Kupffer's vesicle is a ciliated organ of asymmetry in the zebrafish embryo that initiates left-right development of the brain, heart and gut

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals remain poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

Ontogenesis from Embryo to Juvenile in Threadsail Filefish, Stephanolepis cirrhifer

The threadsail filefish, Stephanolepis cirrhifer, is an economically important marine species. However, wild catches have sharply decreased over the past 20 years, causing S. cirrhifer to be added to the IUCN Red List of Threatened Species. Accordingly, this study seeks to promote technological development for artificial breeding and early life-stage farming by defining the morphological characteristics of ontogenesis. The fertilized eggs, with a diameter of 0.62 ± 0.01 mm, were spherical and sticky and contained multiple oil globules of varying sizes. The embryonic development was observed and divided into eight phases, which were cleavage, blastocyst, gastrula, neurula, organogenesis, muscular contraction, heart pulsation, and hatching. At 3 days post-hatching (dph), the yolk sac was completely absorbed. The eye developed rapidly, and the mouth fissure and anus initially formed. Some larvae were fed on S-rotifers (Brachionus plicatilis). At 6-8 dph, the upper and lower jaws of larvae were gradually covered by leathery skin, and the head-to-body proportion increased. At 14-16 dph, the fin differentiation occurred in the dorsal, anal, and pectoral fins, with widespread distribution of yellow and melanin on the body surface. Swim bladder was clear. The swimming ability of larva was enhanced, resulting in an obvious clustering phenomenon. At 22-25 dph, the end of the notochord continued to tilt upwards, forming a tail fin. The trunk was evenly distributed with protruding circular punctate scales. The snout was covered with leathery epidermis, and the mouth began to round. At 40-45 dph, the juvenile completed metamorphosis, with horizontal dark stripes appearing on the trunk. Pigmented spots appeared on the tail fins. The counts of dorsal and anal fin spines were 34-36 and 32-34 dph, respectively. During the development of larvae and juveniles, the growth parameters, such as total length, standard length, body height, and body weight, were made as growth curves. The slopes of growth curves were calculated. We found two inflexion points occurring in the growth curves, which may be associated with metamorphosis and transitions in feeding habits. These results enrich the biological understanding of filefish species while providing guidance for artificial propagation and fry production in S. cirrhifer.

Bilateral cellular flows display asymmetry prior to left-right organizer formation in amniote gastrulation

A bilateral body plan is predominant throughout the animal kingdom. Bilaterality of amniote embryos becomes recognizable as midline morphogenesis begins at gastrulation, bisecting an embryonic field into the left and right sides, and left-right (LR) asymmetry patterning follows. While a series of laterality genes expressed after the LR compartmentalization has been extensively studied, the laterality patterning prior to and at the initiation of midline morphogenesis has remained unclear. Here, through a biophysical quantification in a high spatial and temporal resolution, applied to a chick model system, we show that a large-scale bilateral counterrotating cellular flow, termed "polonaise movements", display LR asymmetries in early gastrulation. This cell movement starts prior to the formation of the primitive streak (PS) (the earliest midline structure) and the subsequent appearance of Hensen's node (the LR organizer). The cellular flow speed and vorticity unravel the location and timing of the LR asymmetries. The bilateral flows displayed a Right dominance after 6 h since the start of cell movements. Mitotic arrest that diminishes PS formation resulted in changes in the bilateral flow pattern, but the Right dominance persisted. Our data indicate that the LR asymmetry in amniote gastrula becomes detectable earlier than suggested by current models, which assume that the asymmetric regulation of the laterality signals at the node leads to the LR patterning. More broadly, our results suggest that physical processes can play an unexpected but significant role in influencing LR laterality during embryonic development.

CIROZ is dispensable in ancestral vertebrates but essential for left-right patterning in humans

Four genes-DAND5, PKD1L1, MMP21, and CIROP-form a genetic module that has specifically evolved in vertebrate species that harbor motile cilia in their left-right organizer (LRO). We find here that CIROZ (previously known as C1orf127) is also specifically expressed in the LRO of mice, frogs, and fish, where it encodes a protein with a signal peptide followed by 3 zona pellucida N domains, consistent with extracellular localization. We report 16 individuals from 10 families with bi-allelic CIROZ inactivation variants, which cause heterotaxy with congenital heart defects. While the knockout of Ciroz in mice also leads to situs anomalies, we unexpectedly find that its targeted inactivation in zebrafish and Xenopus does not lead to observable LR anomalies. Moreover, CIROZ is absent or obsolete in select animals with motile cilia at their LRO, including Carnivora, Atherinomorpha fish, or jawless vertebrates. In summary, this evo-devo study identifies CIROZ as an essential gene for breaking bilateral embryonic symmetry in humans and mice, whereas we witness its contemporary pseudogenization in discrete vertebrate species.

Motile cilia modulate neuronal and astroglial activity in the zebrafish larval brain

The brain uses a specialized system to transport cerebrospinal fluid (CSF), consisting of interconnected ventricles lined by motile ciliated ependymal cells. These cells act jointly with CSF secretion and cardiac pressure gradients to regulate CSF dynamics. To date, the link between cilia-mediated CSF flow and brain function is poorly understood. Using zebrafish larvae as a model system, we identify that loss of ciliary motility does not alter progenitor proliferation, brain morphology, or spontaneous neural activity despite leading to an enlarged telencephalic ventricle. We observe altered neuronal responses to photic stimulations in the optic tectum and hindbrain and brain asymmetry defects in the habenula. Finally, we investigate astroglia since they contact CSF and regulate neuronal activity. Our analyses reveal a reduction in astroglial calcium signals during both spontaneous and light-evoked activity. Our findings highlight a role of motile cilia in regulating brain physiology through the modulation of neural and astroglial networks.

Follistatin like-1 (Fstl1) regulates adipose tissue development in zebrafish

Obesity is a highly prevalent disorder with complex aetiology. Therefore, studying its associated cellular and molecular pathways may be aided by analysing genetic tractable diseases. In this context, the study of ciliopathies such as Bardet-Biedl syndrome has highlighted the relevance of primary cilia in obesity, both in the central nervous system and peripheral tissues. Based on our previous in vitro results supporting the role of a novel Bbs4-cilia-Fstl1 axis in adipocyte differentiation, we evaluated the in vivo relevance of the zebrafish orthologous genes fstl1a and fstl1b in primary cilia and adipose tissue development. Using a combination of knockdowns and a new fstl1a mutant line, we show that fstl1a promotes primary cilia formation in early embryos and participates in adipose tissue formation in larvae. We also show that fstl1b partially compensates for the loss of fstl1a. Moreover, in high fat diet, fstl1a depletion affects the expression of differentiation and mature adipocyte markers. These results agree with our previous in vitro data and provide further support for the role of FSTL1 as a regulator of adipose tissue formation. Dissecting the exact biological role of proteins such as FSTL1 will likely contribute to understand obesity onset and presentation.

