Knowing left from right: the molecular basis of laterality defects

Breaking Left-Right Symmetry by the Interplay of Planar Cell Polarity, Calcium Signaling and Cilia

The formation of the embryonic left-right axis is a fundamental process in animals, which subsequently conditions both the shape and the correct positioning of internal organs. During vertebrate early development, a transient structure, known as the left-right organizer, breaks the bilateral symmetry in a manner that is critically dependent on the activity of motile and immotile cilia or asymmetric cell migration. Extensive studies have partially elucidated the molecular pathways that initiate left-right asymmetric patterning and morphogenesis. Wnt/planar cell polarity signaling plays an important role in the biased orientation and rotational motion of motile cilia. The leftward fluid flow generated in the cavity of the left-right organizer is sensed by immotile cilia through complex mechanisms to trigger left-sided calcium signaling and lateralized gene expression pattern. Disrupted asymmetric positioning or impaired structure and function of cilia leads to randomized left-right axis determination, which is closely linked to laterality defects, particularly congenital heart disease. Despite of the formidable progress made in deciphering the critical contribution of cilia to establishing the left-right asymmetry, a strong challenge remains to understand how cilia generate and sense fluid flow to differentially activate gene expression across the left-right axis. This review analyzes mechanisms underlying the asymmetric morphogenesis and function of the left-right organizer in left-right axis formation. It also aims to identify important questions that are open for future investigations.

Canonical and Non-Canonical Wnt Signaling Generates Molecular and Cellular Asymmetries to Establish Embryonic Axes

The formation of embryonic axes is a critical step during animal development, which contributes to establishing the basic body plan in each particular organism. Wnt signaling pathways play pivotal roles in this fundamental process. Canonical Wnt signaling that is dependent on β-catenin regulates the patterning of dorsoventral, anteroposterior, and left-right axes. Non-canonical Wnt signaling that is independent of β-catenin modulates cytoskeletal organization to coordinate cell polarity changes and asymmetric cell movements. It is now well documented that components of these Wnt pathways biochemically and functionally interact to mediate cell-cell communications and instruct cellular polarization in breaking the embryonic symmetry. The dysfunction of Wnt signaling disrupts embryonic axis specification and proper tissue morphogenesis, and mutations of Wnt pathway genes are associated with birth defects in humans. This review discusses the regulatory roles of Wnt pathway components in embryonic axis formation by focusing on vertebrate models. It highlights current progress in decoding conserved mechanisms underlying the establishment of asymmetry along the three primary body axes. By providing an in-depth analysis of canonical and non-canonical pathways in regulating cell fates and cellular behaviors, this work offers insights into the intricate processes that contribute to setting up the basic body plan in vertebrate embryos.

Three-dimensional morphometric analysis of anterior cranial base in growing patients

INTRODUCTION

The growth of the sphenoethmoidal and sphenofrontal suture in the anterior cranial base (ACB) terminates around 7 years old; therefore, ACB is used as a reference structure for superimposing radiographs in 2-dimensions (2D) and 3-dimensions (3D). The data in the literature regarding the cessation of growth of ACB in 3D is insufficient. The study aimed to evaluate the volumetric changes of ACB in growing patients on cone-beam computed tomography (CBCT) data in 3D.

METHODS

The CBCT sample was obtained from a repository of scans of subjects (n = 30) aged 6-11 years with no craniofacial anomalies or growth-related disorders. CBCTs were taken at 2 time points (approximately 12 months apart). The mean age at the initial scan (T1) was 8.4 ± 0.89 years, and the follow-up scan (T2) was 9.6 ± 0.99 years. The bones of the ACB were segmented, and 3D models of the bones were generated using Mimics software. The volumetric measurement was performed on the 3D-rendered model. Linear measurements were performed on the slices.

RESULTS

The volumetric analysis of the ACB between T1 and T2 showed a significant change (P <0.0001). There were no significant differences in volumetric changes of the ACB between the male and female subjects. The linear measurements on the right side of the cranial base showed continued growth between T1 and T2.

CONCLUSIONS

In the studied sample, growth-related changes of ACB were observed through volumetric analysis after 7 years old.

HMCES modulates the transcriptional regulation of nodal/activin and BMP signaling in mESCs

Despite the fundamental roles of TGF-β family signaling in cell fate determination in all metazoans, the mechanism by which these signals are spatially and temporally interpreted remains elusive. The cell-context-dependent function of TGF-β signaling largely relies on transcriptional regulation by SMAD proteins. Here, we discover that the DNA repair-related protein, HMCES, contributes to early development by maintaining nodal/activin- or BMP-signaling-regulated transcriptional network. HMCES binds with R-SMAD proteins, co-localizing at active histone marks. However, HMCES chromatin occupancy is independent on nodal/activin or BMP signaling. Mechanistically, HMCES competitively binds chromatin to limit binding by R-SMAD proteins, thereby forcing their dissociation and resulting in repression of their regulatory effects. In Xenopus laevis embryo, hmces KD causes dramatic development defects with abnormal left-right axis asymmetry along with increasing expression of lefty1. These findings reveal HMCES transcriptional regulatory function in the context of TGF-β family signaling.

