Insights into the establishment of left–right asymmetries in vertebrates

Impact of gastrointestinal comorbidities in patients with right and left atrial isomerism

BACKGROUND AND AIM

Heterotaxy syndrome, being right atrial isomerism (RAI) or left atrial isomerism (LAI), often presents with Congenital Heart Disease (CHD). Intestinal abnormalities, including malrotation are common. We assessed the spectrum of gut abnormalities and their impact on medium-term outcome in a cohort of patients with fetal and postnatal diagnoses of heterotaxy syndrome.

METHODS

We reviewed the cardiology records of heterotaxy syndrome patients from two centres, regarding the presence of CHD, time for cardiac intervention, presence of gastrointestinal abnormalities, and type/time of surgery. A questionnaire about gastrointestinal status was sent to patients <18 years old. Kaplan-Meier curves were derived for survival data and freedom from intervention.

RESULTS

Data were included for 182 patients (49 RAI and 133 LAI) of 247 identified. Questionnaires were sent to 77 families and 47 replied. CHD was present in all RAI and 61.7% of LAI cases. Thirty-eight patients had abdominal surgery (20.9%), similar for RAI and LAI (20.4% versus 21%, p> 0.99): Ladd procedure in 17 (44.7%), non-Ladd in 12 (31.5%), and both procedures in 9 (23.7%). Ten-year freedom from Ladd procedure for all was 86% for the whole cohort (RAI = 87%; LAI = 85%, p = 0.98). Freedom from any gastrointestinal surgery at 10 years was 79% for the whole cohort (RAI = 77%; LAI = 80%, p = 0.54). Ten-year freedom from cardiac surgery was 31% for the whole cohort (RAI = 6%; LAI = 43%, p < 0.0001).

CONCLUSIONS

In our cohort, one in five patients required abdominal surgery, mostly in their first year of life, similar for RAI and LAI. Between 1 and 10 years of follow-up, the impact of gastrointestinal abnormalities on outcome was minimal. Medium term survival was related to CHD.

The myosin ID pathway and left-right asymmetry in Drosophila

Drosophila is a classical model to study body patterning, however left-right (L/R) asymmetry had remained unexplored, until recently. The discovery of the conserved myosin ID gene as a major determinant of L/R asymmetry has revealed a novel L/R pathway involving the actin cytoskeleton and the adherens junction. In this process, the HOX gene Abdominal-B plays a major role through the control of myosin ID expression and therefore symmetry breaking. In this review, we present organs and markers showing L/R asymmetry in Drosophila and discuss our current understanding of the underlying molecular genetic mechanisms. Drosophila represents a valuable model system revealing novel strategies to establish L/R asymmetry in invertebrates and providing an evolutionary perspective to the problem of laterality in bilateria.

Sept6 is required for ciliogenesis in Kupffer's vesicle, the pronephros, and the neural tube during early embryonic development

Septins are conserved filament-forming GTP-binding proteins that act as cellular scaffolds or diffusion barriers in a number of cellular processes. However, the role of septins in vertebrate development remains relatively obscure. Here, we show that zebrafish septin 6 (sept6) is first expressed in the notochord and then in nearly all of the ciliary organs, including Kupffer's vesicle (KV), the pronephros, eye, olfactory bulb, and neural tube. Knockdown of sept6 in zebrafish embryos results in reduced numbers and length of cilia in KV. Consequently, cilium-related functions, such as the left-right patterning of internal organs and nodal/spaw signaling, are compromised. Knockdown of sept6 also results in aberrant cilium formation in the pronephros and neural tube, leading to cilium-related defects in pronephros development and Sonic hedgehog (Shh) signaling. We further demonstrate that SEPT6 associates with acetylated α-tubulin in vivo and localizes along the axoneme in the cilia of zebrafish pronephric duct cells as well as cultured ZF4 cells. Our study reveals a novel role of sept6 in ciliogenesis during early embryonic development in zebrafish.

