Evolutionary conservation of mechanisms upstream of asymmetric Nodal expression: reconciling chick and Xenopus

The Secrets of the Frogs Heart

The heart of the African clawed frog has a double-inlet and single-outlet ventricle supporting systemic and pulmonary circulations via a truncus, and a lifespan of 25-30 years. We sought to understand the unique cardiac anatomic and physiologic characteristics, with balanced circulation and low metabolic rate, by comparing the basic anatomy structures with focused echocardiography and cardiac magnetic resonance imaging. Twenty-four adult female African clawed frogs were randomly subjected to anatomic dissection (n = 4), echocardiography (n = 10), and cardiac magnetic resonance (n = 10). All anatomical features were confirmed and compared with echocardiography and cardiac magnetic resonance imaging. The main characteristics of the cardiovascular circulation in frogs are the following: Intact interatrial septum, with two separate atrio-ventricular valves, preventing atrial mixing of oxygenated and desaturated blood. Single spongiform ventricular cavity, non-conducive for homogeneous mixing. Single outlet with a valve-like mobile spiral structure, actively streaming into systemic and pulmonary arteries. Intact interatrial septum, spongiform ventricle, and valve-like spiral in the conus arteriosus are likely responsible for balanced systemic and pulmonary circulation in frogs, in spite of double-inlet and single-outlet ventricle.

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.

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.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

Pitx2c attenuation results in cardiac defects and abnormalities of intestinal orientation in developing Xenopus laevis

The experimental manipulation of early embryologic events, resulting in the misexpression of the homeobox transcription factor pitx2, is associated with subsequent defects of laterality in a number of vertebrate systems. To clarify the role of one pitx2 isoform, pitx2c, in determining the left-right axis of amphibian embryos, we examined the heart and gut morphology of Xenopus laevis embryos after attenuating pitx2c mRNA levels using chemically modified antisense oligonucleotides. We demonstrate that the partial depletion of pitx2c mRNA in these embryos results in alteration of both cardiac morphology and intestinal coiling. The most common cardiac abnormality seen was a failure of rightward migration of the outflow tract, while the most common intestinal laterality phenotype seen was a full reversal in the direction of coiling, each present in 23% of embryos injected with the pitx2c antisense oligonucleotide. An abnormality in either the heart or gut further predisposed to a malformation in the other. In addition, a number of other cardiac anomalies were observed after pitx2c mRNA attenuation, including abnormalities of atrial septation, extracellular matrix restriction, relative atrial-ventricular chamber positioning, and restriction of ventricular development. Many of these findings correlate with cardiac defects previously reported in pitx2 null and hypomorphic mice, but can now be assigned specifically to attenuation of the pitx2c isoform in Xenopus.

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.

Embryonic and larval staging of summer flounder (Paralichthys dentatus)

Early development of flatfishes such as the summer flounder Paralichthys dentatus (Pleuronectiformes) has not been extensively documented, largely because of a dearth of material; however, the recent expansion of flatfish aquaculture has made embryos of P. dentatus readily available for developmental studies. We divide development of P. dentatus embryos and larvae into two main periods, pre- and posthatching, and assign stages within each of those primary divisions. Stages from fertilization to hatching loosely follow the general teleost staging scheme suggested by Shardo ([1995] J Morphol 225:125-167); stages from hatching through metamorphosis are aligned with the series used for Japanese flounder, P. olivaceus (Minami [1982] Nippon Suisan Gakkaishi 48:1581-1588; Fukuhara [1986] Nippon Suisan Gakkaishi 52:81-91). Although length, width, and age may serve as approximate indicators of developmental progression in summer flounder, these characteristics are too variable to form the sole basis of a staging table. Therefore, we define stages by morphological criteria drawn from the development of the jaw apparatus and digestive system, eye migration, and notochord tip flexion. Examination of these morphological features in hatched larvae allows accurate and consistent assessment of developmental stage despite variation in timing and size. The staging scheme for flounder embryonic and larval development presented here should facilitate both experimental and comparative research on summer flounder and other flatfish species.

BMP signaling positively regulates Nodal expression during left right specification in the chick embryo

Exogenous application of BMP to the lateral plate mesoderm (LPM) of chick embryos at the early somite stage had a positive effect on Nodal expression. BMP applications into the right LPM were followed by a rapid activation of Nodal, while applications into the left LPM resulted in expansion of the normal domain of Nodal expression. Conversely, blocking of BMP signaling by Noggin in the left LPM interfered with the activation of Nodal expression. These results support a positive role for endogenous BMP on Nodal expression in the LPM. We also report that BMP positively regulates the expression of Caronte, Snail and Cfc in both the left and right LPM. BMP-treated embryos had molecular impairment of the midline with downregulation of Lefty1, Brachyury and Shh but we also show that the midline defect was not sufficient to induce ectopic Nodal expression. We discuss our findings in the context of the known molecular control of the specification of left-right asymmetry.

