A molecular pathway determining left-right asymmetry in chick embryogenesis

Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology

It has been estimated that 25% of monozygotic ("identical") twin pairs exhibit reverse asymmetry (RA) or "mirroring" of minor anatomical features as a result of delayed zygote division. Here, we examine whether identical twin mirroring accounts for patterns of dental asymmetry in a sample of monozygotic and dizygotic ("fraternal") twins. We focus on crown morphology to approach the following question: is there an association between dental RA frequency and twin type suggestive of the presence of mirror image twins in our sample? Data were collected from 208 deciduous and 196 permanent dentitions of participants of the University of Adelaide Twin Study using Arizona State University Dental Anthropology System standards. RA frequencies were compared across morphological complexes (deciduous, permanent), twin types (monozygotic, dizygotic), and traits. Fisher's exact tests were performed to formally evaluate the association between twin type and dental RA. Across the entire dataset, RA rates failed to exceed 8% for any twin type. In monozygotic twins, deciduous mirroring totaled 5.3% of observed cases, while permanent mirroring totaled 7.8% of observed cases. We found no statistically significant association between RA and twin type for any morphological character (p-value range: 0.07-1.00). Our results suggest the timing of monozygotic twin division does not explain the structure of asymmetry for our morphology dataset and that published estimates of identical twin mirroring rates may be inflated or contingent upon phenotype. Instead, rates reported for this sample more closely align with the proposed etiology of this condition.

Bilateral cellular flows display asymmetry prior to left-right organizer formation in amniote gastrulation

A bilateral body plan is predominant throughout the animal kingdom. Bilaterality of amniote embryos becomes recognizable as midline morphogenesis begins at gastrulation, bisecting an embryonic field into the left and right sides, and left-right asymmetry patterning follows. While a series of laterality genes expressed after the left-right compartmentalization has been extensively studied, the laterality patterning prior to and at the initiation of midline morphogenesis has remained unclear. Here, through a biophysical quantification in a high spatial and temporal resolution, applied to a chick model system, we show that a large-scale bilateral counter-rotating cellular flow, termed as 'polonaise movements', display left-right asymmetries in early gastrulation. This cell movement starts prior to the formation of the primitive streak, which is the earliest midline structure, and earlier than expression of laterality genes. The cellular flow speed and vorticity unravel the location and timing of the left-right asymmetries. The bilateral flows displayed a Right dominance after six hours since the start of cell movements. Mitotic arrest that diminishes primitive streak formation resulted in changes in the bilateral flow pattern, but the Right dominance persisted. Our data indicate that the left-right asymmetry in amniote gastrula becomes detectable prior to the point when the asymmetric regulation of the laterality signals at the node leads to the left-right patterning. More broadly, our results suggest that physical processes can play an unexpected but significant role in influencing left-right laterality during embryonic development.

Significance Statement

Bilaterians are defined by a bilaterally symmetrical body plan. Vertebrates exhibit external bilateral symmetry but display left-right (LR) asymmetry in their internal organs. In amniote embryos, the initiation of LR symmetry breaking is not well understood. Here, we study LR symmetry breaking in the chick embryo due to its easy accessibility and similarity to human development. Our biophysical approaches to quantify cellular flows inferred that LR symmetry breaking occurs prior to the formation of Hensen's node, a LR organizer, which serves as a signaling center for LR patterning programs. Our work demonstrates that quantitative biophysical parameters can help unravel the initiation of LR symmetry breaking, suggesting involvement of physical mechanisms in this critical biological patterning process.

The evolution of developmental biology through conceptual and technological revolutions

Developmental biology-the study of the processes by which cells, tissues, and organisms develop and change over time-has entered a new golden age. After the molecular genetics revolution in the 80s and 90s and the diversification of the field in the early 21st century, we have entered a phase when powerful technologies provide new approaches and open unexplored avenues. Progress in the field has been accelerated by advances in genomics, imaging, engineering, and computational biology and by emerging model systems ranging from tardigrades to organoids. We summarize how revolutionary technologies have led to remarkable progress in understanding animal development. We describe how classic questions in gene regulation, pattern formation, morphogenesis, organogenesis, and stem cell biology are being revisited. We discuss the connections of development with evolution, self-organization, metabolism, time, and ecology. We speculate how developmental biology might evolve in an era of synthetic biology, artificial intelligence, and human engineering.

Regulation of long-range BMP gradients and embryonic polarity by propagation of local calcium-firing activity

Many amniote vertebrate species including humans can form identical twins from a single embryo, but this only occurs rarely. It has been suggested that the primitive-streak-forming embryonic region emits signals that inhibit streak formation elsewhere but the signals involved, how they are transmitted and how they act has not been elucidated. Here we show that short tracks of calcium firing activity propagate through extraembryonic tissue via gap junctions and prevent ectopic primitive streak formation in chick embryos. Cross-regulation of calcium activity and an inhibitor of primitive streak formation (Bone Morphogenetic Protein, BMP) via NF-κB and NFAT establishes a long-range BMP gradient spanning the embryo. This mechanism explains how embryos of widely different sizes can maintain positional information that determines embryo polarity. We provide evidence for similar mechanisms in two different human embryo models and in Drosophila, suggesting an ancient evolutionary origin.

Shh-Gli2-Runx2 inhibits vascular calcification

BACKGROUND

In patients with chronic kidney disease (CKD), vascular calcification (VC) is common and is associated with a higher risk of all-cause mortality. Shh, one ligand for Hedgehog (Hh) signaling, participates in osteogenesis and several cardiovascular diseases. However, it remains unclear whether Shh is implicated in the development of VC.

METHODS

Inorganic phosphorus 2.6 mM was used to induce vascular smooth muscle cells (VSMCs) calcification. Mice were fed with adenine diet supplement with 1.2% phosphorus to induce VC.

RESULTS

Shh was decreased in VSMCs exposed to inorganic phosphorus, calcified arteries in mice fed with an adenine diet, as well as radial arteries from patients with CKD presenting VC. Overexpression of Shh inhibited VSMCs ostosteoblastic differentiation and calcification, whereas its silencing accelerated these processes. Likewise, mice treated with smoothened agonist (SAG; Hh signaling agonist) showed alleviated VC, and mice treated with cyclopamine (CPN; Hh signaling antagonist) exhibited severe VC. Additionally, overexpression of Gli2 significantly reversed the pro-calcification effect of Shh silencing on VSMCs, suggesting that Shh inhibited VC via Gli2. Mechanistically, Gli2 interacted with Runx2 and promoted its ubiquitin proteasomal degradation, therefore protecting against VC. Of interest, the pro-degradation effect of Gli2 on Runx2 was independent of Smurf1 and Cullin4B.

