Nodal paralogues underlie distinct mechanisms for visceral left-right asymmetry in reptiles and mammals

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.

Function of nodal cilia in left-right determination: Mechanical regulation in initiation of symmetry breaking

Visceral organs in vertebrates are arranged with left-right asymmetry; for example, the heart is located on the left side of the body. Cilia at the node of mouse early embryos play an essential role in determining this left-right asymmetry. Using information from the anteroposterior axis, motile cilia at the central region of the node generate leftward nodal flow. Immotile cilia at the periphery of the node mechanically sense the direction of leftward nodal flow in a manner dependent on the polarized localization of Pkd2, which is localized on the dorsal side of cilia. Therefore, only left-side cilia are activated by leftward nodal flow. This activation results in frequent calcium transients in the cilia via the Pkd2 channel, which leads to the degradation of Dand5 mRNA only at the left-side crown-cells. This process is the mechanism of initial determination of the left-side-specific signal. In this review, we provide an overview of initial left-right symmetry breaking that occurs at the node, focusing mainly on a recent biophysical study that revealed the function of nodal immotile cilia using advanced microscopic techniques, such as optical tweezers and super-resolution microscopy.

Decrypting the phylogenetics history of EGF-CFC proteins Cripto and Cryptic

EGF-CFC proteins are obligate coreceptors for Nodal signaling and are thus required for gastrulation and left-right patterning. Species with multiple family members show evidence of specialization. For example, mouse Cripto is required for gastrulation, whereas Cryptic is involved in left-right patterning. However, the members of the family across model organisms have little sequence conservation beyond the EGF-CFC domain, posing challenges for determining their evolutionary history and functional conservation. In this study we outline the evolutionary history of the EGF-CFC family of proteins. We traced the EGF-CFC gene family from a single gene in the deuterostome ancestor through its expansion and functional specialization in tetrapods, and subsequent gene loss and translocation in eutherian mammals. Mouse Cripto and Cryptic, zebrafish Tdgf1, and all three Xenopus EGF-CFC genes (Tdgf1, Tdgf1.2 and Cripto.3) and are all descendants of the ancestral Tdgf1 gene. We propose that subsequent to the family expansion in tetrapods, Tdgf1B (Xenopus Tdgf1.2) acquired specialization in the left-right patterning cascade, and after its translocation in eutherians to a different chromosomal location, Cfc1/Cryptic has maintained that specialization.

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.

ETV2 induces endothelial, but not hematopoietic, lineage specification in birds

Cardiovascular system develops from the lateral plate mesoderm. Its three primary cell lineages (hematopoietic, endothelial, and muscular) are specified by the sequential actions of conserved transcriptional factors. ETV2, a master regulator of mammalian hemangioblast development, however, is absent in the chicken genome and acts downstream of NPAS4L in zebrafish. Here, we investigated the epistatic relationship between NPAS4L and ETV2 in avian hemangioblast development. We showed that ETV2 is deleted in all 363 avian genomes analyzed. Mouse ETV2 induced LMO2, but not NPAS4L or SCL, expression in chicken mesoderm. Squamate (lizards, geckos, and snakes) genomes contain both NPAS4L and ETV2 In Madagascar ground gecko, both genes were expressed in developing hemangioblasts. Gecko ETV2 induced only LMO2 in chicken mesoderm. We propose that both NPAS4L and ETV2 were present in ancestral amniote, with ETV2 acting downstream of NPAS4L in endothelial lineage specification. ETV2 may have acted as a pioneer factor by promoting chromatin accessibility of endothelial-specific genes and, in parallel with NPAS4L loss in ancestral mammals, has gained similar function in regulating blood-specific genes.

A reverse genetic approach in geckos with the CRISPR/Cas9 system by oocyte microinjection

Reptiles are important model organisms in developmental and evolutionary biology, but are used less widely than other amniotes such as mouse and chicken. One of the main reasons for this is that has proven difficult to conduct CRISPR/Cas9-mediated genome editing in many reptile species despite the widespread use of this technology in other taxa. Certain features of reptile reproductive systems make it difficult to access one-cell or early-stage zygotes, which represents a key impediment to gene editing techniques. Recently, Rasys and colleagues reported a genome editing method using oocyte microinjection that allowed them to produce genome-edited Anolis lizards. This method opened a new avenue to reverse genetics studies in reptiles. In the present article, we report the development of a related method for genome editing in the Madagascar ground gecko (Paroedura picta), a well-established experimental model, and describe the generation of Tyr and Fgf10 gene-knockout geckos in the F0 generation.

