Organizer induction determines left-right asymmetry in Xenopus

Phenotypically Discordant Anomalies in Conjoined Twins: Quirks of Nature Governed by Molecular Pathways?

A multitude of additional anomalies can be observed in virtually all types of symmetrical conjoined twins. These concomitant defects can be divided into different dysmorphological patterns. Some of these patterns reveal their etiological origin through their topographical location. The so-called shared anomalies are traceable to embryological adjustments and directly linked to the conjoined-twinning mechanism itself, inherently located within the boundaries of the coalescence area. In contrast, discordant patterns are anomalies present in only one of the twin members, intrinsically distant from the area of union. These dysmorphological entities are much more difficult to place in a developmental perspective, as it is presumed that conjoined twins share identical intra-uterine environments and intra-embryonic molecular and genetic footprints. However, their existence testifies that certain developmental fields and their respective developmental pathways take different routes in members of conjoined twins. This observation remains a poorly understood phenomenon. This article describes 69 cases of external discordant patterns within different types of otherwise symmetrical mono-umbilical conjoined twins and places them in a developmental perspective and a molecular framework. Gaining insights into the phenotypes and underlying (biochemical) mechanisms could potentially pave the way and generate novel etiological visions in the formation of conjoined twins itself.

Xenopus: Driving the Discovery of Novel Genes in Patient Disease and Their Underlying Pathological Mechanisms Relevant for Organogenesis

Frog model organisms have been appreciated for their utility in exploring physiological phenomena for nearly a century. Now, a vibrant community of biologists that utilize this model organism has poised Xenopus to serve as a high throughput vertebrate organism to model patient-driven genetic diseases. This has facilitated the investigation of effects of patient mutations on specific organs and signaling pathways. This approach promises a rapid investigation into novel mechanisms that disrupt normal organ morphology and function. Considering that many disease states are still interrogated in vitro to determine relevant biological processes for further study, the prospect of interrogating genetic disease in Xenopus in vivo is an attractive alternative. This model may more closely capture important aspects of the pathology under investigation such as cellular micro environments and local forces relevant to a specific organ's development and homeostasis. This review aims to highlight recent methodological advances that allow investigation of genetic disease in organ-specific contexts in Xenopus as well as provide examples of how these methods have led to the identification of novel mechanisms and pathways important for understanding human disease.

Two is a Crowd: Two is a Crowd: On the Enigmatic Etiopathogenesis of Conjoined Twinning

In this article, we provide a comprehensive overview of multiple facets in the puzzling genesis of symmetrical conjoined twins. The etiopathogenesis of conjoined twins remains matter for ongoing debate and is currently cited-in virtually every paper on conjoined twins-as partial fission or secondary fusion. Both theories could potentially be extrapolated from embryological adjustments exclusively seen in conjoined twins. Adoption of these, seemingly factual, theoretical proposals has (unconsciously) resulted in crystallized patterns of verbal and graphic representations concerning the enigmatic genesis of conjoined twins. Critical evaluation on their plausibility and solidity remains however largely absent. As it appears, both the fission and fusion theories cannot be applied to the full range of conjunction possibilities and thus remain matter for persistent inconclusiveness. We propose that initial duplication of axially located morphogenetic potent primordia could be the initiating factor in the genesis of ventrally, laterally, and caudally conjoined twins. The mutual position of two primordia results in neo-axial orientation and/or interaction aplasia. Both these embryological adjustments result in conjunction patterns that may seemingly appear as being caused by fission or fusion. However, as we will substantiate, neither fission nor fusion are the cause of most conjoined twinning types; rather what is interpreted as fission or fusion is actually the result of the twinning process itself. Furthermore, we will discuss the currently held views on the origin of conjoined twins and its commonly assumed etiological correlation with monozygotic twinning. Finally, considerations are presented which indicate that the dorsal conjunction group is etiologically and pathogenetically different from other symmetric conjoined twins. This leads us to propose that dorsally united twins could actually be caused by secondary fusion of two initially separate monozygotic twins. An additional reason for the ongoing etiopathogenetic debate on the genesis of conjoined twins is because different types of conjoined twins are classically placed in one overarching receptacle, which has hindered the quest for answers. Clin. Anat. 32:722-741, 2019. © 2019 Wiley Periodicals, Inc.

Xenopus, an ideal model organism to study laterality in conjoined twins

Conjoined twins occur at low frequency in all vertebrates including humans. Many twins fused at the chest or abdomen display a very peculiar laterality defect: while the left twin is normal with respect to asymmetric organ morphogenesis and placement (situs solitus), the organ situs is randomized in right twins. Although this phenomenon has fascinated already some of the founders of experimental embryology in the 19th and early 20th century, such as Dareste, Fol, Warynsky and Spemann, its embryological basis has remained enigmatic. Here we summarize historical experiments and interpretations as well as current models, argue that the frog Xenopus is the only vertebrate model organism to tackle the issue, and outline suitable experiments to address the question of twin laterality in the context of cilia-based symmetry breakage.