CONCLUSIONS

Our study provided deeper insight to the pathogenesis of VC, and Shh might be a novel potential target for VC treatment.

Molecular Pathways and Animal Models of Defects in Situs

Left-right patterning is among the least well understood of the three axes defining the body plan, and yet it is no less important, with left-right patterning defects causing structural birth defects with high morbidity and mortality, such as complex congenital heart disease, biliary atresia, or intestinal malrotation. The cell signaling pathways governing left-right asymmetry are highly conserved and involve multiple components of the TGF-β superfamily of cell signaling molecules. Central to left-right patterning is the differential activation of Nodal on the left, and BMP signaling on the right. In addition, a plethora of other cell signaling pathways including Shh, FGF, and Notch also contribute to the regulation of left-right patterning. In vertebrate embryos such as the mouse, frog, or zebrafish, the specification of left-right identity requires the left-right organizer (LRO) containing cells with motile and primary cilia that mediate the left-sided propagation of Nodal signaling, followed by left-sided activation of Lefty and then Pitx2, a transcription factor that specifies visceral organ asymmetry. While this overall scheme is well conserved, there are striking species differences, including the finding that motile cilia do not play a role in left-right patterning in some vertebrates. Surprisingly, the direction of heart looping, one of the first signs of organ left-right asymmetry, was recently shown to be specified by intrinsic cell chirality, not Nodal signaling, possibly a reflection of the early origin of Nodal signaling in radially symmetric organisms. How this intrinsic chirality interacts with downstream molecular pathways regulating visceral organ asymmetry will need to be further investigated to elucidate how disturbance in left-right patterning may contribute to complex CHD.

A collagen-rich arch in the urochordate notochord coordinates cell shaping and multi-tissue elongation

Cell and tissue reshaping is crucial for coordinating three-dimensional pattern formation, in which the size and shape of the cells must be accurately regulated via signal transport and communication among tissues. However, the identity of signaling and transportation mechanisms in this process remains elusive. In our study, we identified an extracellular matrix (ECM) structure with a vertebra-like shape surrounding the central notochord tissue in the larval tail of the urochordate Ciona. Additionally, we verified that the ECM structure was formed de novo, mainly from collagens secreted by notochord cells. Fluorescence recovery after photobleaching and simulation results revealed that this structure was formed via diffusional collagen flow from a notochord that was restricted and molded in the spaces among tail tissues. We revealed that the collagen structure was essential for notochord cell arrangement and elongation. Furthermore, we observed that the central notochord connects with the epidermis through this ECM structure. The disruption of this structure by collagen knockdown and loss-of-collagen function caused the failure of notochord elongation. More importantly, the epidermis could not elongate proportionally with notochord, indicating that the collagen-rich structure serves as a scaffold to coordinate the concurrent elongation of the tail tissues. These findings provide insights into how the central tissue forms and molds its surrounding ECM structure, by not only regulating its own morphogenesis but also functioning as a scaffold for signal transmission to orchestrate the coordinated morphologic reshaping of the surrounding tissues.

Prenatal CFAP53-related laterality defect: case report and review of the literature

Laterality defects include morphological anomalies with impaired left-right asymmetry induction, such as dextrocardia, situs inversus abdominis, situs inversus totalis and situs ambiguus. The different arrangement of major organs is called heterotaxy. We describe for the first time a fetus with situs viscerum inversus and azygos continuation of the inferior vena cava, due to previously unreported variants in compound heterozygosity in the CFAP53 gene, whose product is implied in cilial motility. Prenatal trio exome sequencing was performed with turn-around time during the pregnancy. The fetuses with laterality defects are suitable candidates for prenatal exome sequencing due to the emerging high diagnostic rate of this group of morphological anomalies. A timely molecular diagnosis plays a fundamental role in genetic counseling, regarding couple decisions on the ongoing pregnancy, providing recurrence risks, and in predicting possible respiratory complications due to ciliary dyskinesia.

Translational control of furina by an RNA regulon is important for left-right patterning, heart morphogenesis and cardiac valve function

Heart development is a complex process that requires asymmetric positioning of the heart, cardiac growth and valve morphogenesis. The mechanisms controlling heart morphogenesis and valve formation are not fully understood. The pro-convertase FurinA functions in heart development across vertebrates. How FurinA activity is regulated during heart development is unknown. Through computational analysis of the zebrafish transcriptome, we identified an RNA motif in a variant FurinA transcript harbouring a long 3' untranslated region (3'UTR). The alternative 3'UTR furina isoform is expressed prior to organ positioning. Somatic deletions in the furina 3'UTR lead to embryonic left-right patterning defects. Reporter localisation and RNA-binding assays show that the furina 3'UTR forms complexes with the conserved RNA-binding translational repressor, Ybx1. Conditional ybx1 mutant embryos show premature and increased Furin reporter expression, abnormal cardiac morphogenesis and looping defects. Mutant ybx1 hearts have an expanded atrioventricular canal, abnormal sino-atrial valves and retrograde blood flow from the ventricle to the atrium. This is similar to observations in humans with heart valve regurgitation. Thus, the furina 3'UTR element/Ybx1 regulon is important for translational repression of FurinA and regulation of heart development.

Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here, we clarify the mechanisms underlying Vg1 retention, processing, secretion, and signaling and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: Chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as an in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers.

Hypoplastic Left Heart Syndrome: Signaling & Molecular Perspectives, and the Road Ahead

Hypoplastic left heart syndrome (HLHS) is a lethal congenital heart disease (CHD) affecting 8-25 per 100,000 neonates globally. Clinical interventions, primarily surgical, have improved the life expectancy of the affected subjects substantially over the years. However, the etiological basis of HLHS remains fundamentally unclear to this day. Based upon the existing paradigm of studies, HLHS exhibits a multifactorial mode of etiology mediated by a complicated course of genetic and signaling cascade. This review presents a detailed outline of the HLHS phenotype, the prenatal and postnatal risks, and the signaling and molecular mechanisms driving HLHS pathogenesis. The review discusses the potential limitations and future perspectives of studies that can be undertaken to address the existing scientific gap. Mechanistic studies to explain HLHS etiology will potentially elucidate novel druggable targets and empower the development of therapeutic regimens against HLHS in the future.