The molecular basis for functional divergence of duplicated SOX factors controlling endoderm formation and left-right patterning in zebrafish

Endoderm, one of three primary germ layers of vertebrate embryos, makes major contributions to the respiratory and gastrointestinal tracts and associated organs, including liver and pancreas. In mammals, the transcription factor SOX17 is vital for endoderm organ formation and can induce endoderm progenitor identity. Duplication of ancestral sox17 in the teleost lineage produced the paralogues sox32 and sox17 in zebrafish. Sox32 is required for specification of endoderm and progenitors of the left-right organiser (Kupffer's Vesicle, KV), with Sox17 a downstream target of Sox32 that is implicated in further KV development. Phenotypic evidence therefore suggests functional similarities between zebrafish Sox32 and Sox17 and mammalian SOX17. Here, we directly compare these orthologues and paralogues, using the early zebrafish embryo as a biological platform for functional testing. Our results indicate that, unlike Sox32, human SOX17 cannot induce endoderm specification in zebrafish. Furthermore, using hybrid protein functional analyses, we show that Sox32 specificity for the endoderm gene regulatory network is linked to evolutionary divergence in its DNA-binding HMG domain from its paralogue Sox17. Additionally, changes in the C-terminal regions of Sox32 and Sox17 underpin their differing target specificities. Finally, we establish that specific conserved peptides in the C-terminal domain are essential for the role of Sox17 in establishing correct organ asymmetry. Overall, our results illuminate the molecular basis for functional divergence of Sox32 and Sox17 in vertebrate endoderm development and left-right patterning, and the evolution of SoxF transcription factor function.

Proliferation associated 2G4 is required for the ciliation of vertebrate motile cilia

Motile cilia are critical structures that regulate early embryonic development and tissue homeostasis through synchronized ciliary motility. The formation of motile cilia is dependent on precisely controlled sequential processes including the generation, migration, and docking of centrioles/basal bodies as well as ciliary growth. Using the published proteomics data from various organisms, we identified proliferation-associated 2G4 as a novel regulator of ciliogenesis. Loss-of-function studies using Xenopus laevis as a model system reveal that Pa2G4 is essential for proper ciliogenesis and synchronized movement of cilia in multiciliated cells (MCCs) and the gastrocoel roof plate (GRP). Pa2G4 morphant MCCs exhibit defective basal body docking to the surface as a result of compromised Rac1 activity, apical actin network formation, and immature distal appendage generation. Interestingly, the regions that include the RNA-binding domain and the C-terminus of Pa2G4 are necessary for ciliogenesis in both MCCs and GRP cells. Our findings may provide insights into motile cilia-related genetic diseases such as Primary Ciliary Dyskinesia.

Specific Mitotic Events Drive Cytoskeletal Remodeling Required for Left-Right Organizer Development

Cellular proliferation is vital for tissue development, including the Left-Right Organizer (LRO), a transient organ critical for establishing the vertebrate LR body plan. This study investigates cell redistribution and the role of specific progenitor cells in LRO formation, focusing on cell lineage and behavior. Using zebrafish as a model, we mapped all mitotic events in Kupffer's Vesicle (KV), revealing an FGF-dependent, anteriorly enriched mitotic pattern. With a KV-specific fluorescent microtubule (MT) line, we observed that mitotic spindles align along the KV's longest axis until the rosette stage, spindles that form after spin, and are excluded from KV. Early aligned spindles assemble cytokinetic bridges that point MT bundles toward a tight junction where a rosette will initially form. Post-abscission, repurposed MT bundles remain targeted at the rosette center, facilitating actin recruitment. Additional cells, both cytokinetic and non-cytokinetic, are incorporated into the rosette, repurposing or assembling MT bundles before actin recruitment. These findings show that initial divisions are crucial for rosette assembly, MT patterning, and actin remodeling during KV development.

Calaxin is a key factor for calcium-dependent waveform control in zebrafish sperm

Calcium is critical for regulating the waveform of motile cilia and flagella. Calaxin is currently the only known molecule involved in the calcium-dependent regulation in ascidians. We have recently shown that Calaxin stabilizes outer arm dynein (OAD), and the knockout of Calaxin results in primary ciliary dyskinesia phenotypes in vertebrates. However, from the knockout experiments, it was not clear which functions depend on calcium and how Calaxin regulates the waveform. To address this question, here, we generated transgenic zebrafish expressing a mutant E130A-Calaxin deficient in calcium binding. E130A-Calaxin restored the OAD reduction of calaxin -/- sperm and the abnormal movement of calaxin -/- left-right organizer cilia, showing that Calaxin's stabilization of OADs is calcium-independent. In contrast, our quantitative analysis of E130A-Calaxin sperms showed that the calcium-induced asymmetric beating was not restored, linking Calaxin's calcium-binding ability with an asymmetric flagellar beating for the first time. Our data show that Calaxin is a calcium-dependent regulator of the ciliary beating and a calcium-independent OAD stabilizer.

Cdon is essential for organ left-right patterning by regulating dorsal forerunner cells clustering and Kupffer's vesicle morphogenesis

Cdon and boc are members of the cell adhesion molecule subfamily III Ig/fibronectin. Although they have been reported to be involved in muscle and neural development at late developmental stage, their early roles in embryonic development remain unknown. Here, we discovered that in zebrafish, cdon, but not boc, is expressed in dorsal forerunner cells (DFCs) and the epithelium of Kupffer's vesicle (KV), suggesting a potential role for cdon in organ left-right (LR) patterning. Further data showed that liver and heart LR patterning were disrupted in cdon morphants and cdon mutants. Mechanistically, we found that loss of cdon function led to defect in DFCs clustering, reduced KV lumen, and defective cilia, resulting in randomized Nodal/spaw signaling and subsequent organ LR patterning defects. Additionally, predominant distribution of a cdon morpholino (MO) in DFCs caused defects in DFC clustering, KV morphogenesis, cilia number/length, Nodal/spaw signaling, and organ LR asymmetry, similar to those observed in cdon morphants and cdon -/- embryos, indicating a cell-autonomous role for cdon in regulating KV formation during LR patterning. In conclusion, our data demonstrate that during gastrulation and early somitogenesis, cdon is essential for proper DFC clustering, KV formation, and normal cilia, thereby playing a critical role in establishing organ LR asymmetry.