Immotile cilia mechanically sense the direction of fluid flow for left-right determination

Immotile cilia at the ventral node of mouse embryos are required for sensing leftward fluid flow that breaks left-right symmetry of the body. However, the flow-sensing mechanism has long remained elusive. In this work, we show that immotile cilia at the node undergo asymmetric deformation along the dorsoventral axis in response to the flow. Application of mechanical stimuli to immotile cilia by optical tweezers induced calcium ion transients and degradation of Dand5 messenger RNA (mRNA) in the targeted cells. The Pkd2 channel protein was preferentially localized to the dorsal side of immotile cilia, and calcium ion transients were preferentially induced by mechanical stimuli directed toward the ventral side. Our results uncover the biophysical mechanism by which immotile cilia at the node sense the direction of fluid flow.

Morphological changes and two Nodal paralogs drive left-right asymmetry in the squamate veiled chameleon (C. calyptratus)

The ancestral mode of left-right (L-R) patterning involves cilia in the L-R organizer. However, the mechanisms regulating L-R patterning in non-avian reptiles remains an enigma, since most squamate embryos are undergoing organogenesis at oviposition. In contrast, veiled chameleon (Chamaeleo calyptratus) embryos are pre-gastrula at oviposition, making them an excellent organism for studying L-R patterning evolution. Here we show that veiled chameleon embryos lack motile cilia at the time of L-R asymmetry establishment. Thus, the loss of motile cilia in the L-R organizers is a synapomorphy of all reptiles. Furthermore, in contrast to avians, geckos and turtles, which have one Nodal gene, veiled chameleon exhibits expression of two paralogs of Nodal in the left lateral plate mesoderm, albeit in non-identical patterns. Using live imaging, we observed asymmetric morphological changes that precede, and likely trigger, asymmetric expression of the Nodal cascade. Thus, veiled chameleons are a new and unique model for studying the evolution of L-R patterning.

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.

Abnormal left-right organizer and laterality defects in Xenopus embryos after formin inhibitor SMIFH2 treatment

Left-right symmetry breaking in most studied vertebrates makes use of so-called leftward flow, a mechanism which was studied in detail especially in mouse and Xenopus laevis embryos and is based on rotation of monocilia on specialized epithelial surface designated as left-right organizer or laterality coordinator. However, it has been argued that prior to emergence of leftward flow an additional mechanism operates during early cleavage stages in Xenopus embryo which is based on cytoskeletal processes. Evidence in favour of this early mechanism was supported by left-right abnormalities after chemical inhibition of cytoskeletal protein formin. Here we analyzed temporal dimension of this effect in detail and found that reported abnormalities arise only after treatment at gastrula-neurula stages, i.e. just prior to and during the operation of left-right organizer. Moreover, molecular and morphological analysis of the left-right organizer reveals its abnormal development. Our results strongly indicate that left-right abnormalities reported after formin inhibition cannot serve as support of models based on early symmetry breaking event in Xenopus embryo.

Left-right asymmetric expression of the Nodal-Lefty-Pitx2 module in developing turtle forebrain

The epithalamus of zebrafish shows morphological and molecular left-right (L-R) asymmetry, but such asymmetry is not apparent in tetrapods. To provide further insight into the evolutionary diversity of brain L-R asymmetry, we have now examined the developing brains of reptile embryos for expression of Nodal, Lefty, and Pitx2. Two turtle species, the Chinese softshell turtle and the red-eared slider turtle, showed left-sided expression of these three genes in the developing forebrain, with this expression occurring after Nodal expression at the lateral plate and the L-R organizer has disappeared. Nodal activity, as revealed by the detection of phosphorylated Smad2/3, was also apparent in the neural epithelium on the left side in both turtle species. In the Chinese softshell turtle, the habenula did not show apparent asymmetry in size and the parapineal organ was absent, but the expression of Kctd12 in the habenula showed a small yet reproducible asymmetry. In contrast to the turtles, L-R asymmetric expression of Nodal, Lefty, Pitx2, or Kctd12 was not detected in the developing brain of the Madagascar ground gecko. The transcriptional enhancer (ASE) responsible for the asymmetric expression of Nodal, Lefty, and Pitx2 was conserved among reptiles, including the Chinese softshell turtle and Madagascar ground gecko. Our findings suggest that Nodal, Lefty, and Pitx2 have the potential to be asymmetrically expressed in the developing brain of vertebrates, but that their expression varies even among reptiles.