Leftward Flow Determines Laterality in Conjoined Twins

Conjoined twins fused at the thorax display an enigmatic left-right defect: although left twins are normal, laterality is disturbed in one-half of right twins [1-3]. Molecularly, this randomization corresponds to a lack of asymmetric Nodal cascade induction in right twins [4]. We studied leftward flow [5, 6] at the left-right organizer (LRO) [7, 8] in thoracopagus twins in Xenopus, which displayed a duplicated, fused, and ciliated LRO. Cilia were motile and produced a leftward flow from the right LRO margin of the right to the left margin of the left twin. Motility was required for correct laterality in left twins, as knockdown of dynein motor dnah9 prevented Nodal cascade induction. Nodal was rescued by parallel knockdown of the inhibitor dand5 [9, 10] on the left side of the left twin. Lack of Nodal induction in the right twin, despite the presence of flow, was due to insufficient suppression of dand5. Knockdown of dand5 at the center of the fused LRO resulted in asymmetric Nodal cascade induction in the right twin as well. Manipulation of leftward flow and dand5 in a targeted and sided manner induced the Nodal cascade in a predictable manner, in the left twin, the right one, both, or neither. Laterality in conjoined twins thus was determined by cilia-driven leftward fluid flow like in single embryos, which solves a century-old riddle, as the phenomenon was already studied by some of the founders of experimental embryology, including Dareste [11], Fol and Warynsky [12], and Spemann and Falkenberg [13] (reviewed in [14]).

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

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

It's never too early to get it Right: A conserved role for the cytoskeleton in left-right asymmetry

For centuries, scientists and physicians have been captivated by the consistent left-right (LR) asymmetry of the heart, viscera, and brain. A recent study implicated tubulin proteins in establishing laterality in several experimental models, including asymmetric chemosensory receptor expression in C. elegans neurons, polarization of HL-60 human neutrophil-like cells in culture, and asymmetric organ placement in Xenopus. The same mutations that randomized asymmetry in these diverse systems also affect chirality in Arabidopsis, revealing a remarkable conservation of symmetry-breaking mechanisms among kingdoms. In Xenopus, tubulin mutants only affected LR patterning very early, suggesting that this axis is established shortly after fertilization. This addendum summarizes and extends the knowledge of the cytoskeleton's role in the patterning of the LR axis. Results from many species suggest a conserved role for the cytoskeleton as the initiator of asymmetry, and indicate that symmetry is first broken during early embryogenesis by an intracellular process.

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

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

β-Catenin 1 and β-catenin 2 play similar and distinct roles in left-right asymmetric development of zebrafish embryos

β-Catenin-mediated canonical Wnt signaling has been found to be required for left-right (LR) asymmetric development. However, the implication of endogenous β-catenin in LR development has not been demonstrated by loss-of-function studies. In zebrafish embryos, two β-catenin genes, β-catenin 1 (ctnnb1) and β-catenin 2 (ctnnb2) are maternally expressed and their zygotic expression occurs in almost all types of tissues, including Kupffer's vesicle (KV), an essential organ that initiates LR development in teleost fish. We demonstrate here that morpholino-mediated knockdown of ctnnb1, ctnnb2, or both, in the whole embryo or specifically in dorsal forerunner cells (DFCs) interrupts normal asymmetry of the heart, liver and pancreas. Global knockdown of ctnnb2 destroys the midline physical and molecular barrier, while global knockdown of ctnnb1 impairs the formation of the midline molecular barrier. Depletion of either gene or both in DFCs/KV leads to poor KV cell proliferation, abnormal cilia formation and disordered KV fluid flow with downregulation of ntl and tbx16 expression. ctnnb1 and ctnnb2 in DFCs/KV differentially regulate the expression of charon, a Nodal antagonist, and spaw, a key Nodal gene for laterality development in zebrafish. Loss of ctnnb1 in DFCs/KV inhibits the expression of charon around KV and of spaw in the posterior lateral plate mesoderm, while ctnnb2 knockdown results in loss of spaw expression in the anterior lateral plate mesoderm with little alteration of charon expression. Taken together, our findings suggest that ctnnb1 and ctnnb2 regulate multiple processes of laterality development in zebrafish embryos through similar and distinct mechanisms.

Polarity proteins are required for left-right axis orientation and twin-twin instruction

Two main classes of models address the earliest steps of left-right patterning: those postulating that asymmetry is initiated via cilia-driven fluid flow in a multicellular tissue at gastrulation, and those postulating that asymmetry is amplified from intrinsic chirality of individual cells at very early embryonic stages. A recent study revealed that cultured human cells have consistent left-right (LR) biases that are dependent on apical-basal polarity machinery. The ability of single cells to set up asymmetry suggests that cellular chirality could be converted to embryonic laterality by cilia-independent polarity mechanisms in cell fields. To examine the link between cellular polarity and LR patterning in a vertebrate model organism, we probed the roles of apical-basal and planar polarity proteins in the orientation of the LR axis in Xenopus. Molecular loss-of-function targeting these polarity pathways specifically randomizes organ situs independently of contribution to the ciliated organ. Alterations in cell polarity also disrupt tight junction integrity, localization of the LR signaling molecule serotonin, the normally left-sided expression of Xnr-1, and the LR instruction occurring between native and ectopic organizers. We propose that well-conserved polarity complexes are required for LR asymmetry and that cell polarity signals establish the flow of laterality information across the early blastoderm independently of later ciliary functions. genesis 50:219-234, 2012. © 2011 Wiley Periodicals, Inc.

Regulation of basal body and ciliary functions by Diversin

The centrosome is essential for the formation of the cilia and has been implicated in cell polarization and signaling during early embryonic development. A number of Wnt pathway components were found to localize at the centrosome, but how this localization relates to their signaling functions is unclear. In this study, we assessed a role for Diversin, a putative Wnt pathway mediator, in developmental processes that involve cilia. We find that Diversin is specifically localized to the basal body compartment near the base of the cilium in Xenopus multi-ciliated skin cells. Overexpression of Diversin RNA disrupted basal body polarization in these cells, suggesting that tightly regulated control of Diversin levels is crucial for this process. In cells depleted of endogenous Diversin, basal body structure appeared abnormal and this was accompanied by disrupted polarity, shortened or absent cilia and defective ciliary flow. These results are consistent with the involvement of Diversin in processes that are related to the acquisition of cell polarity and require ciliary functions.