An atypical basement membrane forms a midline barrier in left-right asymmetric gut development

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo-without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are not well understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

Spatial transcriptome profiling uncovers metabolic regulation of left-right patterning

Left-right patterning disturbance can cause severe birth defects, but it remains least understood of the three body axes. We uncovered an unexpected role for metabolic regulation in left-right patterning. Analysis of the first spatial transcriptome profile of left-right patterning revealed global activation of glycolysis, accompanied by right-sided expression of Bmp7 and genes regulating insulin growth factor signaling. Cardiomyocyte differentiation was left-biased, which may underlie the specification of heart looping orientation. This is consistent with known Bmp7 stimulation of glycolysis and glycolysis suppression of cardiomyocyte differentiation. Liver/lung laterality may be specified via similar metabolic regulation of endoderm differentiation. Myo1d , found to be left-sided, was shown to regulate gut looping in mice, zebrafish, and human. Together these findings indicate metabolic regulation of left-right patterning. This could underlie high incidence of heterotaxy-related birth defects in maternal diabetes, and the association of PFKP, allosteric enzyme regulating glycolysis, with heterotaxy. This transcriptome dataset will be invaluable for interrogating birth defects involving laterality disturbance.

Novel homozygous variant in ARL2BP associated with retinitis pigmentosa, situs inversus, and male infertility in a Chinese patient

ARL2BP is a ciliary gene associated with multiple ciliopathy phenotypes. On comprehensive clinical examinations using molecular methods, we identified a Chinese patient from a consanguineous family carrying a novel homozygous variant c.22_23delAG (p.S8Lfs*10) in ARL2BP, presenting with retinitis pigmentosa (RP), situs inversus totalis, and oligozoospermia. Situs inversus and male infertility have never been reported in the same patient with ARL2BP variants; therefore, this a novel ARL2BP-associated phenotypic triad of RP, situs inversus, and male infertility. Moreover, this patient likely had olfactory dysfunction susceptibility and presented with anosmia. We found reduced patient-derived fibroblast proliferation and ciliary length. Our findings expand the genotypic spectrum and reveal abnormal cell proliferation and ciliogenesis in ARL2BP-associated patients.

Characterizing the origin of blood plasma proteins from organ tissues in rainbow trout (Oncorhynchus mykiss) using a comparative non-targeted proteomics approach

Protein expression patterns adapt to various cues to meet the needs of an organism. The dynamicity of an organism's proteome can therefore reveal information about an organism's health. Proteome databases contain limited information regarding organisms outside of medicinal biology. The UniProt human and mouse proteomes are extensively reviewed and ∼50 % of both proteomes include tissue specificity, while >99 % of the rainbow trout proteome lacks tissue specificity. This study aimed to expand knowledge on the rainbow trout proteome with a focus on understanding the origin of blood plasma proteins. Blood, brain, heart, liver, kidney, and gills were collected from adult rainbow trout, plasma and tissue proteins were analyzed using liquid chromatography tandem mass spectrometry. Over 10,000 proteins were identified across all groups. Our data indicated that the majority of the plasma proteome is shared amongst multiple tissue types, though 4-7 % of the plasma proteome is uniquely originated from each tissue (gill > heart > liver > kidney > brain).

Transpositions of the great arteries versus aortic dextropositions. A review of some embryogenetic and morphological aspects

This review examines and discusses the morphology and embryology of two main groups of conotruncal cardiac malformations: (a) transposition of the great arteries (complete transposition and incomplete/partial transposition namely double outlet right ventricle), and (b) aortic dextroposition defects (tetralogy of Fallot and Eisenmenger malformation). In both groups, persistent truncus arteriosus was included because maldevelopment of the neural crest cell supply to the outflow tract, contributing to the production of the persistent truncus arteriosus, is shared by both groups of malformations. The potentially important role of the proximal conal cushions in the rotatory sequence of the conotruncus is emphasized. Most importantly, this study emphasizes the differentiation between the double-outlet right ventricle, which is a partial or incomplete transposition of the great arteries, and the Eisenmenger malformation, which is an aortic dextroposition. Special emphasis is also given to the leftward shift of the conoventricular junction, which covers an important morphogenetic role in both aortic dextropositions and transposition defects as well as in normal development, and whose molecular genetic regulation seems to remain unclear at present. Emphasis is placed on the distinct and overlapping roles of Tbx1 and Pitx2 transcription factors in modulating the development of the cardiac outflow tract.

Unfolding a sequence of sensory influences and interactions in the development of functional brain laterality

Evidence of sensory experience influencing the development of lateralized brain and behavior is reviewed. The epigenetic role of light exposure during two specific stages of embryonic development of precocial avian species is a particular focus of the research discussed. Two specific periods of light sensitivity (in early versus late incubation), each depending on different subcellular and cellular processes, affect lateralized behavior after hatching. Auditory and olfactory stimulation during embryonic development is also discussed with consideration of interactions with light-generated visual lateralization.

Cilia function as calcium-mediated mechanosensors that instruct left-right asymmetry

The breaking of bilateral symmetry in most vertebrates is critically dependent upon the motile cilia of the embryonic left-right organizer (LRO), which generate a directional fluid flow; however, it remains unclear how this flow is sensed. Here, we demonstrated that immotile LRO cilia are mechanosensors for shear force using a methodological pipeline that combines optical tweezers, light sheet microscopy, and deep learning to permit in vivo analyses in zebrafish. Mechanical manipulation of immotile LRO cilia activated intraciliary calcium transients that required the cation channel Polycystin-2. Furthermore, mechanical force applied to LRO cilia was sufficient to rescue and reverse cardiac situs in zebrafish that lack motile cilia. Thus, LRO cilia are mechanosensitive cellular levers that convert biomechanical forces into calcium signals to instruct left-right asymmetry.

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

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

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

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

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

Pitx2 patterns an accelerator-brake mechanical feedback through latent TGFβ to rotate the gut

The vertebrate intestine forms by asymmetric gut rotation and elongation, and errors cause lethal obstructions in human infants. Rotation begins with tissue deformation of the dorsal mesentery, which is dependent on left-sided expression of the Paired-like transcription factor Pitx2. The conserved morphogen Nodal induces asymmetric Pitx2 to govern embryonic laterality, but organ-level regulation of Pitx2 during gut asymmetry remains unknown. We found Nodal to be dispensable for Pitx2 expression during mesentery deformation. Intestinal rotation instead required a mechanosensitive latent transforming growth factor-β (TGFβ), tuning a second wave of Pitx2 that induced reciprocal tissue stiffness in the left mesentery as mechanical feedback with the right side. This signaling regulator, an accelerator (right) and brake (left), combines biochemical and biomechanical inputs to break gut morphological symmetry and direct intestinal rotation.