Congenital anomalies of the tubular gastrointestinal tract

Congenital anomalies of the tubular gastrointestinal tract are an important cause of morbidity not only in infants, but also in children and adults. The gastrointestinal (GI) tract, composed of all three primitive germ layers, develops early during embryogenesis. Two major steps in its development are the formation of the gut tube (giving rise to the foregut, the midgut and the hindgut), and the formation of individual organs with specialized cell types. Formation of an intact and functioning GI tract is under strict control from various molecular pathways. Disruption of any of these crucial mechanisms involved in the cell-fate decision along the dorsoventral, anteroposterior, left-right and radial axes, can lead to numerous congenital anomalies, most of which occur and present in infancy. However, they may run undetected during childhood. Therapy is surgical, which in some cases must be performed urgently, and prognosis depends on early diagnosis and suitable treatment. A precise pathologic macroscopic or microscopic diagnosis is important, not only for the immediate treatment and management of affected individuals, but also for future counselling of the affected individual and their family. This is even more true in cases of multiple anomalies or syndromic patterns. We discuss some of the more frequent or clinically important congenital anomalies of the tubular GI, including atresia's, duplications, intestinal malrotation, Meckel's diverticulum and Hirschsprung's Disease.

Large intestine embryogenesis: Molecular pathways and related disorders (Review)

The large intestine, part of the gastrointestinal tract (GI), is composed of all three germ layers, namely the endoderm, the mesoderm and the ectoderm, forming the epithelium, the smooth muscle layers and the enteric nervous system, respectively. Since gastrulation, these layers develop simultaneously during embryogenesis, signaling to each other continuously until adult age. Two invaginations, the anterior intestinal portal (AIP) and the caudal/posterior intestinal portal (CIP), elongate and fuse, creating the primitive gut tube, which is then patterned along the antero‑posterior (AP) axis and the radial (RAD) axis in the context of left‑right (LR) asymmetry. These events lead to the formation of three distinct regions, the foregut, midgut and hindgut. All the above‑mentioned phenomena are under strict control from various molecular pathways, which are critical for the normal intestinal development and function. Specifically, the intestinal epithelium constitutes a constantly developing tissue, deriving from the progenitor stem cells at the bottom of the intestinal crypt. Epithelial differentiation strongly depends on the crosstalk with the adjacent mesoderm. Major molecular pathways that are implicated in the embryogenesis of the large intestine include the canonical and non‑canonical wingless‑related integration site (Wnt), bone morphogenetic protein (BMP), Notch and hedgehog systems. The aberrant regulation of these pathways inevitably leads to several intestinal malformation syndromes, such as atresia, stenosis, or agangliosis. Novel theories, involving the regulation and homeostasis of intestinal stem cells, suggest an embryological basis for the pathogenesis of colorectal cancer (CRC). Thus, the present review article summarizes the diverse roles of these molecular factors in intestinal embryogenesis and related disorders.

Long-Pentraxin 3 Affects Primary Cilium in Zebrafish Embryo and Cancer Cells via the FGF System

Primary cilium drives the left-right asymmetry process during embryonic development. Moreover, its dysregulation contributes to cancer progression by affecting various signaling pathways. The fibroblast growth factor (FGF)/FGF receptor (FGFR) system modulates primary cilium length and plays a pivotal role in embryogenesis and tumor growth. Here, we investigated the impact of the natural FGF trap long-pentraxin 3 (PTX3) on the determination of primary cilium extension in zebrafish embryo and cancer cells. The results demonstrate that down modulation of the PTX3 orthologue ptx3b causes the shortening of primary cilium in zebrafish embryo in a FGF-dependent manner, leading to defects in the left-right asymmetry determination. Conversely, PTX3 upregulation causes the elongation of primary cilium in FGF-dependent cancer cells. Previous observations have identified the PTX3-derived small molecule NSC12 as an orally available FGF trap with anticancer effects on FGF-dependent tumors. In keeping with the non-redundant role of the FGF/FGR system in primary cilium length determination, NSC12 induces the elongation of primary cilium in FGF-dependent tumor cells, thus acting as a ciliogenic anticancer molecule in vitro and in vivo. Together, these findings demonstrate the ability of the natural FGF trap PTX3 to exert a modulatory effect on primary cilium in embryonic development and cancer. Moreover, they set the basis for the design of novel ciliogenic drugs with potential implications for the therapy of FGF-dependent tumors.