A second-generation device for automated training and quantitative behavior analyses of molecularly-tractable model organisms

A deep understanding of cognitive processes requires functional, quantitative analyses of the steps leading from genetics and the development of nervous system structure to behavior. Molecularly-tractable model systems such as Xenopus laevis and planaria offer an unprecedented opportunity to dissect the mechanisms determining the complex structure of the brain and CNS. A standardized platform that facilitated quantitative analysis of behavior would make a significant impact on evolutionary ethology, neuropharmacology, and cognitive science. While some animal tracking systems exist, the available systems do not allow automated training (feedback to individual subjects in real time, which is necessary for operant conditioning assays). The lack of standardization in the field, and the numerous technical challenges that face the development of a versatile system with the necessary capabilities, comprise a significant barrier keeping molecular developmental biology labs from integrating behavior analysis endpoints into their pharmacological and genetic perturbations. Here we report the development of a second-generation system that is a highly flexible, powerful machine vision and environmental control platform. In order to enable multidisciplinary studies aimed at understanding the roles of genes in brain function and behavior, and aid other laboratories that do not have the facilities to undergo complex engineering development, we describe the device and the problems that it overcomes. We also present sample data using frog tadpoles and flatworms to illustrate its use. Having solved significant engineering challenges in its construction, the resulting design is a relatively inexpensive instrument of wide relevance for several fields, and will accelerate interdisciplinary discovery in pharmacology, neurobiology, regenerative medicine, and cognitive science.

Consistent left-right asymmetry cannot be established by late organizers in Xenopus unless the late organizer is a conjoined twin

How embryos consistently orient asymmetries of the left-right (LR) axis is an intriguing question, as no macroscopic environmental cues reliably distinguish left from right. Especially unclear are the events coordinating LR patterning with the establishment of the dorsoventral (DV) axes and midline determination in early embryos. In frog embryos, consistent physiological and molecular asymmetries manifest by the second cell cleavage; however, models based on extracellular fluid flow at the node predict correct de novo asymmetry orientation during neurulation. We addressed these issues in Xenopus embryos by manipulating the timing and location of dorsal organizer induction: the primary dorsal organizer was ablated by UV irradiation, and a new organizer was induced at various locations, either early, by mechanical rotation, or late, by injection of lithium chloride (at 32 cells) or of the transcription factor XSiamois (which functions after mid-blastula transition). These embryos were then analyzed for the position of three asymmetric organs. Whereas organizers rescued before cleavage properly oriented the LR axis 90% of the time, organizers induced in any position at any time after the 32-cell stage exhibited randomized laterality. Late organizers were unable to correctly orient the LR axis even when placed back in their endogenous location. Strikingly, conjoined twins produced by late induction of ectopic organizers did have normal asymmetry. These data reveal that although correct LR orientation must occur no later than early cleavage stages in singleton embryos, a novel instructive influence from an early organizer can impose normal asymmetry upon late organizers in the same cell field.

A transient asymmetric distribution of XNOA 36 mRNA and the associated spectrin network bisects Xenopus laevis stage I oocytes along the future A/V axis

In Xenopus oogenesis, the mechanisms governing the localisation of molecules crucial for primary axis determination have been uncovered in recent years. In stage I oocytes, the mitochondrial cloud (MC) entraps RNAs implicated in germ line specification and other RNAs, such as Xwnt-11 and Xlsirts, that are later delivered to the vegetal pole. Microfilaments and microtubules gradually develop in the cytoplasm, sustaining organelles as well as the MC. At stage III, other mRNAs migrate to the vegetal hemisphere through a microtubule-dependent mechanism. We report here the isolation of a cDNA encoding XNOA 36, a highly conserved protein, whose function is to date not fully understood. The XNOA 36 transcript is abundantly accumulated in stage I oocytes where it decorates a filamentous network. At the end of stage I the transcript gradually segregates in a sector of the oocyte surrounding the MC and opposite the ovarian hylum. Here, XNOA 36 mRNA distributes in a gradient-like pattern extending from a peripheral network towards the interior of the oocyte. This distribution is similar to that of alpha-spectrin mRNA. Both mRNAs are segregated in one half of the 250 microm oocytes, with the MC located between the XNOA 36/alpha-spectrin mRNA-labelled and unlabelled regions. XNOA 36 mRNA localisation was uncoupled from that of alpha-spectrin mRNA by cytochalasin B or ice-nocodazole treatments, suggesting that the two transcripts rely on different mechanisms for their localisation. However, immunolocalisation experiments coupled with in situ hybridisation revealed that the XNOA 36 transcript co-localises with the protein spectrin. This observation, together with the finding that XNOA 36 mRNA co-precipitates with spectrin, indicates that these two molecules interact physically. In conclusion, our data suggest that XNOA 36 mRNA is localized and/or anchored in the oocyte through a cytoskeletal network containing spectrin. The putative implications of this finding are discussed.