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

Myosin1G promotes Nodal signaling to control zebrafish left-right asymmetry

Myosin1D (Myo1D) has recently emerged as a conserved regulator of animal Left-Right (LR) asymmetry that governs the morphogenesis of the vertebrate central LR Organizer (LRO). In addition to Myo1D, the zebrafish genome encodes the closely related Myo1G. Here we show that while Myo1G also controls LR asymmetry, it does so through an entirely different mechanism. Myo1G promotes the Nodal-mediated transfer of laterality information from the LRO to target tissues. At the cellular level, Myo1G is associated with endosomes positive for the TGFβ signaling adapter SARA. myo1g mutants have fewer SARA-positive Activin receptor endosomes and a reduced responsiveness to Nodal ligands that results in a delay of left-sided Nodal propagation and tissue-specific laterality defects in organs that are most distant from the LRO. Additionally, Myo1G promotes signaling by different Nodal ligands in specific biological contexts. Our findings therefore identify Myo1G as a context-dependent regulator of the Nodal signaling pathway.

Dynamical forces drive organ morphology changes during embryonic development

Organs and tissues must change shape in precise ways during embryonic development to execute their functions. Multiple mechanisms including biochemical signaling pathways and biophysical forces help drive these morphology changes, but it has been difficult to tease apart their contributions, especially from tissue-scale dynamic forces that are typically ignored. We use a combination of mathematical models and in vivo experiments to study a simple organ in the zebrafish embryo called Kupffer's vesicle. Modeling indicates that dynamic forces generated by tissue movements in the embryo produce shape changes in Kupffer's vesicle that are observed during development. Laser ablations in the zebrafish embryo that alter these forces result in altered organ shapes matching model predictions. These results demonstrate that dynamic forces sculpt organ shape during embryo development.

Natural reversal of cavefish heart asymmetry is controlled by Sonic Hedgehog effects on the left-right organizer

The direction of left-right visceral asymmetry is conserved in vertebrates. Deviations of the standard asymmetric pattern are rare, and the underlying mechanisms are not understood. Here, we use the teleost Astyanax mexicanus, consisting of surface fish with normal left-oriented heart asymmetry and cavefish with high levels of reversed right-oriented heart asymmetry, to explore natural changes in asymmetry determination. We show that Sonic Hedgehog (Shh) signaling is increased at the posterior midline, Kupffer's vesicle (the teleost left-right organizer) is enlarged and contains longer cilia, and the number of dorsal forerunner cells is increased in cavefish. Furthermore, Shh increase in surface fish embryos induces asymmetric changes resembling the cavefish phenotype. Asymmetric expression of the Nodal antagonist Dand5 is equalized or reversed in cavefish, and Shh increase in surface fish mimics changes in cavefish dand5 asymmetry. Shh decrease reduces the level of right-oriented heart asymmetry in cavefish. Thus, naturally occurring modifications in cavefish heart asymmetry are controlled by the effects of Shh signaling on left-right organizer function.

Male germ cell-associated kinase is required for axoneme formation during ciliogenesis in zebrafish photoreceptors

Vertebrate photoreceptors are highly specialized retinal neurons that have cilium-derived membrane organelles called outer segments, which function as platforms for phototransduction. Male germ cell-associated kinase (MAK) is a cilium-associated serine/threonine kinase, and its genetic mutation causes photoreceptor degeneration in mice and retinitis pigmentosa in humans. However, the role of MAK in photoreceptors is not fully understood. Here, we report that zebrafish mak mutants show rapid photoreceptor degeneration during embryonic development. In mak mutants, both cone and rod photoreceptors completely lacked outer segments and underwent apoptosis. Interestingly, zebrafish mak mutants failed to generate axonemes during photoreceptor ciliogenesis, whereas basal bodies were specified. These data suggest that Mak contributes to axoneme development in zebrafish, in contrast to mouse Mak mutants, which have elongated photoreceptor axonemes. Furthermore, the kinase activity of Mak was found to be critical in ciliary axoneme development and photoreceptor survival. Thus, Mak is required for ciliogenesis and outer segment formation in zebrafish photoreceptors to ensure intracellular protein transport and photoreceptor survival.

Zebrafish Congenital Heart Disease Models: Opportunities and Challenges

Congenital heart defects (CHDs) are common human birth defects. Genetic mutations potentially cause the exhibition of various pathological phenotypes associated with CHDs, occurring alone or as part of certain syndromes. Zebrafish, a model organism with a strong molecular conservation similar to humans, is commonly used in studies on cardiovascular diseases owing to its advantageous features, such as a similarity to human electrophysiology, transparent embryos and larvae for observation, and suitability for forward and reverse genetics technology, to create various economical and easily controlled zebrafish CHD models. In this review, we outline the pros and cons of zebrafish CHD models created by genetic mutations associated with single defects and syndromes and the underlying pathogenic mechanism of CHDs discovered in these models. The challenges of zebrafish CHD models generated through gene editing are also discussed, since the cardiac phenotypes resulting from a single-candidate pathological gene mutation in zebrafish might not mirror the corresponding human phenotypes. The comprehensive review of these zebrafish CHD models will facilitate the understanding of the pathogenic mechanisms of CHDs and offer new opportunities for their treatments and intervention strategies.

Deciphering the function of the fifth class of Gα proteins: regulation of ionic homeostasis as unifying hypothesis

Trimeric G proteins transduce signals from a superfamily of receptors and each G protein controls a wide range of cellular and systemic functions. Their highly conserved alpha subunits fall in five classes, four of which have been well investigated (Gs, Gi, G12, Gq). In contrast, the function of the fifth class, Gv is completely unknown, despite its broad occurrence and evolutionary ancient origin (older than metazoans). Here we show a dynamic presence of Gv mRNA in several organs during early development of zebrafish, including the hatching gland, the pronephros and several cartilage anlagen, employing in situ hybridisation. Next, we generated a Gv frameshift mutation in zebrafish and observed distinct phenotypes such as reduced oviposition, premature hatching and craniofacial abnormalities in bone and cartilage of larval zebrafish. These phenotypes could suggest a disturbance in ionic homeostasis as a common denominator. Indeed, we find reduced levels of calcium, magnesium and potassium in the larvae and changes in expression levels of the sodium potassium pump atp1a1a.5 and the sodium/calcium exchanger ncx1b in larvae and in the adult kidney, a major osmoregulatory organ. Additionally, expression of sodium chloride cotransporter slc12a3 and the anion exchanger slc26a4 is altered in complementary ways in adult kidney. It appears that Gv may modulate ionic homeostasis in zebrafish during development and in adults. Our results constitute the first insight into the function of the fifth class of G alpha proteins.