Nodal asymmetry and hedgehog signaling during vertebrate left–right symmetry breaking

Development of visceral left–right asymmetry in bilateria is based on initial symmetry breaking followed by subsequent asymmetric molecular patterning. An important step is the left-sided expression of transcription factor pitx2 which is mediated by asymmetric expression of the nodal morphogen in the left lateral plate mesoderm of vertebrates. Processes leading to emergence of the asymmetric nodal domain differ depending on the mode of symmetry breaking. In Xenopus laevis and mouse embryos, the leftward fluid flow on the ventral surface of the left–right organizer leads through intermediate steps to enhanced activity of the nodal protein on the left side of the organizer and subsequent asymmetric nodal induction in the lateral plate mesoderm. In the chick embryo, asymmetric morphogenesis of axial organs leads to paraxial nodal asymmetry during the late gastrulation stage. Although it was shown that hedgehog signaling is required for initiation of the nodal expression, the mechanism of its asymmetry remains to be clarified. In this study, we established the activation of hedgehog signaling in early chick embryos to further study its role in the initiation of asymmetric nodal expression. Our data reveal that hedgehog signaling is sufficient to induce the nodal expression in competent domains of the chick embryo, while treatment of Xenopus embryos led to moderate nodal inhibition. We discuss the role of symmetry breaking and competence in the initiation of asymmetric gene expression.

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.

Discovery of a genetic module essential for assigning left-right asymmetry in humans and ancestral vertebrates

The vertebrate left-right axis is specified during embryogenesis by a transient organ: the left-right organizer (LRO). Species including fish, amphibians, rodents and humans deploy motile cilia in the LRO to break bilateral symmetry, while reptiles, birds, even-toed mammals and cetaceans are believed to have LROs without motile cilia. We searched for genes whose loss during vertebrate evolution follows this pattern and identified five genes encoding extracellular proteins, including a putative protease with hitherto unknown functions that we named ciliated left-right organizer metallopeptide (CIROP). Here, we show that CIROP is specifically expressed in ciliated LROs. In zebrafish and Xenopus, CIROP is required solely on the left side, downstream of the leftward flow, but upstream of DAND5, the first asymmetrically expressed gene. We further ascertained 21 human patients with loss-of-function CIROP mutations presenting with recessive situs anomalies. Our findings posit the existence of an ancestral genetic module that has twice disappeared during vertebrate evolution but remains essential for distinguishing left from right in humans.

Shark and ray genomics for disentangling their morphological diversity and vertebrate evolution

Developmental studies of sharks and rays (elasmobranchs) have provided much insight into the process of morphological evolution of vertebrates. Although those studies are supposedly fueled by large-scale molecular sequencing information, whole-genome sequences of sharks and rays were made available only recently. One compelling difficulty of elasmobranch developmental biology is the low accessibility to embryonic study materials and their slow development. Another limiting factor is the relatively large size of their genomes. Moreover, their large body sizes restrict sustainable captive breeding, while their high body fluid osmolarity prevents reproducible cell culturing for in vitro experimentation, which has also limited our knowledge of their chromosomal organization for validation of genome sequencing products. This article focuses on egg-laying elasmobranch species used in developmental biology and provides an overview of the characteristics of the shark and ray genomes revealed to date. Developmental studies performed on a gene-by-gene basis are also reviewed from a whole-genome perspective. Among the popular regulatory genes studied in developmental biology, I scrutinize shark homologs of Wnt genes that highlight vanishing repertoires in many other vertebrate lineages, as well as Hox genes that underwent an unexpected modification unique to the elasmobranch lineage. These topics are discussed together with insights into the reconstruction of developmental programs in the common ancestor of vertebrates and its subsequent evolutionary trajectories that mark the features that are unique to, and those characterizing the diversity among, cartilaginous fishes.

Exploration of possible cell chirality using material techniques of surface patterning

Consistent left-right (LR) asymmetry or chirality is critical for embryonic development and function maintenance. While chirality on either molecular or organism level has been well established, that on the cellular level has remained an open question for a long time. Although it remains unclear whether chirality exists universally on the cellular level, valuable efforts have recently been made to explore this fundamental topic pertinent to both cell biology and biomaterial science. The development of material fabrication techniques, surface patterning, in particular, has afforded a unique platform to study cell-material interactions. By using patterning techniques, chirality on the cellular level has been examined for cell clusters and single cells in vitro in well-designed experiments. In this review, we first introduce typical fabrication techniques of surface patterning suitable for cell studies and then summarize the main aspects of preliminary evidence of cell chirality on patterned surfaces to date. We finally indicate the limitations of the studies conducted thus far and describe the perspectives of future research in this challenging field. STATEMENT OF SIGNIFICANCE: While both biomacromolecules and organisms can exhibit chirality, it is not yet conclusive whether a cell has left-right (LR) asymmetry. It is important yet challenging to study and reveal the possible existence of cell chirality. By using the technique of surface patterning, the recent decade has witnessed progress in the exploration of possible cell chirality within cell clusters and single cells. Herein, some important preliminary evidence of cell chirality is collected and analyzed. The open questions and perspectives are also described to promote further investigations of cell chirality in biomaterials.