Setdb2 restricts dorsal organizer territory and regulates left-right asymmetry through suppressing fgf8 activity

Dorsal organizer formation is one of the most critical steps in early embryonic development. Several genes and signaling pathways that positively regulate the dorsal organizer development have been identified; however, little is known about the factor(s) that negatively regulates the organizer formation. Here, we show that Setdb2, a SET domain-containing protein possessing potential histone H3K9 methyltransferase activity, restricts dorsal organizer development and regulates left-right asymmetry by suppressing fibroblast growth factor 8 (fgf8) expression. Knockdown of Setdb2 results in a massive expansion of dorsal organizer markers floating head (flh), goosecoid (gsc), and chordin (chd), as well as a significant increase of fgf8, but not fgf4 mRNAs. Consequently, disrupted midline patterning and resultant randomization of left-right asymmetry are observed in Setdb2-deficient embryos. These characteristic changes induced by Setdb2 deficiency can be nearly corrected by either overexpression of a dominant-negative fgf receptor or knockdown of fgf8, suggesting an essential role for Setdb2-Fgf8 signaling in restricting dorsal organizer territory and regulating left-right asymmetry. These results provide unique evidence that a SET domain-containing protein potentially involved in the epigenetic control negatively regulates dorsal organizer formation during early embryonic development.

Is left-right asymmetry a form of planar cell polarity?

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

What's left in asymmetry?

Left-right patterning is a fascinating problem of morphogenesis, linking evolutionary and cellular signaling mechanisms across many levels of organization. In the past 15 years, enormous progress has been made in elucidating the molecular details of this process in embryos of several model species. While many outside the field seem to believe that the fundamental aspects of this pathway are now solved, workers on asymmetry are faced with considerable uncertainties over the details of specific mechanisms, a lack of conceptual unity of mechanisms across phyla, and important questions that are not being pursued in any of the popular model systems. Here, we suggest that data from clinical syndromes, cryptic asymmetries, and bilateral gynandromorphs, while not figuring prominently in the mainstream work on LR asymmetry, point to crucial and fundamental gaps of knowledge about asymmetry. We identify 12 big questions that provide exciting opportunities for fundamental new advances in this field.

Perspectives and open problems in the early phases of left-right patterning

Embryonic left-right (LR) patterning is a fascinating aspect of embryogenesis. The field currently faces important questions about the origin of LR asymmetry, the mechanisms by which consistent asymmetry is imposed on the scale of the whole embryo, and the degree of conservation of early phases of LR patterning among model systems. Recent progress on planar cell polarity and cellular asymmetry in a variety of tissues and species provides a new perspective on the early phases of LR patterning. Despite the huge diversity in body-plans over which consistent LR asymmetry is imposed, and the apparent divergence in molecular pathways that underlie laterality, the data reveal conservation of physiological modules among phyla and a basic scheme of cellular chirality amplified by a planar cell polarity-like pathway over large cell fields.

Wnt/Axin1/beta-catenin signaling regulates asymmetric nodal activation, elaboration, and concordance of CNS asymmetries

Nodal activity in the left lateral plate mesoderm (LPM) is required to activate left-sided Nodal signaling in the epithalamic region of the zebrafish forebrain. Epithalamic Nodal signaling subsequently determines the laterality of neuroanatomical asymmetries. We show that overactivation of Wnt/Axin1/beta-catenin signaling during late gastrulation leads to bilateral epithalamic expression of Nodal pathway genes independently of LPM Nodal signaling. This is consistent with a model whereby epithalamic Nodal signaling is normally bilaterally repressed, with Nodal signaling from the LPM unilaterally alleviating repression. We suggest that Wnt signaling regulates the establishment of the bilateral repression. We identify a second role for the Wnt pathway in the left/right regulation of LPM Nodal pathway gene expression, and finally, we show that at later stages Axin1 is required for the elaboration of concordant neuroanatomical asymmetries.

Anteriorward shifting of asymmetric Xnr1 expression and contralateral communication in left-right specification in Xenopus

Transient asymmetric Nodal signaling in the left lateral plate mesoderm (L LPM) during tailbud/early somitogenesis stages is associated in all vertebrates examined with the development of stereotypical left-right (L-R) organ asymmetry. In Xenopus, asymmetric expression of Nodal-related 1 (Xnr1) begins in the posterior L LPM shortly after the initiation of bilateral perinotochordal expression in the posterior tailbud. The L LPM expression domain rapidly shifts forward to cover much of the flank of the embryo before being progressively downregulated, also in a posterior-to-anterior direction. The mechanisms underlying the initiation and propagation of Nodal/Xnr1 expression in the L LPM, and its transient nature, are not well understood. Removing the posterior tailbud domain prevents Xnr1 expression in the L LPM, consistent with the idea that normal embryos respond to a posteriorly derived asymmetrically acting positive inductive signal. The forward propagation of asymmetric Xnr1 expression occurs LPM-autonomously via planar tissue communication. The shifting is prevented by Nodal signaling inhibitors, implicating an underlying requirement for Xnr1-to-Xnr1 induction. It is also unclear how asymmetric Nodal signals are modulated during L-R patterning. Small LPM grafts overexpressing Xnr1 placed into the R LPM of tailbud embryos induced the expression of the normally L-sided genes Xnr1, Xlefty, and XPitx2, and inverted body situs, demonstrating the late-stage plasticity of the LPM. Orthogonal Xnr1 signaling from the LPM strongly induced Xlefty expression in the midline, consistent with recent findings in the mouse and demonstrating for the first time in another species conservation in the mechanism that induces and maintains the midline barrier. Our findings suggest that there is long-range contralateral communication between L and R LPM, involving Xlefty in the midline, over a substantial period of tailbud embryogenesis, and therefore lend further insight into how, and for how long, the midline maintains a L versus R status in the LPM.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