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.

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.

Left-right development from embryos to brains

Bilateran animals have external bilateral symmetry along the dorsoventral (DV) and anteroposterior (AP) axes. Internal left-right asymmetries appear to be consistently aligned along the left-right (LR) axis with respect to the other axes. Left-right development is most apparent in the directional looping of the cardiac tube, the coiling and placement of the intestines, the positioning of internal organs such as liver, gallbladder, pancreas, and stomach. In addition, there are obvious morphological asymmetries in the brains of some vertebrates and functional left-right asymmetries in the activities of the brain, as assessed by psychological testing, MRI, and the analysis of lesions. There are several fundamental questions: What are the origins of the left-right axis, and are they highly conserved across metazoans? Once the left-right axis is established by the initial breaking of bilateral symmetry, what is the genetic pathway that perpetrates left-right development? What are the cellular and tissue mechanics that lead to morphogenesis during, for example, the looping of the cardiac tube, the coiling of the gut, or asymmetric brain development? Finally, do the asymmetric developmental pathways of each organ system take register from the same initial event that establishes the left-right axis, or are there separate mechanisms that orient heart, gut, and brain left-right asymmetry with respect to the DV and AP axes? These questions are beginning to be experimentally addressed, and papers in this issue of Developmental Genetics make contributions to several aspects in the burgeoning field of left-right development. Recent reviews have summarized the emerging genes and pathways in vertebrate left-right development [Wood, 1997; Harvey, 1998; Ramsdell and Yost, 1998]. Here, I give an overview of the contributions in this issue to the fundamental questions in left-right development.

Left-right development: the roles of nodal cilia

Cilia on the ventral side of the mouse node have been implicated in initiating the left-right axis during embryonic development, but how cilia relate to other factors in the left-right pathway and the mechanism by which cilia convey patterning information remain uncertain.

Cardiac looping and the vertebrate left-right axis: antagonism of left-sided Vg1 activity by a right-sided ALK2-dependent BMP pathway

The rightward looping of the primary heart tube is dependent upon upstream patterning events that establish the vertebrate left-right axis. In Xenopus, a left-sided Vg1 signaling pathway has been implicated in instructing cells to adopt a ‘left-sided identity’; however, it is not known whether ‘right-sided identity’ is acquired by a default pathway or by antagonism of Vg1 signaling. Here, we propose that an antagonistic, BMP/ALK2/Smad-mediated signaling pathway is active on the right side of the Xenopus embryo. Truncated ALK2 receptor expression on the right side of the blastula elicits heart reversals and altered nodal expression. Consistent with these findings, constitutively active ALK2 (CA-ALK2) receptor expression on the left side of the blastula also elicits heart reversals and altered nodal expression. Coexpression of CA-ALK2 with mature Vg1 ligand results in predominantly left-sided nodal expression patterns and normal heart looping, demonstrating that the ALK2 pathway can ‘rescue’ left-right reversals that otherwise occur following right-sided misexpression of mature Vg1 ligand alone. Results with chimeric precursor proteins indicate that the mature domain of BMP ligands can mimic the ability of the ALK2 signaling pathway to antagonize the Vg1 pathway. Consistent with the observed antagonism between BMP and Vg1 ligands, left-sided ectopic expression of Xolloid results in heart reversals. Moreover, ectopic expression of Smad1 or Smad7 identified two downstream modulators of the BMP/ALK2 signaling pathway that also can regulate cardiac orientation. Collectively, these results define a BMP/ALK2-mediated pathway on the right side of the Xenopus embryo and, moreover, suggest that left-right patterning preceding cardiac morphogenesis involves the activation of two distinct and antagonistic, left- and right-sided TGF(beta)-related signaling pathways.

Gap junction-mediated transfer of left-right patterning signals in the early chick blastoderm is upstream of Shh asymmetry in the node