Multi-modal effects of BMP signaling on Nodal expression in the lateral plate mesoderm during left-right axis formation in the chick embryo

During development of left-right asymmetry in the vertebrate embryo, Nodal plays a central role for determination of left-handedness. Bone morphogenetic protein (BMP) signaling has an important role for regulation of Nodal expression, although there is controversy over whether BMP signaling has a positive or negative effect on Nodal expression in the chick embryo. As BMP is a morphogen, we speculated that different concentrations might induce different responses in the cells of the lateral plate mesoderm (LPM). To test this hypothesis, we analyzed the effects of various concentrations of BMP4 and NOGGIN on Nodal expression in the LPM. We found that the effect on Nodal expression varied in a complex fashion with the concentration of BMP. In agreement with previous reports, we found that a high level of BMP signaling induced Nodal expression in the LPM, whereas a low level inhibited expression. However, a high intermediate level of BMP signaling was found to suppress Nodal expression in the left LPM, whereas a low intermediate level induced Nodal expression in the right LPM. Thus, the high and the low intermediate levels of BMP signaling up-regulated Nodal expression, but the high intermediate and low levels of BMP signaling down-regulated Nodal expression. Next, we sought to identify the mechanisms of this complex regulation of Nodal expression by BMP signaling. At the low intermediate level of BMP signaling, regulation depended on a NODAL positive-feedback loop suggesting the possibility of crosstalk between BMP and NODAL signaling. Overexpression of a constitutively active BMP receptor, a constitutively active ACTIVIN/NODAL receptor and SMAD4 indicated that SMAD1 and SMAD2 competed for binding to SMAD4 in the cells of the LPM. Nodal regulation by the high and low levels of BMP signaling was dependent on Cfc up-regulation or down-regulation, respectively. We propose a model for the variable effects of BMP signaling on Nodal expression in which different levels of BMP signaling regulate Nodal expression by a balance between BMP-pSMAD1/4 signaling and NODAL-pSMAD2/4 signaling.

Autotaxin/Lpar3 signaling regulates Kupffer's vesicle formation and left-right asymmetry in zebrafish

Left-right (L-R) patterning is essential for proper organ morphogenesis and function. Calcium fluxes in dorsal forerunner cells (DFCs) are known to regulate the formation of Kupffer's vesicle (KV), a central organ for establishing L-R asymmetry in zebrafish. Here, we identify the lipid mediator lysophosphatidic acid (LPA) as a regulator of L-R asymmetry in zebrafish embryos. LPA is produced by Autotaxin (Atx), a secreted lysophospholipase D, and triggers various cellular responses through activation of specific G protein-coupled receptors (Lpar1-6). Knockdown of Atx or LPA receptor 3 (Lpar3) by morpholino oligonucleotides perturbed asymmetric gene expression in lateral plate mesoderm and disrupted organ L-R asymmetries, whereas overexpression of lpar3 partially rescued those defects in both atx and lpar3 morphants. Similar defects were observed in embryos treated with the Atx inhibitor HA130 and the Lpar1-3 inhibitor Ki16425. Knockdown of either Atx or Lpar3 impaired calcium fluxes in DFCs during mid-epiboly stage and compromised DFC cohesive migration, KV formation and ciliogenesis. Application of LPA to DFCs rescued the calcium signal and laterality defects in atx morphants. This LPA-dependent L-R asymmetry is mediated via Wnt signaling, as shown by the accumulation of β-catenin in nuclei at the dorsal side of both atx and lpar3 morphants. Our results suggest a major role for the Atx/Lpar3 signaling axis in regulating KV formation, ciliogenesis and L-R asymmetry via a Wnt-dependent pathway.

Defective Nodal and Cerl2 expression in the Arl13b(hnn) mutant node underlie its heterotaxia

Specification of the left-right axis during embryonic development is critical for the morphogenesis of asymmetric organs such as the heart, lungs, and stomach. The first known left-right asymmetry to occur in the mouse embryo is a leftward fluid flow in the node that is created by rotating cilia on the node surface. This flow is followed by asymmetric expression of Nodal and its inhibitor Cerl2 in the node. Defects in cilia and/or fluid flow in the node lead to defective Nodal and Cerl2 expression and therefore incorrect visceral organ situs. Here we show the cilia protein Arl13b is required for left right axis specification as its absence results in heterotaxia. We find the defect originates in the node where Cerl2 is not downregulated and asymmetric expression of Nodal is not maintained resulting in symmetric expression of both genes. Subsequently, Nodal expression is delayed in the lateral plate mesoderm (LPM). Symmetric Nodal and Cerl2 in the node could result from defects in either the generation and/ or the detection of Nodal flow, which would account for the subsequent defects in the LPM and organ positioning.

Vertebrate segmentation: from cyclic gene networks to scoliosis

One of the most striking features of the human vertebral column is its periodic organization along the anterior-posterior axis. This pattern is established when segments of vertebrates, called somites, bud off at a defined pace from the anterior tip of the embryo's presomitic mesoderm (PSM). To trigger this rhythmic production of somites, three major signaling pathways--Notch, Wnt/β-catenin, and fibroblast growth factor (FGF)--integrate into a molecular network that generates a traveling wave of gene expression along the embryonic axis, called the "segmentation clock." Recent systems approaches have begun identifying specific signaling circuits within the network that set the pace of the oscillations, synchronize gene expression cycles in neighboring cells, and contribute to the robustness and bilateral symmetry of somite formation. These findings establish a new model for vertebrate segmentation and provide a conceptual framework to explain human diseases of the spine, such as congenital scoliosis.