Mathematical model of morphogen electrophoresis through gap junctions

Gap junctional communication is important for embryonic morphogenesis. However, the factors regulating the spatial properties of small molecule signal flows through gap junctions remain poorly understood. Recent data on gap junctions, ion transporters, and serotonin during left-right patterning suggest a specific model: the net unidirectional transfer of small molecules through long-range gap junctional paths driven by an electrophoretic mechanism. However, this concept has only been discussed qualitatively, and it is not known whether such a mechanism can actually establish a gradient within physiological constraints. We review the existing functional data and develop a mathematical model of the flow of serotonin through the early Xenopus embryo under an electrophoretic force generated by ion pumps. Through computer simulation of this process using realistic parameters, we explored quantitatively the dynamics of morphogen movement through gap junctions, confirming the plausibility of the proposed electrophoretic mechanism, which generates a considerable gradient in the available time frame. The model made several testable predictions and revealed properties of robustness, cellular gradients of serotonin, and the dependence of the gradient on several developmental constants. This work quantitatively supports the plausibility of electrophoretic control of morphogen movement through gap junctions during early left-right patterning. This conceptual framework for modeling gap junctional signaling -- an epigenetic patterning mechanism of wide relevance in biological regulation -- suggests numerous experimental approaches in other patterning systems.

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.

Handedness in twins: a meta-analysis

In the largest meta-analysis of twins and singletons conducted to date we have found a higher incidence of left-handedness in twins compared to singletons. Our analysis revealed no difference in the frequency of left-handedness among monozygotic versus dizygotic twins. However, identical twins were more likely to be concordant for hand preference than non-identical twins, which is consistent with a genetic model of handedness. Prior analyses have not revealed these findings consistently, and this has led to a number of conflicting models of handedness.

Wnt genes define distinct boundaries in the developing human brain: implications for human forebrain patterning

Understanding the factors that govern human forebrain regionalization along the dorsal-ventral and left-right (L-R) axes is likely to be relevant to a wide variety of neurodevelopmental and neuropsychiatric conditions. Recent work in lower vertebrates has identified several critical signaling molecules involved in embryonic patterning along these axes. Among these are the Wingless-Int (WNT) proteins, involved in the formation of dorsal central nervous system (CNS) structures, as well as in visceral L-R asymmetry. We examined the expression of WNT2b and WNT7b in the human brain, because these genes have highly distinctive expression patterns in the embryonic mouse forebrain. In the human fetal telencephalon, WNT2b expression appears to define the cortical hem, a dorsal signaling center previously characterized in mouse, which is also confirmed by BMP7 expression. In diencephalon, WNT2b expression is restricted to medial dorsal structures, including the developing pineal gland and habenular nucleus, both implicated in CNS L-R asymmetry in lower organisms. At 5 weeks gestation, WNT7b is expressed in cerebral cortical and diencephalic progenitor cells. As the cortical plate develops, WNT7b expression shifts, demarcating deep layer neurons of the neocortex and the hippocampal formation. Spatial and temporal expression patterns show startling similarity between human and mouse, suggesting that the developmental roles of these WNT genes may be highly conserved, despite the far greater size and complexity of the human forebrain.

Left-right asymmetry: nodal points

The striking left-right asymmetry of visceral organs is known to depend on left- and right-side-specific cascades of gene expression during early embryogenesis. Now, developmental biologists are characterizing the earliest steps in asymmetry determination that dictate the sidedness of asymmetric gene expression. The proteins and structures involved control fascinating physiological processes, such as extracellular fluid flow and membrane voltage potential and yet little is known about how their activities are coordinated to control laterality. By analogy with intercellular signalling in certain epithelial and endothelial cells, however, it is reasonable to speculate that at least three of these players, monocilia, gap junction communication and the Ca2+ channel polycystin-2, participate in a signalling pathway that propagates left-right cues through multicellular fields.

A protein disulfide isomerase expressed in the embryonic midline is required for left/right asymmetries

Although the vertebrate embryonic midline plays a critical role in determining the left/right asymmetric development of multiple organs, few genes expressed in the midline are known to function specifically in establishing laterality patterning. Here we show that a gene encoding protein disulfide isomerase P5 (PDI-P5) is expressed at high levels in the organizer and axial mesoderm and is required for establishing left/right asymmetries in the zebrafish embryo. pdi-p5 was discovered in a screen to detect genes down-regulated in the zebrafish midline mutant one-eyed pinhead and expressed predominantly in midline tissues of wild-type embryos. Depletion of the pdi-p5 product with morpholino antisense oligonucleotides results in loss of the asymmetric development of the heart, liver, pancreas, and gut. In addition, PDI-P5 depletion results in bilateral expression of all genes known to be expressed asymmetrically in the lateral plate mesoderm and the brain during embryogenesis. The laterality defects caused by pdi-p5 antisense treatment arise solely due to loss of the PDI-P5 protein, as they are reversed when treated embryos are supplied with an exogenous source of the PDI-P5 protein. Thus the spectrum of laterality defects resulting from depletion of the PDI-P5 protein fully recapitulates that resulting from loss of the midline. As loss of PDI-P5 does not appear to interfere with other aspects of midline development or function, we propose that PDI-P5 is specifically involved in the production of midline-derived signals required to establish left/right asymmetry.

Xenopus Brachyury regulates mesodermal expression of Zic3, a gene controlling left-right asymmetry

The Brachyury gene has a critical role in the formation of posterior mesoderm and notochord in vertebrate development. A recent study showed that Brachyury is also responsible for the formation of the left-right (L-R) axis in mouse and zebrafish. However, the role of Brachyury in L-R axis specification is still elusive. Here, it is demonstrated that Brachyury is involved in L-R specification of the Xenopus laevis embryo and regulates expression of Zic3, which controls the L-R specification process. Overexpression of Xenopus Brachyury (Xbra) and dominant-negative type Xbra (Xbra-EnR) altered the orientation of heart and gut looping, concomitant with disturbed laterality of nodal-related 1 (Xnr1) and Pitx2 expression, both of which are normally expressed in the left lateral plate mesoderm. Furthermore, activation of inducible type Xbra (Xbra-GR) induces Zic3 expression within 20 min. These results suggest that a role of Brachyury in L-R specification may be the direct regulation of Zic3 expression.