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

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

Homeostases of epidermis and hair follicle, and development of basal cell carcinoma

Hedgehog signaling (Hh) plays a critical role in embryogenesis. On the other hand, its overactivity may cause basal cell carcinoma (BCC), the most common human cancer. Further, epidermal and hair follicle homeostases may have a key role in the development of BCC. This article describes the importance of different signaling pathways in the different stages of the two processes. The description of the homeostases brought up the importance of the Notch signaling along with the sonic hedgehog (Shh) and the Wnt pathways. Loss of the Notch signaling adversely affects the late stages of hair follicle formation and allows the bulge cells in the hair follicles to take the fate of the keratinocytes in the interfollicular epidermis. Further, the loss of Notch activity upregulates the Shh and Wnt activities, adversely affecting the homeostases. Notably, the Notch signaling is suppressed in BCC, and the peripheral BCC cells, which have low Notch activity, show drug resistance in comparison to the interior suprabasal BCC cells, which have high Notch activity.

Light-induced asymmetries in embryonic retinal gene expression are mediated by the vascular system and extracellular matrix

Left–right asymmetries in the nervous system (lateralisation) influence a broad range of behaviours, from social responses to navigation and language. The role and pathways of endogenous and environmental mechanisms in the ontogeny of lateralisation remains to be established. The domestic chick is a model of both endogenous and experience-induced lateralisation driven by light exposure. Following the endogenous rightward rotation of the embryo, the asymmetrical position in the egg results in a greater exposure of the right eye to environmental light. To identify the genetic pathways activated by asymmetric light stimulation, and their time course, we exposed embryos to different light regimes: darkness, 6 h of light and 24 h of light. We used RNA-seq to compare gene expression in the right and left retinas and telencephalon. We detected differential gene expression in right vs left retina after 6 h of light exposure. This difference was absent in the darkness condition and had already disappeared by 24 h of light exposure, suggesting that light-induced activation is a self-terminating phenomenon. This transient effect of light exposure was associated with a downregulation of the sensitive-period mediator gene DIO2 (iodothyronine deiodinase 2) in the right retina. No differences between genes expressed in the right vs. left telencephalon were detected. Gene networks associated with lateralisation were connected to vascularisation, cell motility, and the extracellular matrix. Interestingly, we know that the extracellular matrix—including the differentially expressed PDGFRB gene—is involved in morphogenesis, sensitive periods, and in the endogenous chiral mechanism of primary cilia, that drives lateralisation. Our data show a similarity between endogenous and experience-driven lateralisation, identifying functional gene networks that affect lateralisation in a specific time window.

Laterality in Horse Training: Psychological and Physical Balance and Coordination and Strength Rather Than Straightness

Simple Summary

For centuries, straightening a horse has been considered a key element in achieving its responsiveness and suppleness and has been a traditional goal in training. However, body asymmetry (natural crookedness), motor laterality (the preference for limbs on one side) and sensory laterality (the preference for sensory organs on one side) are naturally occurring phenomena. In humans, the forced correction of these imbalances, for example, forcing left-handed children to write with their right hands, has been shown to lead to psychological imbalance. In view of this, lateral asymmetries in horses should be accepted, and training should focus on psychological and physical balance, coordination and equal strength on both sides, instead of enforcing “straightness”. To explore this, we conducted a review of the literature on motor and sensory laterality in horses and found that the evidence suggests that enforcing straightness may be stressful and may even be counterproductive by causing psychological and physical imbalance relative to a horse, making it tense and uncooperative. In general, body asymmetry has been shown to have little impact on performance, but increases in motor and sensory laterality can indicate insufficiencies in housing, handling and training. We, therefore, propose that laterality should be recognized as a welfare indicator and that straightness in a horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Abstract

For centuries, a goal of training in many equestrian disciplines has been to straighten the horse, which is considered a key element in achieving its responsiveness and suppleness. However, laterality is a naturally occurring phenomenon in horses and encompasses body asymmetry, motor laterality and sensory laterality. Furthermore, forcibly counterbalancing motor laterality has been considered a cause of psychological imbalance in humans. Perhaps asymmetry and laterality should rather be accepted, with a focus on training psychological and physical balance, coordination and equal strength on both sides instead of enforcing “straightness”. To explore this, we conducted a review of the literature on the function and causes of motor and sensory laterality in horses, especially in horses when trained on the ground or under a rider. The literature reveals that body asymmetry is innate but does not prevent the horse from performing at a high level under a rider. Motor laterality is equally distributed in feral horses, while in domestic horses, age, breed, training and carrying a rider may cause left leg preferences. Most horses initially observe novel persons and potentially threatening objects or situations with their left sensory organs. Pronounced preferences for the use of left sensory organs or limbs indicate that the horse is experiencing increased emotionality or stress, and long-term insufficiencies in welfare, housing or training may result in left shifts in motor and sensory laterality and pessimistic mentalities. Therefore, increasing laterality can be regarded as an indicator for insufficiencies in housing, handling and training. We propose that laterality be recognized as a welfare indicator and that straightening the horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Cerebral Polymorphisms for Lateralisation: Modelling the Genetic and Phenotypic Architectures of Multiple Functional Modules

Recent fMRI and fTCD studies have found that functional modules for aspects of language, praxis, and visuo-spatial functioning, while typically left, left and right hemispheric respectively, frequently show atypical lateralisation. Studies with increasing numbers of modules and participants are finding increasing numbers of module combinations, which here are termed cerebral polymorphisms—qualitatively different lateral organisations of cognitive functions. Polymorphisms are more frequent in left-handers than right-handers, but it is far from the case that right-handers all show the lateral organisation of modules described in introductory textbooks. In computational terms, this paper extends the original, monogenic McManus DC (dextral-chance) model of handedness and language dominance to multiple functional modules, and to a polygenic DC model compatible with the molecular genetics of handedness, and with the biology of visceral asymmetries found in primary ciliary dyskinesia. Distributions of cerebral polymorphisms are calculated for families and twins, and consequences and implications of cerebral polymorphisms are explored for explaining aphasia due to cerebral damage, as well as possible talents and deficits arising from atypical inter- and intra-hemispheric modular connections. The model is set in the broader context of the testing of psychological theories, of issues of laterality measurement, of mutation-selection balance, and the evolution of brain and visceral asymmetries.