Development of the venous pole of the heart in the frog Xenopus laevis: a morphological study with special focus on the development of the venoatrial connections

The heart of lung-breathing vertebrates normally shows an asymmetric arrangement of its venoatrial connections along the left-right (L-R) body axis. The systemic venous tributaries empty into the right atrium while the pulmonary venous tributaries empty into the left atrium. The ways by which this asymmetry evolves from the originally symmetrically arranged embryonic venous heart pole are poorly defined. Here we document the development of the venous heart pole in Xenopus laevis (stages 40-46). We show that, prior to the appearance of the mouth of the common pulmonary vein (MCPV), the systemic venous tributaries empty into a bilaterally symmetric chamber (sinus venosus) that is demarcated from the developing atriums by a circular ridge of tissue (sinu-atrial ridge). A solitary MCPV appears during stage 41. From the time point of its first appearance onwards, the MCPV lies cranial to the sinu-atrial ridge and to the left of the developing interatrial septum and body midline. L-R lineage analysis shows that the interatrial septum and MCPV both derive from the left body half. The CPV, therefore, opens from the beginning into the future left atrium. The definitive venoatrial connections are established by the formation of a septal complex that divides the lumen of the venous heart pole into systemic and pulmonary venous flow pathways. This complex arises from the anlage of the interatrial septum and the left half of the sinu-atrial ridge.

Sox17 and chordin are required for formation of Kupffer's vesicle and left-right asymmetry determination in zebrafish

Kupffer's vesicle (KV), a ciliated fluid-filled sphere in the zebrafish embryo with a critical role in laterality determination, is derived from a group of superficial cells in the organizer region of the gastrula named the dorsal forerunner cells (DFC). We have examined the role of the expression of sox17 and chordin (chd) in the DFC in KV formation and laterality determination. Whereas sox17 was known to be expressed in DFC, its function in these cells was not studied before. Further, expression of chd in these cells has not been reported previously. Targeted knockdown of Sox17 and Chd in DFC led to aberrant Left-Right (L-R) asymmetry establishment, as visualized by the expression of southpaw and lefty, and heart and pancreas placement in the embryo. These defects correlated with the formation of small KVs with apparently diminished cilia, consistent with the known requirement for ciliary function in the laterality organ for the establishment of L-R asymmetry.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Rotation of organizer tissue contributes to left-right asymmetry

Current hypotheses regarding vertebrate left-right asymmetry patterns are based on the presumption that genetic regulatory networks specify sidedness via extracellular morphogens and/or ciliary activity. We show empirical time-lapse evidence for an asymmetric rotation of epiblastic nodal tissue in avian embryos. This rotation spans the interval when initial symmetric expression of Shh and Fgf8 becomes asymmetrical with respect to the midline.

FGF-dependent left-right asymmetry patterning in zebrafish is mediated by Ier2 and Fibp1

Establishment of left-right asymmetry in vertebrates requires nodal, Wnt-PCP and FGF signaling and involves ciliogenesis in a laterality organ. Effector genes through which FGF signaling affects laterality have not been described. We isolated the zebrafish ier2 and fibp1 genes as FGF target genes and show that their protein products interact. Knock down of these factors interferes with establishment of organ laterality and causes defective cilia formation in Kupffer's Vesicle, the zebrafish laterality organ. Cilia are also lost after suppression of FGF8, but can be rescued by injection of ier2 and fibp1 mRNA. We conclude that Ier2 and Fibp1 mediate FGF signaling in ciliogenesis in Kupffer's Vesicle and in the establishment of laterality in the zebrafish embryo.

Balancing segmentation and laterality during vertebrate development

Somites are the mesodermal segments of vertebrate embryos that become the vertebral column, skeletal muscle and dermis. Somites arise within the paraxial mesoderm by the periodic, bilaterally symmetric process of somitogenesis. However, specification of left-right asymmetry occurs in close spatial and temporal proximity to somitogenesis and involves some of the same cell signaling pathways that govern segmentation. Here, we review recent evidence that identifies cross-talk between these processes and that demonstrates a role for retinoic acid in maintaining symmetrical somitogenesis by preventing impingement of left-right patterning signals upon the paraxial mesoderm.