Should I shrink or should I grow: cell size changes in tissue morphogenesis

Cells change shape, move, divide, and die to sculpt tissues. Common to all these cell behaviours are cell size changes, which have recently emerged as key contributors to tissue morphogenesis. Cells can change their mass-the number of macromolecules they contain-or their volume-the space they encompass. Changes in cell mass and volume occur through different molecular mechanisms and at different timescales, slow for changes in mass and rapid for changes in volume. Therefore, changes in cell mass and cell volume, which are often linked, contribute to the development and shaping of tissues in different ways. Here, we review the molecular mechanisms by which cells can control and alter their size, and we discuss how changes in cell mass and volume contribute to tissue morphogenesis. The role that cell size control plays in developing embryos is only starting to be elucidated. Research on the signals that control cell size will illuminate our understanding of the cellular and molecular mechanisms that drive tissue morphogenesis.

Positioning centrioles and centrosomes

Centrosomes are the primary microtubule organizer in eukaryotic cells. In addition to shaping the intracellular microtubule network and the mitotic spindle, centrosomes are responsible for positioning cilia and flagella. To fulfill these diverse functions, centrosomes must be properly located within cells, which requires that they undergo intracellular transport. Importantly, centrosome mispositioning has been linked to ciliopathies, cancer, and infertility. The mechanisms by which centrosomes migrate are diverse and context dependent. In many cells, centrosomes move via indirect motor transport, whereby centrosomal microtubules engage anchored motor proteins that exert forces on those microtubules, resulting in centrosome movement. However, in some cases, centrosomes move via direct motor transport, whereby the centrosome or centriole functions as cargo that directly binds molecular motors which then walk on stationary microtubules. In this review, we summarize the mechanisms of centrosome motility and the consequences of centrosome mispositioning and identify key questions that remain to be addressed.

Cytoglobin regulates NO-dependent cilia motility and organ laterality during development

Cytoglobin is a heme protein with unresolved physiological function. Genetic deletion of zebrafish cytoglobin (cygb2) causes developmental defects in left-right cardiac determination, which in humans is associated with defects in ciliary function and low airway epithelial nitric oxide production. Here we show that Cygb2 co-localizes with cilia and with the nitric oxide synthase Nos2b in the zebrafish Kupffer's vesicle, and that cilia structure and function are disrupted in cygb2 mutants. Abnormal ciliary function and organ laterality defects are phenocopied by depletion of nos2b and of gucy1a, the soluble guanylate cyclase homolog in fish. The defects are rescued by exposing cygb2 mutant embryos to a nitric oxide donor or a soluble guanylate cyclase stimulator, or with over-expression of nos2b. Cytoglobin knockout mice also show impaired airway epithelial cilia structure and reduced nitric oxide levels. Altogether, our data suggest that cytoglobin is a positive regulator of a signaling axis composed of nitric oxide synthase-soluble guanylate cyclase-cyclic GMP that is necessary for normal cilia motility and left-right patterning.

Calcium signaling mediates proliferation of the precursor cells that give rise to the ciliated left-right organizer in the zebrafish embryo

Several of our internal organs, including heart, lungs, stomach, and spleen, develop asymmetrically along the left-right (LR) body axis. Errors in establishing LR asymmetry, or laterality, of internal organs during early embryonic development can result in birth defects. In several vertebrates-including humans, mice, frogs, and fish-cilia play a central role in establishing organ laterality. Motile cilia in a transient embryonic structure called the "left-right organizer" (LRO) generate a directional fluid flow that has been proposed to be detected by mechanosensory cilia to trigger asymmetric signaling pathways that orient the LR axis. However, the mechanisms that control the form and function of the ciliated LRO remain poorly understood. In the zebrafish embryo, precursor cells called dorsal forerunner cells (DFCs) develop into a transient ciliated structure called Kupffer's vesicle (KV) that functions as the LRO. DFCs can be visualized and tracked in the embryo, thereby providing an opportunity to investigate mechanisms that control LRO development. Previous work revealed that proliferation of DFCs via mitosis is a critical step for developing a functional KV. Here, we conducted a targeted pharmacological screen to identify mechanisms that control DFC proliferation. Small molecule inhibitors of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) were found to reduce DFC mitosis. The SERCA pump is involved in regulating intracellular calcium ion (Ca2+) concentration. To visualize Ca2+ in living embryos, we generated transgenic zebrafish using the fluorescent Ca2+ biosensor GCaMP6f. Live imaging identified dynamic cytoplasmic Ca2+ transients ("flux") that occur unambiguously in DFCs. In addition, we report Ca2+ flux events that occur in the nucleus of DFCs. Nuclear Ca2+ flux occurred in DFCs that were about to undergo mitosis. We find that SERCA inhibitor treatments during DFC proliferation stages alters Ca2+ dynamics, reduces the number of ciliated cells in KV, and alters embryo laterality. Mechanistically, SERCA inhibitor treatments eliminated both cytoplasmic and nuclear Ca2+ flux events, and reduced progression of DFCs through the S/G2 phases of the cell cycle. These results identify SERCA-mediated Ca2+ signaling as a mitotic regulator of the precursor cells that give rise to the ciliated LRO.

The centrosome - diverse functions in fertilization and development across species

The centrosome is a non-membrane-bound organelle that is conserved across most animal cells and serves various functions throughout the cell cycle. In dividing cells, the centrosome is known as the spindle pole and nucleates a robust microtubule spindle to separate genetic material equally into two daughter cells. In non-dividing cells, the mother centriole, a substructure of the centrosome, matures into a basal body and nucleates cilia, which acts as a signal-transducing antenna. The functions of centrosomes and their substructures are important for embryonic development and have been studied extensively using in vitro mammalian cell culture or in vivo using invertebrate models. However, there are considerable differences in the composition and functions of centrosomes during different aspects of vertebrate development, and these are less studied. In this Review, we discuss the roles played by centrosomes, highlighting conserved and divergent features across species, particularly during fertilization and embryonic development.