Left-right asymmetry determination in vertebrates

A distinctive and essential feature of the vertebrate body is a pronounced left-right asymmetry of internal organs and the central nervous system. Remarkably, the direction of left-right asymmetry is consistent among all normal individuals in a species and, for many organs, is also conserved across species, despite the normal health of individuals with mirror-image anatomy. The mechanisms that determine stereotypic left-right asymmetry have fascinated biologists for over a century. Only recently, however, has our understanding of the left-right patterning been pushed forward by links to specific genes and proteins. Here we examine the molecular biology of the three principal steps in left-right determination: breaking bilateral symmetry, propagation and reinforcement of pattern, and the translation of pattern into asymmetric organ morphogenesis.

Left-right positioning of the adult rudiment in sea urchin larvae is directed by the right side

Indirect-developing sea urchins eventually form an adult rudiment on the left side through differential left-right development in the late larval stages. Components of the adult rudiment, such as the hydropore canal, the hydrocoel and the primary vestibule, all develop on the left side alone, and are the initial morphological traits that exhibit left-right differences. Although it has previously been shown that partial embryos dissected in cleavage stages correctly determine the normal left-right placement of the adult rudiment, the timing and the mechanism that determine left-right polarity during normal development remain unknown. In order to determine these, we have carried out a series of regional operations in two indirect-developing sea urchin species. We excised all or a part of tissue on the left or right side of the embryos during the early gastrula stage and the two-armed pluteus stage, and examined the left-right position of the adult rudiment, and of its components. Excisions of tissues on the left side of the embryos, regardless of stage, resulted in formation of a left adult rudiment, as in normal development. By contrast, excisions on the right side of the embryos resulted in three different types of impairment in the left-right placement of the adult rudiment in a stage-dependent manner. Generally, when the adult rudiment was definitively formed only on the right side of the larvae, no trace of basic development of the components of the adult rudiment was found on the left side, indicating that a right adult rudiment results from reversal of the initial left-right polarity but not from a later inhibitory effect on the development of an adult rudiment. Thus, we suggest that determination of the left-right placement of the adult rudiment depends on a process, which is directed by the right side, of polarity establishment during the gastrula and the prism stages; however, but commitment of the cell fate to initiate formation of the adult rudiment occurs later than the two-armed pluteus stage.

Wnt signaling and PKA control Nodal expression and left-right determination in the chick embryo

Expression of the Nodal gene, which encodes a member of the TGFβ superfamily of secreted factors, localizes to the left side of the developing embryo in all vertebrates examined so far. This asymmetric pattern correlates with normal development of the left-right axis. We now show that the Wnt and PKA signaling pathways control left-right determination in the chick embryo through Nodal. A Wnt/β-catenin pathway controlsNodal expression in and around Hensen's node, without affecting the upstream regulators Sonic hedgehog, Car and Fibroblast Growth Factor 8. Transcription of Nodal is also positively regulated by a protein kinase A-dependent pathway. Both the adhesion protein N-cadherin and PKI (an endogenous protein kinase A inhibitor) are localized to the right side of the node and may contribute to restrict Nodal activation by Wnt signaling and PKA to the left side of the node.

Classification of left-right patterning defects in zebrafish, mice, and humans

Numerous genes and developmental processes have been implicated in the establishment of the vertebrate left-right axis. Although the mechanisms that initiate left-right patterning may be distinct in different classes of vertebrates, it is clear that the asymmetric gene expression patterns of nodal, lefty, and pitx2 in the left lateral plate mesoderm are conserved and that left-right development of the brain, heart, and gut is tightly linked to the development of the embryonic midline. This review categorizes left-right patterning defects based on asymmetric gene expression patterns, midline phenotypes, and situs phenotypes. In so doing, we hope to provide a framework to assess the genetic bases of laterality defects in humans and other vertebrates.

Zic3 is involved in the left-right specification of the Xenopus embryo

Establishment of left-right (L-R) asymmetry is fundamental to vertebrate development. Several genes involved in L-R asymmetry have been described. In the Xenopus embryo, Vg1/activin signals are implicated upstream of asymmetric nodal related 1 (Xnr1) and Pitx2 expression in L-R patterning. We report here that Zic3 carries the left-sided signal from the initial activin-like signal to determinative factors such as Pitx2. Overexpression of Zic3 on the right side of the embryo altered the orientation of heart and gut looping, concomitant with disturbed laterality of expression of Xnr1 and Pitx2, both of which are normally expressed in the left lateral plate mesoderm. The results indicate that Zic3 participates in the left-sided signaling upstream of Xnr1 and Pitx2. At early gastrula, Zic3 was expressed not only in presumptive neuroectoderm but also in mesoderm. Correspondingly, overexpression of Zic3 was effective in the L-R specification at the early gastrula stage, as revealed by a hormone-inducible Zic3 construct. The Zic3 expression in the mesoderm is induced by activin (beta) or Vg1, which are also involved in the left-sided signal in L-R specification. These findings suggest that an activin-like signal is a potent upstream activator of Zic3 that establishes the L-R axis. Furthermore, overexpression of the zinc-finger domain of Zic3 on the right side is sufficient to disturb the L-R axis, while overexpression of the N-terminal domain on the left side affects the laterality. These results suggest that Zic3 has at least two functionally important domains that play different roles and provide a molecular basis for human heterotaxy, which is an L-R pattern anomaly caused by a mutation in human ZIC3.