A change of heart: new roles for cilia in cardiac development and disease

Although cardiac abnormalities have been observed in a growing class of human disorders caused by defective primary cilia, the function of cilia in the heart remains an underexplored area. The primary function of cilia in the heart was long thought to be restricted to left-right axis patterning during embryogenesis. However, new findings have revealed broad roles for cilia in congenital heart disease, valvulogenesis, myocardial fibrosis and regeneration, and mechanosensation. In this Review, we describe advances in our understanding of the mechanisms by which cilia function contributes to cardiac left-right axis development and discuss the latest findings that highlight a broader role for cilia in cardiac development. Specifically, we examine the growing line of evidence connecting cilia function to the pathogenesis of congenital heart disease. Furthermore, we also highlight research from the past 10 years demonstrating the role of cilia function in common cardiac valve disorders, including mitral valve prolapse and aortic valve disease, and describe findings that implicate cardiac cilia in mechanosensation potentially linking haemodynamic and contractile forces with genetic regulation of cardiac development and function. Finally, given the presence of cilia on cardiac fibroblasts, we also explore the potential role of cilia in fibrotic growth and summarize the evidence implicating cardiac cilia in heart regeneration.

Comparative reconstruction of the predatory feeding structures of the polyphenic nematode Pristionchus pacificus

Pristionchus pacificus is a nematode model for the developmental genetics of morphological polyphenism, especially at the level of individual cells. Morphological polyphenism in this species includes an evolutionary novelty, moveable teeth, which have enabled predatory feeding in this species and others in its family (Diplogastridae). From transmission electron micrographs of serial thin sections through an adult hermaphrodite of P. pacificus, we three‐dimensionally reconstructed all epithelial and myoepithelial cells and syncytia, corresponding to 74 nuclei, of its face, mouth, and pharynx. We found that the epithelia that produce the predatory morphology of P. pacificus are identical to Caenorhabditis elegans in the number of cell classes and nuclei. However, differences in cell form, spatial relationships, and nucleus position correlate with gross morphological differences from C. elegans and outgroups. Moreover, we identified fine‐structural features, especially in the anteriormost pharyngeal muscles, that underlie the conspicuous, left‐right asymmetry that characterizes the P. pacificus feeding apparatus. Our reconstruction provides an anatomical map for studying the genetics of polyphenism, feeding behavior, and the development of novel form in a satellite model to C. elegans.

All cells making the dimorphic, novel form of an animal with cell constancy were identified. Although the number of cells is fully conserved, divergence in form and connectivity—including fixed asymmetries—sheds light on the origins of this trait.

The embryonic node behaves as an instructive stem cell niche for axial elongation

In warm-blooded vertebrate embryos (mammals and birds), the axial tissues of the body form from a growth zone at the tail end, Hensen's node, which generates neural, mesodermal, and endodermal structures along the midline. While most cells only pass through this region, the node has been suggested to contain a small population of resident stem cells. However, it is unknown whether the rest of the node constitutes an instructive niche that specifies this self-renewal behavior. Here, we use heterotopic transplantation of groups and single cells and show that cells not destined to enter the node can become resident and self-renew. Long-term resident cells are restricted to the posterior part of the node and single-cell RNA-sequencing reveals that the majority of these resident cells preferentially express G2/M phase cell-cycle-related genes. These results provide strong evidence that the node functions as a niche to maintain self-renewal of axial progenitors.

The embryonic node behaves as an instructive stem cell niche for axial elongation

Previous studies have suggested that the amniote node (Hensen’s node) contains a small population of self-renewing resident cells whose progeny progressively lay down axial tissues, including notochord and somites. This can only be demonstrated definitively at the level of single cells. Here we ask whether the node is an environment that can confer this behavior on cells that enter it. We challenge single cells in vivo and mRNA-profile these cells to demonstrate that the node can indeed do this, and thus show that the node acts as an instructive niche.

In warm-blooded vertebrate embryos (mammals and birds), the axial tissues of the body form from a growth zone at the tail end, Hensen’s node, which generates neural, mesodermal, and endodermal structures along the midline. While most cells only pass through this region, the node has been suggested to contain a small population of resident stem cells. However, it is unknown whether the rest of the node constitutes an instructive niche that specifies this self-renewal behavior. Here, we use heterotopic transplantation of groups and single cells and show that cells not destined to enter the node can become resident and self-renew. Long-term resident cells are restricted to the posterior part of the node and single-cell RNA-sequencing reveals that the majority of these resident cells preferentially express G2/M phase cell-cycle–related genes. These results provide strong evidence that the node functions as a niche to maintain self-renewal of axial progenitors.

Asymmetric Hapln1a drives regionalized cardiac ECM expansion and promotes heart morphogenesis in zebrafish development

AIMS

Vertebrate heart development requires the complex morphogenesis of a linear tube to form the mature organ, a process essential for correct cardiac form and function, requiring coordination of embryonic laterality, cardiac growth, and regionalized cellular changes. While previous studies have demonstrated broad requirements for extracellular matrix (ECM) components in cardiac morphogenesis, we hypothesized that ECM regionalization may fine tune cardiac shape during heart development.

METHODS AND RESULTS

Using live in vivo light sheet imaging of zebrafish embryos, we describe a left-sided expansion of the ECM between the myocardium and endocardium prior to the onset of heart looping and chamber ballooning. Analysis using an ECM sensor revealed the cardiac ECM is further regionalized along the atrioventricular axis. Spatial transcriptomic analysis of gene expression in the heart tube identified candidate genes that may drive ECM expansion. This approach identified regionalized expression of hapln1a, encoding an ECM cross-linking protein. Validation of transcriptomic data by in situ hybridization confirmed regionalized hapln1a expression in the heart, with highest levels of expression in the future atrium and on the left side of the tube, overlapping with the observed ECM expansion. Analysis of CRISPR-Cas9-generated hapln1a mutants revealed a reduction in atrial size and reduced chamber ballooning. Loss-of-function analysis demonstrated that ECM expansion is dependent upon Hapln1a, together supporting a role for Hapln1a in regionalized ECM modulation and cardiac morphogenesis. Analysis of hapln1a expression in zebrafish mutants with randomized or absent embryonic left-right asymmetry revealed that laterality cues position hapln1a-expressing cells asymmetrically in the left side of the heart tube.

CONCLUSION

We identify a regionalized ECM expansion in the heart tube which promotes correct heart development, and propose a novel model whereby embryonic laterality cues orient the axis of ECM asymmetry in the heart, suggesting these two pathways interact to promote robust cardiac morphogenesis.