The role of cilia during organogenesis in zebrafish

Cilia are hair-like organelles that protrude from the surface of eukaryotic cells and are present on the surface of nearly all human cells. Cilia play a crucial role in signal transduction, organ development and tissue homeostasis. Abnormalities in the structure and function of cilia can lead to a group of human diseases known as ciliopathies. Currently, zebrafish serves as an ideal model for studying ciliary function and ciliopathies due to its relatively conserved structure and function of cilia compared to humans. In this review, we will summarize the different types of cilia that present in embryonic and adult zebrafish, and provide an overview of the advantages of using zebrafish as a vertebrate model for cilia research. We will specifically focus on the roles of cilia during zebrafish organogenesis based on recent studies. Additionally, we will highlight future prospects for ciliary research in zebrafish.

IK is essentially involved in ciliogenesis as an upstream regulator of oral-facial-digital syndrome ciliopathy gene, ofd1

BACKGROUND

The cilia are microtubule-based organelles that protrude from the cell surface. Abnormalities in cilia result in various ciliopathies, including polycystic kidney disease (PKD), Bardet-Biedl syndrome (BBS), and oral-facial-digital syndrome type I (OFD1), which show genetic defects associated with cilia formation. Although an increasing number of human diseases is attributed to ciliary defects, the functions or regulatory mechanisms of several ciliopathy genes remain unclear. Because multi ciliated cells (MCCs) are especially deep in vivo, studying ciliogenesis is challenging. Here, we demonstrate that ik is essential for ciliogenesis in vivo.

RESULTS

In the absence of ik, zebrafish embryos showed various ciliopathy phenotypes, such as body curvature, abnormal otoliths, and cyst formation in the kidney. RNA sequencing analysis revealed that ik positively regulated ofd1 expression required for cilium assembly. In fact, depletion of ik resulted in the downregulation of ofd1 expression with ciliary defects, and these ciliary defects in ik mutants were rescued by restoring ofd1 expression. Interestingly, ik affected ciliogenesis particularly in the proximal tubule but not in the distal tubule in the kidney.

CONCLUSIONS

This study demonstrates the role of ik in ciliogenesis in vivo for the first time. Loss of ik in zebrafish embryos displays various ciliopathy phenotypes with abnormal ciliary morphology in ciliary tissues. Our findings on the ik-ofd1 axis provide new insights into the biological function of ik in clinical ciliopathy studies in humans.

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.

Loss of zebrafish pkd1l1 causes biliary defects that have implications for biliary atresia splenic malformation

Biliary atresia is a fibroinflammatory neonatal disease with no effective therapies. A subset of cases (10-20%) is associated with laterality defects - labeled biliary atresia splenic malformation (BASM) syndrome. Recently, whole-exome sequencing of patients with BASM identified deleterious variants in PKD1L1. PKD1L1 is involved in left-right axis determination; however, its role in cholangiocytes is unknown. We generated the pkd1l1hsc117 allele using CRISPR/Cas9 mutagenesis in zebrafish to determine the role of Pkd1l1 in biliary development and function. Wild-type and mutant larvae were assessed for laterality defects, biliary function and biliary tree architecture at 5 days post fertilization. pkd1l1hsc117 mutant larvae exhibited early left-right patterning defects. The gallbladder was positioned on the left in 47% of mutants compared to 4% of wild-type larvae. Accumulation of PED6 in the gallbladder, an indicator of hepatobiliary function, was significantly reduced in pkd1l1hsc117 mutants (46%) compared to wild-type larvae (4%). pkd1l1hsc117 larvae exhibited fewer biliary epithelial cells and reduced density of the intrahepatic biliary network compared to those in wild-type larvae. These data highlight the essential role of pkd1l1 in normal development and function of the zebrafish biliary system, supporting a role for this gene as a cause of BASM.

CFAP45, a heterotaxy and congenital heart disease gene, affects cilia stability

Congenital heart disease (CHD) is the most common and lethal birth defect, affecting 1.3 million individuals worldwide. During early embryogenesis, errors in Left-Right (LR) patterning called Heterotaxy (Htx) can lead to severe CHD. Many of the genetic underpinnings of Htx/CHD remain unknown. In analyzing a family with Htx/CHD using whole-exome sequencing, we identified a homozygous recessive missense mutation in CFAP45 in two affected siblings. CFAP45 belongs to the coiled-coil domain-containing protein family, and its role in development is emerging. When we depleted Cfap45 in frog embryos, we detected abnormalities in cardiac looping and global markers of LR patterning, recapitulating the patient's heterotaxy phenotype. In vertebrates, laterality is broken at the Left-Right Organizer (LRO) by motile monocilia that generate leftward fluid flow. When we analyzed the LRO in embryos depleted of Cfap45, we discovered "bulges" within the cilia of these monociliated cells. In addition, epidermal multiciliated cells lost cilia with Cfap45 depletion. Via live confocal imaging, we found that Cfap45 localizes in a punctate but static position within the ciliary axoneme, and depletion leads to loss of cilia stability and eventual detachment from the cell's apical surface. This work demonstrates that in Xenopus, Cfap45 is required to sustain cilia stability in multiciliated and monociliated cells, providing a plausible mechanism for its role in heterotaxy and congenital heart disease.

Recruitment of transcription factor ETS1 to activated accessible regions promotes the transcriptional program of cilia genes

Defects in cilia genes, which are critical for cilia formation and function, can cause complicated ciliopathy syndromes involving multiple organs and tissues; however, the underlying regulatory mechanisms of the networks of cilia genes in ciliopathies remain enigmatic. Herein, we have uncovered the genome-wide redistribution of accessible chromatin regions and extensive alterations of expression of cilia genes during Ellis-van Creveld syndrome (EVC) ciliopathy pathogenesis. Mechanistically, the distinct EVC ciliopathy-activated accessible regions (CAAs) are shown to positively regulate robust changes in flanking cilia genes, which are a key requirement for cilia transcription in response to developmental signals. Moreover, a single transcription factor, ETS1, can be recruited to CAAs, leading to prominent chromatin accessibility reconstruction in EVC ciliopathy patients. In zebrafish, the collapse of CAAs driven by ets1 suppression subsequently causes defective cilia proteins, resulting in body curvature and pericardial oedema. Our results depict a dynamic landscape of chromatin accessibility in EVC ciliopathy patients, and uncover an insightful role for ETS1 in controlling the global transcriptional program of cilia genes by reprogramming the widespread chromatin state.