TGFβ family signaling and development

The transforming growth factor β (TGFβ) signaling family is evolutionarily conserved in metazoans. The signal transduction mechanisms of TGFβ family members have been expansively investigated and are well understood. During development and homeostasis, numerous TGFβ family members are expressed in various cell types with temporally changing levels, playing diverse roles in embryonic development, adult tissue homeostasis and human diseases by regulating cell proliferation, differentiation, adhesion, migration and apoptosis. Here, we discuss the molecular mechanisms underlying signal transduction and regulation of the TGFβ subfamily pathways, and then highlight their key functions in mesendoderm induction, dorsoventral patterning and laterality development, as well as in the formation of several representative tissues/organs.

Statistical Validation Verifies That Enantiomorphic States of Chiral Cells Are Determinant Dictating the Left- or Right-Handed Direction of the Hindgut Rotation in Drosophila

In the left–right (LR) asymmetric development of invertebrates, cell chirality is crucial. A left- or right-handed cell structure directs morphogenesis with corresponding LR-asymmetry. In Drosophila, cell chirality is thought to drive the LR-asymmetric development of the embryonic hindgut and other organs. This hypothesis is supported only by an apparent concordance between the LR-directionality of cell chirality and hindgut rotation and by computer simulations that connect the two events. In this article, we mathematically evaluated the causal relationship between the chirality of the hindgut epithelial cells and the LR-direction of hindgut rotation. Our logistic model, drawn from several Drosophila genotypes, significantly explained the correlation between the enantiomorphic (sinistral or dextral) state of chiral cells and the LR-directionality of hindgut rotation—even in individual live mutant embryos with stochastically determined cell chirality and randomized hindgut rotation, suggesting that the mechanism by which cell chirality forms is irrelevant to the direction of hindgut rotation. Thus, our analysis showed that cell chirality, which forms before hindgut rotation, is both sufficient and required for the subsequent rotation, validating the hypothesis that cell chirality causally defines the LR-directionality of hindgut rotation.

The twists and turns of left-right asymmetric gut morphogenesis

Many organs develop left-right asymmetric shapes and positions that are crucial for normal function. Indeed, anomalous laterality is associated with multiple severe birth defects. Although the events that initially orient the left-right body axis are beginning to be understood, the mechanisms that shape the asymmetries of individual organs remain less clear. Here, we summarize new evidence challenging century-old ideas about the development of stomach and intestine laterality. We compare classical and contemporary models of asymmetric gut morphogenesis and highlight key unanswered questions for future investigation.

Diversity of left-right symmetry breaking strategy in animals

Left-right (L-R) asymmetry of visceral organs in animals is established during embryonic development via a stepwise process. While some steps are conserved, different strategies are employed among animals for initiating the breaking of body symmetry. In zebrafish (teleost), Xenopus (amphibian), and mice (mammal), symmetry breaking is elicited by directional fluid flow at the L-R organizer, which is generated by motile cilia and sensed by mechanoresponsive cells. In contrast, birds and reptiles do not rely on the cilia-driven fluid flow. Invertebrates such as Drosophila and snails employ another distinct mechanism, where the symmetry breaking process is underpinned by cellular chirality acquired downstream of the molecular interaction of myosin and actin. Here, we highlight the convergent entry point of actomyosin interaction and planar cell polarity to the diverse L-R symmetry breaking mechanisms among animals.

Molecular and cellular basis of left-right asymmetry in vertebrates

Although the human body appears superficially symmetrical with regard to the left-right (L-R) axis, most visceral organs are asymmetric in terms of their size, shape, or position. Such morphological asymmetries of visceral organs, which are essential for their proper function, are under the control of a genetic pathway that operates in the developing embryo. In many vertebrates including mammals, the breaking of L-R symmetry occurs at a structure known as the L-R organizer (LRO) located at the midline of the developing embryo. This symmetry breaking is followed by transfer of an active form of the signaling molecule Nodal from the LRO to the lateral plate mesoderm (LPM) on the left side, which results in asymmetric expression of Nodal (a left-side determinant) in the left LPM. Finally, L-R asymmetric morphogenesis of visceral organs is induced by Nodal-Pitx2 signaling. This review will describe our current understanding of the mechanisms that underlie the generation of L-R asymmetry in vertebrates, with a focus on mice.