Multiple pathways in the midline regulate concordant brain, heart and gut left-right asymmetry

The embryonic midline in vertebrates has been implicated in left-right development, but the mechanisms by which it regulates left-right asymmetric gene expression and organ morphogenesis are unknown. Zebrafish embryos have three domains of left-right asymmetric gene expression that are useful predictors of organ situs. cyclops (nodal), lefty1 and pitx2 are expressed in the left diencephalon; cyclops, lefty2 and pitx2 are expressed in the left heart field; and cyclops and pitx2 are expressed in the left gut primordium. Distinct alterations of these expression patterns in zebrafish midline mutants identify four phenotypic classes that have different degrees of discordance among the brain, heart and gut. These classes help identify two midline domains and several genetic pathways that regulate left-right development. A cyclops-dependent midline domain, associated with the prechordal plate, regulates brain asymmetry but is dispensable for normal heart and gut left-right development. A second midline domain, associated with the anterior notochord, is dependent on no tail, floating head and momo function and is essential for restricting asymmetric gene expression to the left side. Mutants in spadetail or chordino give discordant gene expression among the brain, heart and gut. one-eyed pinhead and schmalspur are necessary for asymmetric gene expression and may mediate signaling from midline domains to lateral tissues. The different phenotypic classes help clarify the apparent disparity of mechanisms proposed to explain left-right development in different vertebrates.

The TGF-beta family member derrière is involved in regulation of the establishment of left-right asymmetry

Although a number of genes that are involved in the establishment of left-right asymmetry have been identified, earlier events in the molecular pathway developing left-right asymmetry remain to be elucidated. Here we present evidence suggesting that the transforming growth factor-beta family member derrière is involved in the development of left-right asymmetry in Xenopus embryos. Ectopic expression of derrière on the right side can fully invert cardiac and visceral left-right orientation and nodal expression, and expression of a dominant-negative form of derrière on the left side can partially randomize the left-right orientation and nodal expression. Moreover, while expression of the dominant-negative derrière does not inhibit the activity of Vg1 directly, it can rescue the altered left-right orientation induced by Vg1. Vg1 can induce derrière in animal cap explants. These results suggest that derrière is involved in earlier molecular pathways developing the left-right asymmetry.

Pitx2 isoforms: involvement of Pitx2c but not Pitx2a or Pitx2b in vertebrate left-right asymmetry

During vertebrate left-right development the homeobox gene Pitx2 serves as a mediator between transient nodal signaling in the left lateral plate mesoderm (l-LPM) and asymmetric organ morphogenesis. Misexpression of Pitx2 in chick and frog led to alteration of organ situs. Here we report the presence of different Pitx2 isoforms in mouse and frog. Pitx2c but not Pitx2a or Pitx2b was asymmetrically expressed in the l-LPM, heart and gut, and was specifically induced by nodal in Xenopus animal cap explant cultures and whole embryos. Pitx2c induced its own transcription, suggesting a maintenance mechanism following the down-regulation of nodal in the l-LPM. Pitx2c thus represents the left-specific isoform involved in vertebrate left-right asymmetry.

Symmetry and asymmetry in the development of inner organs in parabiotic twins of amphibians (Urodela)

Newt embryos of different developmental stages were combined to parabiotic twins in different positions. The exterior appearance and the symmetry relations, particularly of the internal organs (intestinal tract, heart, nuclei habenulae, and vitelline vein) were studied. Experimentally caused organ inversions allowed conclusions with respect to organ asymmetry and unilateral dominance. There was no direct correlation between appearance and symmetry of the exterior and the internal organs. All internal organs showed a continuous transition between normal and ideally inverse situs. The concordance of the organ situs differs greatly. The "left-hand side" or "right-hand side" dominance is not uniform. It depends on the type of fusion, i.e. the relative position of the parabiotic twins, and is often specific for a given organ. In some cases a non-genetic "symmetrisation factor" appears to be strongly active, depending on the fusion type and resulting in a dominant transindividual organ mirror image symmetry in the parabiotic twins. The older twin generally dominates the processes of determination and induction. The "symmetrisation factor" also acts on members of different families, i.e. genetically completely heterogeneous parabiotic twins. The development of organ asymmetry appears to be a process with several phases.

The homeobox gene Pitx2: mediator of asymmetric left-right signaling in vertebrate heart and gut looping

Left-right asymmetry in vertebrates is controlled by activities emanating from the left lateral plate. How these signals get transmitted to the forming organs is not known. A candidate mediator in mouse, frog and zebrafish embryos is the homeobox gene Pitx2. It is asymmetrically expressed in the left lateral plate mesoderm, tubular heart and early gut tube. Localized Pitx2 expression continues when these organs undergo asymmetric looping morphogenesis. Ectopic expression of Xnr1 in the right lateral plate induces Pitx2 transcription in Xenopus. Misexpression of Pitx2 affects situs and morphology of organs. These experiments suggest a role for Pitx2 in promoting looping of the linear heart and gut.

Molecular mechanisms of vertebrate left-right development

Patterning of all tissues and organs in the vertebrate embryo occurs along the dorsoventral (DV), anteroposterior (AP), and left-right (LR) body axes. Whereas significant progress has been made in identifying the processes underlying DV and AP patterning, relatively little is known about mechanisms guiding LR development. The significant incidence of human disease conditions associated with LR laterality defects, particularly those of the cardiovascular system, underscores the importance of understanding how LR asymmetries become established in the embryo. The focus of this review is on recently identified genes that are involved in generation of vertebrate LR asymmetry, and the proposed cellular and molecular mechanisms by which they might function in initiation, propagation and interpretation of LR patterning information.

