Tbx6 regulates left/right patterning in mouse embryos through effects on nodal cilia and perinodal signaling

Live imaging of mouse embryos

The development of the mouse embryo is a dynamic process that requires the spatial and temporal coordination of multiple cell types as they migrate, proliferate, undergo apoptosis, and differentiate to form complex structures. However, the confined nature of embryos as they develop in utero limits our ability to observe these morphogenetic events in vivo. Previous work has used fixed samples and histological methods such as immunofluorescence or in situ hybridization to address expression or localization of a gene of interest within a developmental time line. However, such methods do not allow us to follow the complex, dynamic movements of individual cells as the embryo develops. Genetic manipulation methods now allow us to label virtually any cell type or protein of interest fluorescently, providing powerful insights into morphogenetic events at cellular and subcellular resolutions. The development of ex vivo embryo culture methods combined with high-resolution imaging now provides a strong platform for observing morphogenetic events as they occur within the developing embryo. In this article, we discuss the advantages of live embryo imaging for observing dynamic morphogenetic events in vivo.

The ATP-sensitive K(+)-channel (K(ATP)) controls early left-right patterning in Xenopus and chick embryos

Consistent left-right asymmetry requires specific ion currents. We characterize a novel laterality determinant in Xenopus laevis: the ATP-sensitive K(+)-channel (K(ATP)). Expression of specific dominant-negative mutants of the Xenopus Kir6.1 pore subunit of the K(ATP) channel induced randomization of asymmetric organ positioning. Spatio-temporally controlled loss-of-function experiments revealed that the K(ATP) channel functions asymmetrically in LR patterning during very early cleavage stages, and also symmetrically during the early blastula stages, a period when heretofore largely unknown events transmit LR patterning cues. Blocking K(ATP) channel activity randomizes the expression of the left-sided transcription of Nodal. Immunofluorescence analysis revealed that XKir6.1 is localized to basal membranes on the blastocoel roof and cell-cell junctions. A tight junction integrity assay showed that K(ATP) channels are required for proper tight junction function in early Xenopus embryos. We also present evidence that this function may be conserved to the chick, as inhibition of K(ATP) in the primitive streak of chick embryos randomizes the expression of the left-sided gene Sonic hedgehog. We propose a model by which K(ATP) channels control LR patterning via regulation of tight junctions.

Dissecting the role of Fgf signaling during gastrulation and left-right axis formation in mouse embryos using chemical inhibitors

Fgf signaling plays pivotal roles in mouse gastrulation and left-right axis formation. However, although genetic analyses have revealed important aspects of Fgf signaling in these processes, the temporal resolution of genetic studies is low. Here, we combined whole-embryo culture with application of chemical compounds to inhibit Fgf signaling at specific time points. We found that sodium chlorate and PD173074 are potent inhibitors of Fgf signaling in early mouse embryos. Fgf signaling is required for the epithelial-to-mesenchymal transition of the primitive streak before the onset of gastrulation. Once gastrulation begins, Fgf signaling specifies mesodermal fates via the Ras/MAPK downstream cascade. Finally, Fgf signaling on the posterior side of the embryo during gastrulation induces Nodal expression in the node via Tbx6-Dll1, the initial event required for Nodal expression in the left lateral plate mesoderm.

Left-right patterning in the mouse requires Epb4.1l5-dependent morphogenesis of the node and midline

The mouse node is a transient early embryonic structure that is required for left-right asymmetry and for generation of the axial midline, which patterns neural and mesodermal tissues. The node is a shallow teardrop-shaped pit that sits at the distal tip of the early headfold (e7.75) embryo. The shape of the node is believed to be important for generation of the coherent leftward fluid flow required for initiation of left-right asymmetry, but little is known about the morphogenesis of the node. Here we show that the FERM domain protein Lulu/Epb4.1l5 is required for left-right asymmetry in the early mouse embryo. Unlike other genes previously shown to be required for left-right asymmetry in the mouse, lulu is not required for specification of node cell identity, for Nodal signaling in the node or for ciliogenesis. Instead, lulu is required for proper morphogenesis of the node and midline. The precursors of the wild-type node undergo a series of rapid morphological transitions. First, node precursors arise from an epithelial-to-mesenchymal transition at the anterior primitive streak. While in the mesenchymal layer, the node precursors form several ciliated rosette-like clusters; they then rapidly undergo a mesenchymal-to-epithelial transition to insert into the outer, endodermal layer of the embryo. In lulu mutants, node precursor cells are specified and form clusters, but those clusters fail to coalesce to make a single continuous node epithelium. The data suggest that the assembly of the contiguous node epithelium from mesenchymal clusters requires a rapid reorganization of apical-basal polarity that depends on Lulu/Epb4.1l5.