A spatiotemporal barrier formed by Follistatin is required for left-right patterning

How left-right (LR) asymmetry emerges in a patterning field along the anterior-posterior axis remains an unresolved problem in developmental biology. Left-biased Nodal emanating from the LR organizer propagates from posterior to anterior (PA) and establishes the LR pattern of the whole embryo. However, little is known about the regulatory mechanism of the PA spread of Nodal and its asymmetric activation in the forebrain. Here, we identify bilaterally expressed Follistatin (Fst) as a regulator blocking the propagation of the zebrafish Nodal ortholog Southpaw (Spaw) in the right lateral plate mesoderm (LPM), and restricting Spaw transmission in the left LPM to facilitate the establishment of a robust LR asymmetric Nodal patterning. In addition, Fst inhibits the Activin-Nodal signaling pathway in the forebrain thus preventing Nodal activation prior to the arrival, at a later time, of Spaw emanating from the left LPM. This contributes to the orderly propagation of asymmetric Nodal activation along the PA axis. The LR regulation function of Fst is further confirmed in chick and frog embryos. Overall, our results suggest that a robust LR patterning emerges by counteracting a Fst barrier formed along the PA axis.

Quantum noise may limit the mechanosensory sensitivity of cilia in the left-right organizer of the vertebrate bodyplan

Could nature be harnessing quantum mechanics in cilia to optimize the sensitivity of the mechanism of left-right symmetry breaking during development in vertebrates? I evaluate whether mechanosensing - i.e., the detection of a left-right asymmetric signal through mechanical stimulation of sensory cilia, as opposed to biochemical signalling - might be functioning in the embryonic left-right organizer of the vertebrate bodyplan through quantum mechanics. I conclude that there is a possible role for quantum biology in mechanosensing in cilia. The system may not be limited by classical thermal noise, but instead by quantum noise, with an amplification process providing active cooling.

Rab8, Rab11, and Rab35 coordinate lumen and cilia formation during zebrafish left-right organizer development

An essential process during Danio rerio's left-right organizer (Kupffer's Vesicle, KV) formation is the formation of a motile cilium by developing KV cells which extends into the KV lumen. Beating of motile cilia within the KV lumen directs fluid flow to establish the embryo's left-right axis. However, the timepoint at which KV cells start to form cilia and how cilia formation is coordinated with KV lumen formation have not been examined. We identified that nascent KV cells form cilia at their centrosomes at random intracellular positions that then move towards a forming apical membrane containing cystic fibrosis transmembrane conductance regulator (CFTR). Using optogenetic clustering approaches, we found that Rab35 positive membranes recruit Rab11 to modulate CFTR delivery to the apical membrane, which is required for lumen opening, and subsequent cilia extension into the lumen. Once the intracellular cilia reach the CFTR positive apical membrane, Arl13b-positive cilia extend and elongate in a Rab8 dependent manner into the forming lumen once the lumen reaches an area of 300 μm2. These studies demonstrate the need to acutely coordinate Rab8, Rab11, and Rab35-mediated membrane trafficking events to ensure appropriate timing in lumen and cilia formation during KV development.

Ecdysone signaling determines lateral polarity and remodels neurites to form Drosophila's left-right brain asymmetry

Left-right (LR) asymmetry of the brain is fundamental to its higher-order functions. The Drosophila brain's asymmetrical body (AB) consists of a structural pair arborized from AB neurons and is larger on the right side than the left. We find that the AB initially forms LR symmetrically and then develops LR asymmetrically by neurite remodeling that is specific to the left AB and is dynamin dependent. Additionally, neuronal ecdysone signaling inhibition randomizes AB laterality, suggesting that ecdysone signaling determines AB's LR polarity. Given that AB's LR asymmetry relates to memory formation, our research establishes AB as a valuable model for studying LR asymmetry and higher-order brain function relationships.

11.4 T ultra-high static magnetic field has no effect on morphology but induces upregulation of TNF signaling pathway based on transcriptome analysis in zebrafish embryos

As magnetic resonance imaging (MRI) scanners with ultra-high field (UHF) have optimal performance, scientists have been working to develop high-performance devices with strong magnetic fields to improve their diagnostic potential. However, whether an MRI scanner with UHF poses a risk to the safety of the organism require further evaluation. This study evaluated the effects of 11.4 Tesla (T) UHF on embryonic development using a zebrafish model. Multiple approaches, including morphological parameters, physiological behaviors, and analyses of the transcriptome at the molecular level, were determined during 5 days after laboratory-controlled exposure from 6 hour post fertilization (hpf) to 24 hpf. No significant effects were observed in embryo mortality, hatching rate, body length, Left-Right patterning, locomotor behavior, etc. RNA-sequencing analysis revealed up-regulated tumor necrosis factor (TNF) inflammatory factors and activated TNF signaling pathways in the 11.4 T exposure group. The results were further validated using qPCR. Our findings indicate that although UHF exposure under 11.4 T has no effect on the development of zebrafish embryos, it has specific effects on the immune response that require further investigation.

Calaxin stabilizes the docking of outer arm dyneins onto ciliary doublet microtubule in vertebrates

Outer arm dynein (OAD) is the main force generator of ciliary beating. Although OAD loss is the most frequent cause of human primary ciliary dyskinesia, the docking mechanism of OAD onto the ciliary doublet microtubule (DMT) remains elusive in vertebrates. Here, we analyzed the functions of Calaxin/Efcab1 and Armc4, the two of five components of vertebrate OAD-DC (docking complex), using zebrafish spermatozoa and cryo-electron tomography. Mutation of armc4 caused complete loss of OAD, whereas mutation of calaxin caused only partial loss of OAD. Detailed structural analysis revealed that calaxin-/- OADs are tethered to DMT through DC components other than Calaxin, and that recombinant Calaxin can autonomously rescue the deficient DC structure and the OAD instability. Our data demonstrate the discrete roles of Calaxin and Armc4 in the OAD-DMT interaction, suggesting the stabilizing process of OAD docking onto DMT in vertebrates.

Engineering Small Molecule Switches of Protein Function in Zebrafish Embryos

Precise temporally regulated protein function directs the highly complex processes that make up embryo development. The zebrafish embryo is an excellent model organism to study development, and conditional control over enzymatic activity is desirable to target chemical intervention to specific developmental events and to investigate biological mechanisms. Surprisingly few, generally applicable small molecule switches of protein function exist in zebrafish. Genetic code expansion allows for site-specific incorporation of unnatural amino acids into proteins that contain caging groups that are removed through addition of small molecule triggers such as phosphines or tetrazines. This broadly applicable control of protein function was applied to activate several enzymes, including a GTPase and a protease, with temporal precision in zebrafish embryos. Simple addition of the small molecule to the media produces robust and tunable protein activation, which was used to gain insight into the development of a congenital heart defect from a RASopathy mutant of NRAS and to control DNA and protein cleavage events catalyzed by a viral recombinase and a viral protease, respectively.