Receptor control by membrane-tethered ubiquitin ligases in development and tissue homeostasis

Paracrine cell-cell communication is central to all developmental processes, ranging from cell diversification to patterning and morphogenesis. Precise calibration of signaling strength is essential for the fidelity of tissue formation during embryogenesis and tissue maintenance in adults. Membrane-tethered ubiquitin ligases can control the sensitivity of target cells to secreted ligands by regulating the abundance of signaling receptors at the cell surface. We discuss two examples of this emerging concept in signaling: (1) the transmembrane ubiquitin ligases ZNRF3 and RNF43 that regulate WNT and bone morphogenetic protein receptor abundance in response to R-spondin ligands and (2) the membrane-recruited ubiquitin ligase MGRN1 that controls Hedgehog and melanocortin receptor abundance. We focus on the mechanistic logic of these systems, illustrated by structural and protein interaction models enabled by AlphaFold. We suggest that membrane-tethered ubiquitin ligases play a widespread role in remodeling the cell surface proteome to control responses to extracellular ligands in diverse biological processes.

Genomic frontiers in congenital heart disease

The application of next-generation sequencing to study congenital heart disease (CHD) is increasingly providing new insights into the causes and mechanisms of this prevalent birth anomaly. Whole-exome sequencing analysis identifies damaging gene variants altering single or contiguous nucleotides that are assigned pathogenicity based on statistical analyses of families and cohorts with CHD, high expression in the developing heart and depletion of damaging protein-coding variants in the general population. Gene classes fulfilling these criteria are enriched in patients with CHD and extracardiac abnormalities, evidencing shared pathways in organogenesis. Developmental single-cell transcriptomic data demonstrate the expression of CHD-associated genes in particular cell lineages, and emerging insights indicate that genetic variants perturb multicellular interactions that are crucial for cardiogenesis. Whole-genome sequencing analyses extend these observations, identifying non-coding variants that influence the expression of genes associated with CHD and contribute to the estimated ~55% of unexplained cases of CHD. These approaches combined with the assessment of common and mosaic genetic variants have provided a more complete knowledge of the causes and mechanisms of CHD. Such advances provide knowledge to inform the clinical care of patients with CHD or other birth defects and deepen our understanding of the complexity of human development. In this Review, we highlight known and candidate CHD-associated human genes and discuss how the integration of advances in developmental biology research can provide new insights into the genetic contributions to CHD.

Impact of Motile Ciliopathies on Human Development and Clinical Consequences in the Newborn

Motile cilia are hairlike organelles that project outward from a tissue-restricted subset of cells to direct fluid flow. During human development motile cilia guide determination of the left-right axis in the embryo, and in the fetal and neonatal periods they have essential roles in airway clearance in the respiratory tract and regulating cerebral spinal fluid flow in the brain. Dysregulation of motile cilia is best understood through the lens of the genetic disorder primary ciliary dyskinesia (PCD). PCD encompasses all genetic motile ciliopathies resulting from over 60 known genetic mutations and has a unique but often underrecognized neonatal presentation. Neonatal respiratory distress is now known to occur in the majority of patients with PCD, laterality defects are common, and very rarely brain ventricle enlargement occurs. The developmental function of motile cilia and the effect and pathophysiology of motile ciliopathies are incompletely understood in humans. In this review, we will examine the current understanding of the role of motile cilia in human development and clinical considerations when assessing the newborn for suspected motile ciliopathies.

Of form and function: Early cardiac morphogenesis across classical and emerging model systems

The heart is the earliest organ to develop during embryogenesis and is remarkable in its ability to function efficiently as it is being sculpted. Cardiac heart defects account for a high burden of childhood developmental disorders with many remaining poorly understood mechanistically. Decades of work across a multitude of model organisms has informed our understanding of early cardiac differentiation and morphogenesis and has simultaneously opened new and unanswered questions. Here we have synthesized current knowledge in the field and reviewed recent developments in the realm of imaging, bioengineering and genetic technology and ex vivo cardiac modeling that may be deployed to generate more holistic models of early cardiac morphogenesis, and by extension, new platforms to study congenital heart defects.

Functional Roles of FGF Signaling in Early Development of Vertebrate Embryos

Fibroblast growth factors (FGFs) comprise a large family of growth factors, regulating diverse biological processes including cell proliferation, migration, and differentiation. Each FGF binds to a set of FGF receptors to initiate certain intracellular signaling molecules. Accumulated evidence suggests that in early development and adult state of vertebrates, FGFs also play exclusive and context dependent roles. Although FGFs have been the focus of research for therapeutic approaches in cancer, cardiovascular disease, and metabolic syndrome, in this review, we mainly focused on their role in germ layer specification and axis patterning during early vertebrate embryogenesis. We discussed the functional roles of FGFs and their interacting partners as part of the gene regulatory network for germ layer specification, dorsal-ventral (DV), and anterior-posterior (AP) patterning. Finally, we briefly reviewed the regulatory molecules and pharmacological agents discovered that may allow modulation of FGF signaling in research.

Hedgehog/GLI Signaling Pathway: Transduction, Regulation, and Implications for Disease

Simple Summary

The Hedgehog/GLI (Hh/GLI) pathway plays a major role during development and it is commonly dysregulated in many diseases, including cancer. This highly concerted series of ligands, receptors, cytoplasmic signaling molecules, transcription factors, and co-regulators is involved in regulating the biological functions controlled by this pathway. Activation of Hh/GLI in cancer is most often through a non-canonical method of activation, independent of ligand binding. This review is intended to summarize our current understanding of the Hh/GLI signaling, non-canonical mechanisms of pathway activation, its implication in disease, and the current therapeutic strategies targeting this cascade.

Abstract

The Hh/GLI signaling pathway was originally discovered in Drosophila as a major regulator of segment patterning in development. This pathway consists of a series of ligands (Shh, Ihh, and Dhh), transmembrane receptors (Ptch1 and Ptch2), transcription factors (GLI1–3), and signaling regulators (SMO, HHIP, SUFU, PKA, CK1, GSK3β, etc.) that work in concert to repress (Ptch1, Ptch2, SUFU, PKA, CK1, GSK3β) or activate (Shh, Ihh, Dhh, SMO, GLI1–3) the signaling cascade. Not long after the initial discovery, dysregulation of the Hh/GLI signaling pathway was implicated in human disease. Activation of this signaling pathway is observed in many types of cancer, including basal cell carcinoma, medulloblastoma, colorectal, prostate, pancreatic, and many more. Most often, the activation of the Hh/GLI pathway in cancer occurs through a ligand-independent mechanism. However, in benign disease, this activation is mostly ligand-dependent. The upstream signaling component of the receptor complex, SMO, is bypassed, and the GLI family of transcription factors can be activated regardless of ligand binding. Additional mechanisms of pathway activation exist whereby the entirety of the downstream signaling pathway is bypassed, and PTCH1 promotes cell cycle progression and prevents caspase-mediated apoptosis. Throughout this review, we summarize each component of the signaling cascade, non-canonical modes of pathway activation, and the implications in human disease, including cancer.