Transcriptional control of left-right patterning in cardiac development

The heart develops from a simple left-right (L-R) symmetrical tube. Through a complex process of looping and remodelling, it becomes a highly L-R asymmetrical organ with distinct asymmetries in both morphology and function. Abnormal cardiac L-R patterning can result in a spectrum of defects that include, dextrocardia (a malposition of the heart to the right), isomerism of the atria (both atria being morphologically right-sided or left-sided), abnormal ventricular topology (e.g. the morphological left ventricle being dextral to the morphological right ventricle) or mirror-image topology (associated with situs inversus). Intermediate forms include abnormalities such as situs ambiguus and heterotaxia. L-R patterning abnormalities are typically associated with cardiac malformations, and it has become clear that an isolated septal, outflow tract and aortic arch malformation may be the only presenting manifestation of an L-R patterning defect. In the last two decades, there have been seminal advances in our understanding of the mechanisms controlling L-R patterning, and how mutations in L-R patterning genes result in human cardiac malformation. In this review, we provide an overview of the transcriptional mechanisms that result in asymmetric gene activation in mammals, how they receive information from signalling pathways, and how this translates to abnormal cardiac development.

Normal bias in the direction of fetal rotation depends on blastomere composition during early cleavage in the mouse

Interest in establishing the basis of left/right asymmetry during embryogenesis has burgeoned in recent years. Relevant studies in mammals, focused largely on the mouse, have revealed involvement of a variety of genes that are common to the process in other animals. In the mouse, lateral differences in gene expression are first evident late in gastrulation when directional rotation of nodal cilia has been implicated in effecting the normally very strong bias in handedness. Reconstructing cleavage stages with correspondingly positioned blastomeres from appropriate numbers of conceptuses with similar division planes provides a way of testing whether they differ in potency without the confounding effects of reduced cell number. In a study using this strategy, 4-cell stage conceptuses reconstructed from blastomeres produced by equatorial as opposed to meridional second cleavage were found to be compromised in their ability to support normal development. Here, in more refined reconstructions undertaken at both the 4- and 8-cell stage, no significant impairment of development to the 9^th or 12^th day of gestation was found for products of equatorial second cleavage or their 8-cell stage progeny. Most surprisingly, however, a significant increase in reversal of the direction of axial rotation was found specifically among fetuses developing from conceptuses reconstructed from 8-cell stage progeny of products of equatorial second cleavage. Hence, manipulations during early cleavage some 6 days before fetal asymmetries are first evident can perturb the normally very strong bias in specification of a facet of left-right asymmetry.

Imaging fluid flow and cilia beating pattern in Xenopus brain ventricles

Brain development and health depends upon the efficient movement of the cerebrospinal fluid inside of brain ventricles. When disrupted either through mutation, disease, or physiological damage, brain function becomes significantly impaired. Here I present a simple method of following cerebrospinal fluid circulation in Xenopus tadpoles using fluorescent microspheres which can be applied to imaging fluid circulation in any transparent embryo. In particular, cilia may be labeled with these microspheres to study their dynamics and movement patterns in vivo while simultaneously measuring bulk fluid flow. This technique will facilitate the analysis of fluid dynamics in developing embryos and aid in understanding the regulation of cilia dependent fluid flow in vivo.