Invariant patterning of left-right asymmetry during embryogenesis depends upon a cascade of inductive and repressive interactions between asymmetrically expressed genes. Different cascades of asymmetric genes distinguish the left and right sides of the embryo and are maintained by a midline barrier. As such, the left and right sides of an embryo can be viewed as distinct and autonomous fields. Here we describe a series of experiments that indicate that the initiation of these programs requires communication between the two sides of the blastoderm. When deprived of either the left or the right lateral halves of the blastoderm, embryos are incapable of patterning normal left-right gene expression at Hensen's node. Not only are both flanks required, suggesting that there is no single signaling source for LR pattern, but the blastoderm must be intact. These results are consistent with our previously proposed model in which the orientation of LR asymmetry in the frog, Xenopus laevis, depends on large-scale partitioning of LR determinants through intercellular gap junction channels (M. Levin and M. Mercola (1998) Developmental Biology 203, 90–105). Here we evaluate whether gap junctional communication is required for the LR asymmetry in the chick, where it is possible to order early events relative to the well-characterized left and right hierarchies of gene expression. Treatment of cultured chick embryos with lindane, which diminishes gap junctional communication, frequently unbiased normal LR asymmetry of Shh and Nodal gene expression, causing the normally left-sided program to be recapitulated symmetrically on the right side of the embryo. A survey of early expression of connexin mRNAs revealed that Cx43 is present throughout the blastoderm at Hamburger-Hamilton stage 2–3, prior to known asymmetric gene expression. Application of antisense oligodeoxynucleotides or blocking antibody to cultured embryos also resulted in bilateral expression of Shh and Nodal transcripts. Importantly, the node and primitive streak at these stages lack Cx43 mRNA. This result, together with the requirement for an intact blastoderm, suggests that the path of communication through gap junction channels circumvents the node and streak. We propose that left-right information is transferred unidirectionally throughout the epiblast by gap junction channels in order to pattern left-sided Shh expression at Hensen's node.

Molecular mechanisms of holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain in humans. Several distinct human genes for holoprosencephaly have now been identified. They include Sonic hedgehog (SHH), ZIC2, and SIX3. Many additional genes involved in forebrain development are rapidly being cloned and characterized in model vertebrate organisms. These include Patched (Ptc), Smoothened (Smo), cubitus interuptus (ci)/Gli, wingless (wg/Wnt, decapentaplegic (dpp)/BMP, Hedgehog interacting protein (Hip), nodal, Smads, One-eyed pinhead (Oep), and TG-Interacting Factor (TGIF). However, further analysis is needed before their roles in HPE can be established. Here we present an overview of the presently known genes causing human holoprosencephaly and describe candidate genes involved in forebrain development identified in other systems. A model is discussed for how these genes may interact within and between several different signaling pathways to direct the formation of the forebrain.

Cerberus regulates left-right asymmetry of the embryonic head and heart

BACKGROUND

Most of the molecules known to regulate left-right asymmetry in vertebrate embryos are expressed on the left side of the future trunk region of the embryo. Members of the protein family comprising Cerberus and the putative tumour suppressor Dan have not before been implicated in left-right asymmetry. In Xenopus, these proteins have been shown to antagonise members of the transforming growth factor beta (TGF-beta) and Wnt families of signalling proteins.

RESULTS

Chick Cerberus (cCer) was found to be expressed in the left head mesenchyme and in the left flank of the embryo. Expression on the left side of the head was controlled by Sonic hedgehog (Shh) acting through the TGF-beta family member Nodal; in the flank, cCer was also regulated by Shh, but independently of Nodal. Surprisingly, although no known targets of Cerberus are expressed asymmetrically on the right side of the embryo at these stages, misexpression of cCer on this side of the embryo led to upregulation of the transcription factor Pitx2 and reversal of the direction of heart and head turning, apparently as independent events. Consistent with the possibility that cCer may be acting on bilaterally expressed TGF-beta family members such as the bone morphogenetic proteins (BMPs), this result was mimicked by right-sided misexpression of the BMP antagonist, Noggin.

CONCLUSIONS

Our findings suggest that cCer maintains a delicate balance of different TGF-beta family members involved in laterality decisions, and reveal the existence of partially overlapping molecular pathways regulating left-right asymmetry in the head and trunk of the embryo.

Diverse initiation in a conserved left-right pathway?

Preceding stereotypical left-right asymmetric morphogenesis, asymmetric gene expression patterns of nodal and pitx2 are very similar in major groups of vertebrates. I propose that these conserved expression patterns are indicative of 'left-right' phylotypic stages' of development. It is not known whether these patterns are initiated by conserved or divergent developmental mechanisms.

Reconstitution of the organizer is both sufficient and required to re-establish a fully patterned body plan in avian embryos

Lateral blastoderm isolates (LBIs) at the late gastrula/early neurula stage (i.e., stage 3d/4) that lack Hensen's node (organizer) and primitive streak can reconstitute a functional organizer and primitive streak within 10–12 hours in culture. We used LBIs to study the initiation and regionalization of the body plan. A complete body plan forms in each LBI by 36 hours in culture, and normal craniocaudal, dorsoventral, and mediolateral axes are re-established. Thus, reconstitution of the organizer is sufficient to re-establish a fully patterned body plan. LBIs can be modified so that reconstitution of the organizer does not occur. In such modified LBIs, tissue-type specific differentiation (with the exception of heart differentiation) and reconstitution of the body plan fail to occur. Thus, the reconstitution of the organizer is not only sufficient to re-establish a fully patterned body plan, it is also required. Finally, our results show that formation and patterning of the heart is under the control of the organizer, and that such control is exerted during the early to mid-gastrula stages (i.e., stages 2–3a), prior to formation of the fully elongated primitive streak.

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.