Right, left and cilia: How asymmetry is established

The initial breaking of left-right (L-R) symmetry in the embryo is controlled by a motile-cilia-driven leftward fluid flow in the left-right organiser (LRO), resulting in L-R asymmetric gene expression flanking the LRO. Ultimately this results in left- but not right-sided activation of the Nodal-Pitx2 pathway in more lateral tissues. While aspects of the initial breaking event clearly vary between vertebrates, events in the Lateral Plate Mesoderm (LPM) are conserved through the vertebrate lineage. Evidence from model systems and humans highlights the role of cilia both in the initial symmetry breaking and in the ability of more lateral tissues to exhibit asymmetric gene expression. In this review we concentrate on the process of L-R determination in mouse and humans.

Transient Nodal Signaling in Left Precursors Coordinates Opposed Asymmetries Shaping the Heart Loop

The secreted factor Nodal, known as a major left determinant, is associated with severe heart defects. Yet, it has been unclear how it regulates asymmetric morphogenesis such as heart looping, which align cardiac chambers to establish the double blood circulation. Here, we report that Nodal is transiently active in precursors of the mouse heart tube poles, before looping. In conditional mutants, we show that Nodal is not required to initiate asymmetric morphogenesis. We provide evidence of a heart-specific random generator of asymmetry that is independent of Nodal. Using 3D quantifications and simulations, we demonstrate that Nodal functions as a bias of this mechanism: it is required to amplify and coordinate opposed left-right asymmetries at the heart tube poles, thus generating a robust helical shape. We identify downstream effectors of Nodal signaling, regulating asymmetries in cell proliferation, differentiation, and extracellular matrix composition. Our study uncovers how Nodal regulates asymmetric organogenesis.

The Biophysics of Regenerative Repair Suggests New Perspectives on Biological Causation

Evolution exploits the physics of non-neural bioelectricity to implement anatomical homeostasis: a process in which embryonic patterning, remodeling, and regeneration achieve invariant anatomical outcomes despite external interventions. Linear "developmental pathways" are often inadequate explanations for dynamic large-scale pattern regulation, even when they accurately capture relationships between molecular components. Biophysical and computational aspects of collective cell activity toward a target morphology reveal interesting aspects of causation in biology. This is critical not only for unraveling evolutionary and developmental events, but also for the design of effective strategies for biomedical intervention. Bioelectrical controls of growth and form, including stochastic behavior in such circuits, highlight the need for the formulation of nuanced views of pathways, drivers of system-level outcomes, and modularity, borrowing from concepts in related disciplines such as cybernetics, control theory, computational neuroscience, and information theory. This approach has numerous practical implications for basic research and for applications in regenerative medicine and synthetic bioengineering.

A Membrane-Tethered Ubiquitination Pathway Regulates Hedgehog Signaling and Heart Development

The etiology of congenital heart defects (CHDs), which are among the most common human birth defects, is poorly understood because of its complex genetic architecture. Here, we show that two genes implicated in CHDs, Megf8 and Mgrn1, interact genetically and biochemically to regulate the strength of Hedgehog signaling in target cells. MEGF8, a transmembrane protein, and MGRN1, a RING superfamily E3 ligase, assemble to form a receptor-like ubiquitin ligase complex that catalyzes the ubiquitination and degradation of the Hedgehog pathway transducer Smoothened. Homozygous Megf8 and Mgrn1 mutations increased Smoothened abundance and elevated sensitivity to Hedgehog ligands. While mice heterozygous for loss-of-function Megf8 or Mgrn1 mutations were normal, double heterozygous embryos exhibited an incompletely penetrant syndrome of CHDs with heterotaxy. Thus, genetic interactions can arise from biochemical mechanisms that calibrate morphogen signaling strength, a conclusion broadly relevant for the many human diseases in which oligogenic inheritance is emerging as a mechanism for heritability.

Fluid flow as a driver of embryonic morphogenesis

Fluid flow is a powerful morphogenic force during embryonic development. The physical forces created by flowing fluids can either create morphogen gradients or be translated by mechanosensitive cells into biological changes in gene expression. In this Primer, we describe how fluid flow is created in different systems and highlight the important mechanosensitive signalling pathways involved for sensing and transducing flow during embryogenesis. Specifically, we describe how fluid flow helps establish left-right asymmetry in the early embryo and discuss the role of flow of blood, lymph and cerebrospinal fluid in sculpting the embryonic cardiovascular and nervous system.

Understanding embryonic development for cell-based therapies of intervertebral disc degeneration: Toward an effort to treat disc degeneration subphenotypes

Chronic low back and neck pain are associated with intervertebral disc degeneration and are major contributors to the global burden of disability. New evidence now suggests that disc degeneration comprises a spectrum of subphenotypes influenced by genetic background, age, and environmental factors, which may be contributing to the mixed outcomes seen in clinical trials of cell-based therapies that aim to treat disc degeneration. This problem is further compounded by the fact that disc degeneration and aging coincide with an exhaustion of endogenous progenitor cells, imposing limitations on the regenerative capacity of the disc. At the bench-side, current work is focused on applying our knowledge of embryonic disc development to direct and refine differentiation of adult and human-induced pluripotent stem cells into notochord-like and nucleus pulposus-like cells for use in novel cell-based therapies. Accordingly, this review presents the salient features of intervertebral disc development, post-natal maintenance, and regeneration, with emphasis on recent advancements. We also discuss how a stratified approach can be undertaken for the development of future cell-based therapies to bring emerging subphenotypes into consideration.

The maternal coordinate system: Molecular-genetics of embryonic axis formation and patterning in the zebrafish

Axis specification of the zebrafish embryo begins during oogenesis and relies on proper formation of well-defined cytoplasmic domains within the oocyte. Upon fertilization, maternally-regulated cytoplasmic flow and repositioning of dorsal determinants establish the coordinate system that will build the structure and developmental body plan of the embryo. Failure of specific genes that regulate the embryonic coordinate system leads to catastrophic loss of body structures. Here, we review the genetic principles of axis formation and discuss how maternal factors orchestrate axis patterning during zebrafish early embryogenesis. We focus on the molecular identity and functional contribution of genes controlling critical aspects of oogenesis, egg activation, blastula, and gastrula stages. We examine how polarized cytoplasmic domains form in the oocyte, which set off downstream events such as animal-vegetal polarity and germ line development. After gametes interact and form the zygote, cytoplasmic segregation drives the animal-directed reorganization of maternal determinants through calcium- and cell cycle-dependent signals. We also summarize how maternal genes control dorsoventral, anterior-posterior, mesendodermal, and left-right cell fate specification and how signaling pathways pattern these axes and tissues during early development to instruct the three-dimensional body plan. Advances in reverse genetics and phenotyping approaches in the zebrafish model are revealing positional patterning signatures at the single-cell level, thus enhancing our understanding of genotype-phenotype interactions in axis formation. Our emphasis is on the genetic interrogation of novel and specific maternal regulatory mechanisms of axis specification in the zebrafish.