Gap junctions are involved in the early generation of left-right asymmetry

Invariant left-right asymmetry of the visceral organs is a fundamental feature of vertebrate embryogenesis. While a cascade of asymmetrically expressed genes has been described, the embryonic mechanism that orients the left-right axis relative to the dorsoventral and anteroposterior axes (a prerequisite for asymmetric gene expression) is unknown. We propose that this process involves dorsoventral differences in cell-cell communication through gap junctions composed of connexin proteins. Global modulation of gap junctional states in Xenopus embryos by pharmacological agents specifically induced heterotaxia involving mirror-image reversals of heart, gut, and gall bladder. Greatest sensitivity was observed between st. 5 and st. 12, well before the onset of organogenesis. Moreover, heterotaxia was also induced following microinjection of dominant negative and wild-type connexin mRNAs to modify the endogenous dorsoventral difference in junctional communication. Heterotaxia was induced by either blocking gap junction communication (GJC) dorsally or by introducing communication ventrally (but not the reverse). Both connexin misexpression and exposure to GJC-modifying drugs altered expression of the normally left-sided gene XNR-1, demonstrating that GJC functions upstream of XNR-1 in the pathway that patterns left-right asymmetry. Finally, lineage analysis to follow the progeny of microinjected cells indicated that they generally do not contribute the asymmetric organs. Together with the early sensitivity window, this suggests that GJC functions as part of a fundamental, early aspect of left-right patterning. In addition, we show that a potential regulatory mutation in Connexin43 is sufficient to cause heterotaxia. Despite uncertainty about the prevalence of the serine364 to proline substitution reported in human patients with laterality defects, the mutant protein is both a mild hypomorph and a potent antimorph as determined by the effect of its expression on left-right patterning. Taken together, our data suggest that endogenous dorsoventral differences in GJC within the early embryo are needed to consistently orient left-right asymmetry.

Inversin, a novel gene in the vertebrate left-right axis pathway, is partially deleted in the inv mouse

Visceral left-right asymmetry occurs in all vertebrates, but the inversion of embryo turning (inv) mouse, which resulted following a random transgene insertion, is the only model in which these asymmetries are consistently reversed. We report positional cloning of the gene underlying this recessive phenotype. Although transgene insertion was accompanied by neighbouring deletion and duplication events, our YAC phenotype rescue studies indicate that the mutant phenotype results from the deletion. After extensively characterizing the 47-kb deleted region and flanking sequences from the wild-type mouse genome, we found evidence for only one gene sequence in the deleted region. We determined the full-length 5.5-kb cDNA sequence and identified 16 exons, of which exons 3-11 were eliminated by the deletion, causing a frameshift. The novel gene specifies a 1062-aa product with tandem ankyrin-like repeat sequences. Characterization of complementing and non-complementing YAC transgenic families revealed that correction of the inv mutant phenotype was concordant with integration and intact expression of this novel gene, which we have named inversin (Invs).

Links in the left/right axial pathway

We now have a sketch of the vertebrate L/R pathway from egg to organ. There is a lot to learn and a lot to explain, but the beauty of the pathway is already evident. We also have an inkling of its fragility. Set against the complex laterality disorders seen in humans, efforts to understand this pathway should continue to challenge our intellectual and experimental dexterity.

The transcription factor Pitx2 mediates situs-specific morphogenesis in response to left-right asymmetric signals

The mechanism by which asymmetric signals induce left-right-specific morphogenesis has been elusive. Pitx2 encodes a transcription factor expressed throughout the left lateral plate mesoderm and subsequently on the left side of asymmetric organs such as the heart and gut during organogenesis in the chick embryo. Pitx2 is induced by the asymmetric signals encoded by Nodal and Sonic hedgehog, and its expression is blocked by prior treatment with an antibody against Sonic hedgehog. Misexpression of Pitx2 on the right side of the embryo is sufficient to produce reversed heart looping and heart isomerisms, reversed body rotation, and reversed gut situs.

The left-right coordinator: the role of Vg1 in organizing left-right axis formation

The asymmetries of internal organs are consistently oriented along the left-right axis in all vertebrates, and perturbations of left-right orientation lead to significant congenital disease. We propose a model in which a "left-right coordinator" interacts with the Spemann organizer to coordinate the evolutionarily conserved three-dimensional asymmetries in the embryo. The Vg1 cell-signaling pathway plays a central role in left-right coordinator function. Antagonists of Vg1 alter left-right development; antagonists of other members of the TGFbeta family do not. Cell-lineage directed expression of Vg1 protein can fully invert the left-right axis (situs inversus), can randomize left-right asymmetries, or can "rescue" a perturbed left-right axis in conjoined twins to normal orientation (situs solitus), indicating that Vg1 can mimic left-right coordinator activity. These are the first molecular manipulations in any vertebrate by which the left-right axis can be reliably controlled.

The transfer of left-right positional information during chick embryogenesis

The earliest known left-right asymmetric genes are expressed at Hensen's node during chick gastrulation. Gene expression following reorientation of the node shows asymmetry is instructed by adjacent tissue, hence left-right information originates outside the node. Subsequently, the node signals back to the lateral tissue, initiating a cascade leading to left-sided expression of nodal in the lateral plate mesoderm. Loss of nodal expression in the presence of blocking antibodies confirms that Sonic hedgehog is the key signal conveying left-right information from the node; however, manipulation of explant cultures suggests that the induction of nodal requires secondary signals produced in the paraxial mesoderm. These experiments establish the time of action of these signals to and from Hensen's node in establishing left-right asymmetry.