Polarized transport of membrane and secreted proteins during lumen morphogenesis

A ubiquitous feature of animal development is the formation of fluid-filled cavities or lumina, which transport gases and fluids across tissues and organs. Among different species, lumina vary drastically in size, scale, and complexity. However, all lumen formation processes share key morphogenetic principles that underly their development. Fundamentally, a lumen simply consists of epithelial cells that encapsulate a continuous internal space, and a common way of building a lumen is via opening and enlarging by filling it with fluid and/or macromolecules. Here, we discuss how polarized targeting of membrane and secreted proteins regulates lumen formation, mainly focusing on ion transporters in vertebrate model systems. We also discuss mechanistic differences observed among invertebrates and vertebrates and describe how the unique properties of the Na+/K+-ATPase and junctional proteins can promote polarization of immature epithelia to build lumina de novo in developing organs.

Rassf7a promotes spinal cord regeneration and controls spindle orientation in neural progenitor cells

Spinal cord injury (SCI) can cause long-lasting disability in mammals due to the lack of axonal regrowth together with the inability to reinitiate spinal neurogenesis at the injury site. Deciphering the mechanisms that regulate the proliferation and differentiation of neural progenitor cells is critical for understanding spinal neurogenesis after injury. Compared with mammals, zebrafish show a remarkable capability of spinal cord regeneration. Here, we show that Rassf7a, a member of the Ras-association domain family, promotes spinal cord regeneration after injury. Zebrafish larvae harboring a rassf7a mutation show spinal cord regeneration and spinal neurogenesis defects. Live imaging shows abnormal asymmetric neurogenic divisions and spindle orientation defects in mutant neural progenitor cells. In line with this, the expression of rassf7a is enriched in neural progenitor cells. Subcellular analysis shows that Rassf7a localizes to the centrosome and is essential for cell cycle progression. Our data indicate a role for Rassf7a in modulating spindle orientation and the proliferation of neural progenitor cells after spinal cord injury.

Dand5 is involved in zebrafish tailbud cell movement

During vertebrate development, symmetry breaking occurs in the left-right organizer (LRO). The transfer of asymmetric molecular information to the lateral plate mesoderm is essential for the precise patterning of asymmetric internal organs, such as the heart. However, at the same developmental time, it is crucial to maintain symmetry at the somite level for correct musculature and vertebrae specification. We demonstrate how left-right signals affect the behavior of zebrafish somite cell precursors by using live imaging and fate mapping studies in dand5 homozygous mutants compared to wildtype embryos. We describe a population of cells in the vicinity of the LRO, named Non-KV Sox17:GFP+ Tailbud Cells (NKSTCs), which migrate anteriorly and contribute to future somites. We show that NKSTCs originate in a cluster of cells aligned with the midline, posterior to the LRO, and leave that cluster in a left-right alternating manner, primarily from the left side. Fate mapping revealed that more NKSTCs integrated somites on the left side of the embryo. We then abolished the asymmetric cues from the LRO using dand5-/- mutant embryos and verified that NKSTCs no longer displayed asymmetric patterns. Cell exit from the posterior cluster became bilaterally synchronous in dand5-/- mutants. Our study revealed a new link between somite specification and Dand5 function. The gene dand5 is well known as the first asymmetric gene involved in vertebrate LR development. This study revealed a new link for Dand5 as a player in cell exit from the maturation zone into the presomitic mesoderm, affecting the expression patterns of myogenic factors and tail size.

Cilia function as calcium-mediated mechanosensors that instruct left-right asymmetry

The breaking of bilateral symmetry in most vertebrates is critically dependent upon the motile cilia of the embryonic left-right organizer (LRO), which generate a directional fluid flow; however, it remains unclear how this flow is sensed. Here, we demonstrated that immotile LRO cilia are mechanosensors for shear force using a methodological pipeline that combines optical tweezers, light sheet microscopy, and deep learning to permit in vivo analyses in zebrafish. Mechanical manipulation of immotile LRO cilia activated intraciliary calcium transients that required the cation channel Polycystin-2. Furthermore, mechanical force applied to LRO cilia was sufficient to rescue and reverse cardiac situs in zebrafish that lack motile cilia. Thus, LRO cilia are mechanosensitive cellular levers that convert biomechanical forces into calcium signals to instruct left-right asymmetry.

The challenge of dissecting gene function in model organisms: Tools to characterize genetic mutants and assess transcriptional adaptation in zebrafish

Genome editing technologies including the CRISPR/Cas9 system have greatly improved our knowledge of gene function and biological processes, however, these approaches have also brought new challenges to determining genotype-phenotype correlations. In this chapter, we briefly review gene-editing technologies used in zebrafish and discuss the differences in phenotypes that can arise when gene expression is inhibited by anti-sense or by gene editing techniques. We outline possible explanations for why knockout phenotypes are milder, tissue-restricted, or even absent, compared with severe knockdown phenotypes. One proposed explanation is transcriptional adaptation, a form of genetic robustness that is induced by deleterious mutations but not gene knockdowns. Although much is unknown about what triggers this process, its relevance in shaping genome expression has been shown in multiple animal models. We recently explored if transcriptional adaptation could explain genotype-phenotype discrepancies seen between two zebrafish models of the centrosomal protein Cep290 deficiency. We compared cilia-related phenotypes in knockdown (anti-sense) and knockout (mutation) Cep290 models and showed that only cep290 gene mutation induces the upregulation of genes encoding the cilia-associated small GTPases Arl3, Arl13b, and Unc119b. Importantly, the ectopic expression of Arl3, Arl13b, and Unc119b in cep290 morphant zebrafish embryos rescued cilia defects. Here we provide protocols and experimental approaches that can be used to explore if transcriptional adaptation may be modulating gene expression in a zebrafish ciliary mutant model.

Appearing and disappearing acts of cilia

The past few decades have seen a rise in research on vertebrate cilia and ciliopathy, with interesting collaborations between basic and clinical scientists. This work includes studies on ciliary architecture, composition, evolution, and organelle generation and its biological role. The human body has cells that harbour any of the following four types of cilia: 9+0 motile, 9+0 immotile, 9+2 motile, and 9+2 immotile. Depending on the type, cilia play an important role in cell/fluid movement, mating, sensory perception, and development. Defects in cilia are associated with a wide range of human diseases afflicting the brain, heart, kidneys, respiratory tract, and reproductive system. These are commonly known as ciliopathies and affect millions of people worldwide. Due to their complex genetic etiology, diagnosis and therapy have remained elusive. Although model organisms like Chlamydomonas reinhardtii have been a useful source for ciliary research, reports of a fascinating and rewarding translation of this research into mammalian systems, especially humans, are seen. The current review peeks into one of the complex features of this organelle, namely its birth, the common denominators across the formation of both 9+0 and 9+2 ciliary types, the molecules involved in ciliogenesis, and the steps that go towards regulating their assembly and disassembly.