A Multidisciplinary Review of the Roles of Cripto in the Scientific Literature Through a Bibliometric Analysis of its Biological Roles

Cripto is a small glycosylphosphatidylinisitol (GPI)-anchored and secreted oncofetal protein that plays important roles in regulating normal physiological processes, including stem cell differentiation, embryonal development, and tissue growth and remodeling, as well as pathological processes such as tumor initiation and progression. Cripto functions as a co-receptor for TGF-β ligands such as Nodal, GDF1, and GDF3. Soluble and secreted forms of Cripto also exhibit growth factor-like activity and activate SRC/MAPK/PI3K/AKT pathways. Glucose-Regulated Protein 78 kDa (GRP78) binds Cripto at the cell surface and has been shown to be required for Cripto signaling via both TGF-β and SRC/MAPK/PI3K/AKT pathways. To provide a comprehensive overview of the scientific literature related to Cripto, we performed, for the first time, a bibliometric analysis of the biological roles of Cripto as reported in the scientific literature covering the last 10 years. We present different fields of knowledge in comprehensive areas of research on Cripto, ranging from basic to translational research, using a keyword-driven approach. Our ultimate aim is to aid the scientific community in conducting targeted research by identifying areas where research has been conducted so far and, perhaps more importantly, where critical knowledge is still missing.

Cardioprotective effect of sonic hedgehog ligand in pig models of ischemia reperfusion

Sonic hedgehog (SHH) signaling pathway is involved in embryonic tissue patterning and development. Our previous work identified, in small rodent model of ischemia reperfusion, SHH as a specific efficient tool to reduce infarct size and subsequent arrhythmias by preventing ventricular repolarization abnormalities. The goal of the present study was to provide a proof of concept of the cardioprotective effect of SHH ligand in a porcine model of acute ischemia.

Methods: The antiarrhythmic effect of SHH, either by a recombinant peptide (N-SHH) or shed membrane microparticles harboring SHH ligand (MPs^SHH+), was evaluated in a first set of pigs following a short (25 min) coronary artery occlusion (CAO) followed by 24 hours-reperfusion (CAR) (Protocol A). The infarct-limiting effect was evaluated on a second set of pigs with 40 min of coronary artery occlusion followed by 24 hours reperfusion (Protocol B). Electrocardiogram (ECG) was recorded and arrhythmia's scores were evaluated. Area at risk and myocardial infarct size were quantified.

Results: In protocol A, administration of N-SHH 15 min. after the onset of coronary occlusion significantly reduced the occurrence of ventricular fibrillation compared to control group. Evaluation of arrhythmic score showed that N-SHH treatment significantly reduced the overall occurrence of arrhythmias. In protocol B, massive infarction was observed in control animals. Either N-SHH or MPs^SHH+ treatment reduced significantly the infarct size with a concomitant increase of salvaged area. The reduction in infarct size was both accompanied by a significant decrease in systemic biomarkers of myocardial injury, i.e., cardiac troponin I and fatty acid-binding protein and an increase of eNOS activation.

Conclusions: We show for the first time in a large mammalian model that the activation of the SHH pathway by N-SHH or MPs^SHH+ offers a potent protection of the heart to ischemia-reperfusion by preventing the reperfusion arrhythmias, reducing the infarct area and the circulating levels of biomarkers for myocardial injury. These data open up potentially theranostic prospects for patients suffering from myocardial infarction to prevent the occurrence of arrhythmias and reduce myocardial tissue damage.

Cilia-driven asymmetric Hedgehog signalling determines the amphioxus left-right axis by controlling Dand5 expression

Cilia rotation-driven nodal flow is crucial for the left-right (L-R) break in symmetry in most vertebrates. However, the mechanism by which the flow signal is translated to asymmetric gene expression has been insufficiently addressed. Here, we show that Hedgehog (Hh) signalling is asymmetrically activated (L<R) in the region in which initial asymmetric Dand5 expression is detected. Upregulation of Hh signalling on the left side of wild-type embryos induces ectopic Dand5 expression on the left side, and the unilateral recovery of Hh signalling in Hh homozygous mutants induces Dand5 expression in the Hh signal recovery side. Immunofluorescence analysis results revealed that Hh fusion protein is asymmetrically enriched in the anterior-right paraxial mesoderm at the early neurula stage. Inhibiting embryonic cilia motility using methylcellulose (MC) blocks Hh protein enrichment on the right hand side and randomizes Dand5 expression and organ positioning along the L-R axis. These findings present a model showing that cilia movement is crucial for the symmetry breaks in amphioxus through asymmetric Hh protein transport. The resultant asymmetric Hh signalling provides a clue into the induction of asymmetric Dand5 expression.This article has an associated 'The people behind the papers' interview.

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

Unidirectional fluid flow generated by motile cilia at the left-right organizer (LRO) breaks left-right (L-R) symmetry during early embryogenesis in mouse, frog and zebrafish. The chick embryo, however, does not require motile cilia for L-R symmetry breaking. The diversity of mechanisms for L-R symmetry breaking among vertebrates and the trigger for such symmetry breaking in non-mammalian amniotes have remained unknown. Here we examined how L-R asymmetry is established in two reptiles, Madagascar ground gecko and Chinese softshell turtle. Both of these reptiles appear to lack motile cilia at the LRO. The expression of the Nodal gene at the LRO in the reptilian embryos was found to be asymmetric, in contrast to that in vertebrates such as mouse that are dependent on cilia for L-R patterning. Two paralogues of the Nodal gene derived from an ancient gene duplication are retained and expressed differentially in cilia-dependent and cilia-independent vertebrates. The expression of these two Nodal paralogues is similarly controlled in the lateral plate mesoderm but regulated differently at the LRO. Our in-depth analysis of reptilian embryos thus suggests that mammals and non-mammalian amniotes deploy distinct strategies dependent on different Nodal paralogues for rendering Nodal activity asymmetric at the LRO.