and lungs - is under the genetic control of a conserved signaling pathway in the left lateral plate mesoderm (LPM). A key feature of this pathway, shared by embryos of all non-avian vertebrate classes analyzed to date (e.g. fish, amphibia and mammals) is the formation of a transitory midline epithelial structure. Remarkably, the motility of cilia projecting from this epithelium produce a leftward-directed movement of extracellular liquid. This leftward flow precedes any sign of asymmetry in gene expression. Numerous analyses have shown that this leftward flow is not only necessary, but indeed sufficient to direct laterality. Interestingly, however, cilia-independent mechanisms acting much earlier in development in the frog Xenopus have been reported during the earliest cleavage stages, a period before any major zygotic gene transcription. The relationship between these two distinct mechanisms is not understood. In this review we present the conserved and critical steps of Xenopus LR axis formation. Next, we address the basic question of how an early asymmetric activity might contribute to, feed into, or regulate the conserved cilia-dependent pathway. Finally, we discuss the possibility that Spemann's organizer is itself polarized in the left-right dimension. In attempting to reconcile the sufficiency of the cilia-dependent pathway with potential earlier-acting asymmetries, we offer a general practical experimental checklist for the Xenopus community working on the process of left-right determination. This approach indicates areas where work still needs to be done to clarify the relationship between early determinants and cilia-driven leftward flow.

The multiple roles of Notch signaling during left-right patterning

The establishment of left-right (LR) asymmetry is regulated by intricate signaling mechanisms during embryogenesis and this asymmetry is critical for morphogenesis as well as the positioning of internal organs within the organism. Recent progress including elucidation of ion transporters, leftward nodal flow, and regulation of asymmetric gene expression contributes to our understanding of how the breaking of the symmetry is initiated and how this laterality information is subsequently transmitted to the organ primordium. A number of developmental signaling pathways have been implicated in this complex process. In this review, we will focus on the roles of the Notch signaling pathway during development of LR asymmetry. The Notch signaling pathway is a short-range communication system between neighboring cells. While Notch signaling plays essential roles in regulating the morphogenesis of the node and left-specific expression of Nodal in the lateral plate mesoderm, a hallmark gene in LR patterning, Notch signaling also suppresses the expression of Pitx2 that is a direct downstream target of Nodal during later stages of development. This negative activity of Notch signaling towards left-specific activity was recently shown to be inhibited by the B cell lymphoma 6 (BCL6)/BCL6 co-repressor (BcoR) transcriptional repressor complex in a target-specific manner. The complex regulation of Notch-dependent gene expression for LR asymmetry will be highlighted in this review.

Pkd1l1 complexes with Pkd2 on motile cilia and functions to establish the left-right axis

The internal organs of vertebrates show distinctive left-right asymmetry. Leftward extracellular fluid flow at the node (nodal flow), which is generated by the rotational movement of node cilia, is essential for left-right patterning in the mouse and other vertebrates. However, the identity of the pathways by which nodal flow is interpreted remains controversial as the molecular sensors of this process are unknown. In the current study, we show that the medaka left-right mutant abecobe (abc) is defective for left-right asymmetric expression of southpaw, lefty and charon, but not for nodal flow. We identify the abc gene as pkd1l1, the expression of which is confined to Kupffer's vesicle (KV, an organ equivalent to the node). Pkd1l1 can interact and interdependently colocalize with Pkd2 at the cilia in KV. We further demonstrate that all KV cilia contain Pkd1l1 and Pkd2 and left-right dynein, and that they are motile. These results suggest that Pkd1l1 and Pkd2 form a complex that functions as the nodal flow sensor in the motile cilia of the medaka KV. We propose a new model for the role of cilia in left-right patterning in which the KV cilia have a dual function: to generate nodal flow and to interpret it through Pkd1l1-Pkd2 complexes.

The art of medaka genetics and genomics: what makes them so unique?

The medaka fish, Oryzias latipes, is an emerging vertebrate model and now has a high quality draft genome and a number of unique mutants. The long history of medaka research in Japan has provided medaka with unique features, which are complementary to other vertebrate models. A large collection of spontaneous mutants collected over a century, the presence of highly polymorphic inbred lines established over decades, and the recently completed genome sequence all give the medaka a big boost. This review focuses on the state of the art in medaka genetics and genomics, such as the first isolation of active transposons in vertebrates, the influence of chromatin structure on sequence variation, fine quantitative trait locus (QTL) analysis, and versatile mutants as human disease models.

Notch signalling regulates left-right asymmetry through ciliary length control

The importance of cilia in embryonic development and adult physiology is emphasized by human ciliopathies. Despite its relevance, molecular signalling pathways behind cilia formation are poorly understood. We show that Notch signalling is a key pathway for cilia length control. In deltaD zebrafish mutants, cilia length is reduced in Kupffer's vesicle and can be rescued by the ciliogenic factor foxj1a. Conversely, cilia length increases when Notch signalling is hyperactivated. Short cilia found in deltaD mutants reduce the fluid flow velocity inside Kupffer's vesicle, thus compromising the asymmetric expression of the flow sensor charon. Notch signalling brings together ciliary length control and fluid flow hydrodynamics with transcriptional activation of laterality genes. In addition, our deltaD mutant analysis discloses an uncoupling between gut and heart laterality.

Zebrafish Nkd1 promotes Dvl degradation and is required for left-right patterning

The establishment of the left-right (LR) axis in zebrafish embryos relies on signals from the dorsal forerunner cells (DFC) and the Kupffer's vesicle (KV). While the Wnt signaling network influences many aspects of embryonic development, its precise role in LR patterning is still unclear. One branch of the Wnt network leads to stabilization of β-catenin and activation of downstream target genes. Other Wnt ligands appear to act independently of β-catenin to modulate calcium release and influence cell polarity. Central to regulation of β-catenin and coordination of convergent extension (CE) movements is Dishevelled (Dvl). Naked Cuticle (Nkd) binds Dvl and modulates β-catenin-dependent and independent Wnt signaling. Here, we analyze the expression patterns of three zebrafish Nkd homologs and find enriched expression of nkd1 in DFCs and KV. Dvl is degraded upon Nkd1 overexpression in zebrafish. Knockdown of Nkd1 specifically in the DFC results in β-catenin nuclear localization and transcriptional activation as well as alterations to DFC migration, KV formation, ciliogenesis and LR patterning. Furthermore, we identify asymmetric expression of the Nodal antagonist charon around the KV and show that Nkd1 knockdown impacts asymmetric charon expression. Our findings show that Nkd1 acts as a β-catenin antagonist in the DFCs necessary for LR patterning.

Dynein axonemal intermediate chain 2 is required for formation of the left-right body axis and kidney in medaka

Ciliary defects lead to various diseases, such as primary ciliary dyskinesia (PCD) and polycystic kidney disease (PKD). We isolated a medaka mutant mii, which exhibits defects in the left-right (LR) polarity of organs, and found that mii encodes dynein axonemal intermediate chain 2a (dnai2a). Ortholog mutations were recently reported to cause PCD in humans. mii mutant embryos exhibited loss of nodal flow in Kupffer's Vesicle (KV), which is equivalent to the mammalian node, and abnormal expression of the left-specific gene. KV cilia in the mii mutant were defective in their outer dynein arms (ODAs), indicating that Dnai2a is required for ODA formation in KV cilia. While the mii mutant retained motility of the renal cilia and failed to show PKD, the loss of dnai2a and another dnai2 ortholog dnai2b led to PKD. These findings demonstrate that Dnai2 proteins control LR polarity and kidney formation through regulation of ciliary motility.