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'.

Variable phenotypes and penetrance between and within different zebrafish ciliary transition zone mutants

Meckel syndrome, nephronophthisis, Joubert syndrome and Bardet-Biedl syndrome are caused by mutations in proteins that localize to the ciliary transition zone (TZ). The phenotypically distinct syndromes suggest that these TZ proteins have differing functions. However, mutations in a single TZ gene can result in multiple syndromes, suggesting that the phenotype is influenced by modifier genes. We performed a comprehensive analysis of ten zebrafish TZ mutants, including mks1, tmem216, tmem67, rpgrip1l, cc2d2a, b9d2, cep290, tctn1, nphp1 and nphp4, as well as mutants in ift88 and ift172. Our data indicate that variations in phenotypes exist between different TZ mutants, supporting different tissue-specific functions of these TZ genes. Further, we observed phenotypic variations within progeny of a single TZ mutant, reminiscent of multiple disease syndromes being associated with mutations in one gene. In some mutants, the dynamics of the phenotype became complex with transitory phenotypes that are corrected over time. We also demonstrated that multiple-guide-derived CRISPR/Cas9 F0 'crispant' embryos recapitulate zygotic null phenotypes, and rapidly identified ciliary phenotypes in 11 cilia-associated gene candidates (ankfn1, ccdc65, cfap57, fhad1, nme7, pacrg, saxo2, c1orf194, ttc26, zmynd12 and cfap52).

Structures and functions of cilia during vertebrate embryo development

Cilia are hair-like structures that project from the surface of cells. In vertebrates, most cells have an immotile primary cilium that mediates cell signaling, and some specialized cells assemble one or multiple cilia that are motile and beat synchronously to move fluids in one direction. Gene mutations that alter cilia structure or function cause a broad spectrum of disorders termed ciliopathies that impact virtually every system in the body. A wide range of birth defects associated with ciliopathies underscores critical functions for cilia during embryonic development. In many cases, the mechanisms underlying cilia functions during development and disease remain poorly understood. This review describes different types of cilia in vertebrate embryos and discusses recent research results from diverse model systems that provide novel insights into how cilia form and function during embryo development. The work discussed here not only expands our understanding of in vivo cilia biology, but also opens new questions about cilia and their roles in establishing healthy embryos.

The inner junction protein CFAP20 functions in motile and non-motile cilia and is critical for vision

Motile and non-motile cilia are associated with mutually-exclusive genetic disorders. Motile cilia propel sperm or extracellular fluids, and their dysfunction causes primary ciliary dyskinesia. Non-motile cilia serve as sensory/signalling antennae on most cell types, and their disruption causes single-organ ciliopathies such as retinopathies or multi-system syndromes. CFAP20 is a ciliopathy candidate known to modulate motile cilia in unicellular eukaryotes. We demonstrate that in zebrafish, cfap20 is required for motile cilia function, and in C. elegans, CFAP-20 maintains the structural integrity of non-motile cilia inner junctions, influencing sensory-dependent signalling and development. Human patients and zebrafish with CFAP20 mutations both exhibit retinal dystrophy. Hence, CFAP20 functions within a structural/functional hub centered on the inner junction that is shared between motile and non-motile cilia, and is distinct from other ciliopathy-associated domains or macromolecular complexes. Our findings suggest an uncharacterised pathomechanism for retinal dystrophy, and potentially for motile and non-motile ciliopathies in general.

Biallelic DAW1 variants cause a motile ciliopathy characterized by laterality defects and subtle ciliary beating abnormalities

PURPOSE

The clinical spectrum of motile ciliopathies includes laterality defects, hydrocephalus, and infertility as well as primary ciliary dyskinesia when impaired mucociliary clearance results in otosinopulmonary disease. Importantly, approximately 30% of patients with primary ciliary dyskinesia lack a genetic diagnosis.

METHODS

Clinical, genomic, biochemical, and functional studies were performed alongside in vivo modeling of DAW1 variants.

RESULTS

In this study, we identified biallelic DAW1 variants associated with laterality defects and respiratory symptoms compatible with motile cilia dysfunction. In early mouse embryos, we showed that Daw1 expression is limited to distal, motile ciliated cells of the node, consistent with a role in left-right patterning. daw1 mutant zebrafish exhibited reduced cilia motility and left-right patterning defects, including cardiac looping abnormalities. Importantly, these defects were rescued by wild-type, but not mutant daw1, gene expression. In addition, pathogenic DAW1 missense variants displayed reduced protein stability, whereas DAW1 loss-of-function was associated with distal type 2 outer dynein arm assembly defects involving axonemal respiratory cilia proteins, explaining the reduced cilia-induced fluid flow in particle tracking velocimetry experiments.

CONCLUSION

Our data define biallelic DAW1 variants as a cause of human motile ciliopathy and determine that the disease mechanism involves motile cilia dysfunction, explaining the ciliary beating defects observed in affected individuals.

PHLPP1 regulates CFTR activity and lumen expansion through AMPK

Complex organ development depends on single lumen formation and its expansion during tubulogenesis. This can be achieved by correct mitotic spindle orientation during cell division, combined with luminal fluid filling that generates hydrostatic pressure. Using a human 3D cell culture model, we have identified two regulators of these processes. We find that pleckstrin homology leucine-rich repeat protein phosphatase (PHLPP) 2 regulates mitotic spindle orientation, and thereby midbody positioning and maintenance of a single lumen. Silencing the sole PHLPP family phosphatase in Drosophila melanogaster, phlpp, resulted in defective spindle orientation in Drosophila neuroblasts. Importantly, cystic fibrosis transmembrane conductance regulator (CFTR) is the main channel regulating fluid transport in this system, stimulated by phosphorylation by protein kinase A and inhibited by the AMP-activated protein kinase AMPK. During lumen expansion, CFTR remains open through the action of PHLPP1, which stops activated AMPK from inhibiting ion transport through CFTR. In the absence of PHLPP1, the restraint on AMPK activity is lost and this tips the balance in the favour of channel closing, resulting in the lack of lumen expansion and accumulation of mucus.