The dynamic cilium in human diseases

Cilia are specialized organelles protruding from the cell surface of almost all mammalian cells. They consist of a basal body, composed of two centrioles, and a protruding body, named the axoneme. Although the basic structure of all cilia is the same, numerous differences emerge in different cell types, suggesting diverse functions. In recent years many studies have elucidated the function of 9+0 primary cilia. The primary cilium acts as an antenna for the cell, and several important pathways such as Hedgehog, Wnt and planar cell polarity (PCP) are transduced through it. Many studies on animal models have revealed that during embryogenesis the primary cilium has an essential role in defining the correct patterning of the body. Cilia are composed of hundreds of proteins and the impairment or dysfunction of one protein alone can cause complete loss of cilia or the formation of abnormal cilia. Mutations in ciliary proteins cause ciliopathies which can affect many organs at different levels of severity and are characterized by a wide spectrum of phenotypes. Ciliary proteins can be mutated in more than one ciliopathy, suggesting an interaction between proteins. To date, little is known about the role of primary cilia in adult life and it is tempting to speculate about their role in the maintenance of adult organs. The state of the art in primary cilia studies reveals a very intricate role. Analysis of cilia-related pathways and of the different clinical phenotypes of ciliopathies helps to shed light on the function of these sophisticated organelles. The aim of this review is to evaluate the recent advances in cilia function and the molecular mechanisms at the basis of their activity.

Live-imaging fluorescent proteins in mouse embryos: multi-dimensional, multi-spectral perspectives

Microscopy has always been an obligate tool in the field of developmental biology, a goal of which is to elucidate the essential cellular and molecular interactions that coordinate the specification of different cell types and the establishment of body plans. The 2008 Nobel Prize in chemistry was awarded 'for the discovery and development of the green fluorescent protein, GFP' in recognition that the discovery of genetically encoded fluorescent proteins (FPs) has spearheaded a revolution in applications for imaging of live cells. With the development of more-sophisticated imaging technology and availability of FPs with different spectral characteristics, dynamic processes can now be live-imaged at high resolution in situ in embryos. Here, we review some recent advances in this rapidly evolving field as applied to live-imaging capabilities in the mouse, the most genetically tractable mammalian model organism for embryologists.

Teasing out T-box targets in early mesoderm

T-box transcription factor genes are widely conserved in metazoan development and widely involved in developmental processes. With the phase of T-box gene discovery winding down, the phase of transcriptional target discovery for T-box transcription factors is finally taking off and yielding rich rewards. Mutant phenotypes in mouse and zebrafish as well as morpholino studies in zebrafish have helped to link the T-box genes to a variety of signaling pathways through diverse target genes and feedback loops. Particularly in early mesoderm development, it is emerging that a network of T-box genes interacts with Wnt/beta-catenin and Notch/Delta signaling pathways, among others, to control the important processes of mesoderm specification, somite segmentation, and left/right body axis determination.

Cilia multifunctional organelles at the center of vertebrate left-right asymmetry

Cilia establish the vertebrate left-right (LR) axis and are integral to the development and function of the kidney, liver, and brain. Left-right asymmetry is established in the ciliated ventral node cells of the mouse. The chiral structure of the cilium provides a reference asymmetry to impose handed LR asymmetric development on the bilaterally symmetric vertebrate embryo. A ciliary mechanism of LR development is evolutionarily conserved, as ciliated organs essential to LR axis formation, called LR organizers, are found in other vertebrates, including rabbit, fish, and Xenopus. Mice with mutations affecting ciliary biogenesis, motility, or sensory function have abnormal LR development and abnormal development of the heart. The axonemal dynein heavy chain left-right dynein (lrd) localizes to the LR organizer and drives counterclockwise movement of node primary cilia. Node primary cilia are an admixture of 9 + 2 and 9 + 0 cilia. Mutations in lrd result in structurally normal, immotile node monocilia. In the mouse, coordinated, directional beating of motile node monocilia at the neural fold stage generates leftward flow of extraembryonic fluid surrounding the node (nodal flow). Nodal flow triggers a rise in intracellular calcium in cells at the left side of the node. The perinodal asymmetric rise in intracellular calcium generated by nodal flow subsequently leads to asymmetric gene expression and morphogenesis.