Kartagener syndrome, current data on a classical disease. Case report

Introducción. El presente artículo aborda los aspectos generales (fisiopatología, embriología, presentación clínica y pronóstico) del síndrome de Kartagener (SK). Presentación del caso. Paciente masculino de 26 años, con antecedente de sinusitis complicada con absceso cerebral y epilepsia secundaria, quien consulta al Hospital Universitario Nacional de Colombia por cefalea, fiebre y expectoración mucosa. La presencia de situs inverso, sinusitis crónica y bronquiectasias sugieren diagnóstico de discinesia ciliar primaria y SK. Discusión. Los diagnósticos diferenciales del SK deben enmarcarse en la relación de causalidad posible con la discinesia ciliar primaria y de otros diagnósticos asociados a disfunción ciliar secundaria como fibrosis quística, inmunodeficiencia y condiciones anatómicas-funcionales con compromiso rinosenopulmonar. La sospecha clínica del SK se da cuando se ausculta el corazón a la derecha y se palpa el hígado a la izquierda. Su confirmación es mediante métodos de imagen que comprueban la heterotaxia visceral, por métodos indirectos de mal funcionamiento del barrido ciliar (óxido nítrico nasal, video microscopia) y por biopsia ciliar que demuestra el defecto de la ultraestructura ciliar. Conclusiones. El compromiso infeccioso respiratorio presentado por los pacientes que cursan con SK se explica por la alteración en la cilia, que conlleva tanto a la malposición de algunos órganos como a la alteración estructural y funcional de otros. Se detallan los hallazgos en las imágenes diagnósticas, se explican las alteraciones anatómicas, las bases moleculares y la relación con enfermedades respiratorias del síndrome. 

Childhood intelligence and early tooth wear patterns

OBJECTIVE

The aim of this study is to explore the relationships between early dental wear patterns and preschool IQ (Intelligence Quotient, by Stanford-Binet) of the child to illuminate the historic relationship of mental queries and bruxism.

METHODS

The dental study participants were 864 Euro-American preschool and school children whose IQs were tested for school maturity purposes at the age of 4 years, followed by dental data in a cross-sectional manner at the mean age of 7.8 years. Worn dentitions were classified as "symmetric" or "right-" and "left-sided," based on the faceting of the teeth.

RESULTS

In general, the relationships of tooth wear and intelligence were scarce, reflecting social background factors. Statistically significant results between asymmetric wear and gender groups suggest that direction of jaw function has a role in the regulation of processes responsible for individual mental performance in childhood.

DISCUSSION

Increased left-side tooth wear and early advantage in the intelligence test in girls is intriguing due to the fact that they reach maturity earlier than boys in verbal articulation, controlled in most cases by the limited area on the left side of the brain.

Detection and Correction of Laterality Errors in Radiology Reports

The objectives of the study are to introduce the development of supervising software for double-checking of laterality error in radiology reports and to evaluate the usefulness of detection and correction software by applying it to radiology report systems. An AutoHotkey macro program was applied to the design for double-checking of laterality errors. The software was performed according to the flowchart below: (1) detecting laterality discrepancies between radiologic examination names and the context of the radiology report and (2) providing conditioned discrepancy correction with a pop-up window. The accuracy of the detection was evaluated with 300 radiologic examinations that include the intended discrepancies and concordance of lateralities. The number of detections and corrections were quantified, and the confidence intervals were calculated for accuracy. We also applied this module to previous radiology reports with laterality errors from the radiologic examination database to validate the module. The AutoHotkey-scripted macro program functioned well in the reading workstation, and it was acted successfully as additional software. The detection accuracy was 99.67% (95% CI; 99.01-%) in the 300 radiologic examinations from the radiologic reading session. There was one running failure, caused by a temporary lag in the hospital's computer network, but no failures resulted during the second trial. We found that there were laterality errors in 0.048% (n = 14/29,257) of the examinations from the database. We developed detection and correction software aimed at double-checking for laterality errors. This method can be successfully adopted in any hospital software and is expected to be included for a better radiologic reading environment.

Laterality disturbance and hypopituitarism. A case report of co-existing situs inversus totalis and combined pituitary hormone deficiency

The authors present the case history of a 52-yr-old male patient with a unique association of combined pituitary hormone deficiency (CPHD) and situs inversus totalis. Except for signs and symptoms of pituitary hormone deficiency, the patient had no dysmorphic features, and hearing impairment, primary mental or neurological defects were also absent. Pituitary magnetic resonance imaging (MRI) scan showed hypoplasia of the anterior lobe of the pituitary gland and an ectopic posterior pituitary lobe. Despite the presence of situs inversus totalis, the patient was right-handed and functional MRI demonstrated left-hemisphere activation during language tests. Kartagener syndrome was considered, but immunofluorescence analysis showed normal localization of the outer dynein arm protein in respiratory epithelial cells obtained from the nasal mucosa. Direct DNA sequencing of all coding exons of the pituitary transcription factor 1 (PIT1) and prophet of PIT1 (PROP1) genes failed to detect disease-causing mutations, suggesting that these genes were not involved in the development of CPHD in our patient. More interestingly, the potential role of the paired like homeodomain transcription factor 2 (PITX2) gene, which has been implicated not only in CPHD, but also in left-right patterning in animal models, was also excluded, as sequencing showed the absence of mutations in coding exons of this gene. To our knowledge, PITX2 gene mutations have not been investigated in CPHD patients who had situs inversus totalis. We conclude that in contrast to animal models, the PITX2 gene is not involved in the development of situs inversus totalis, at least not in our CPHD patient.

A novel homeobox mutation in the PITX2 gene in a family with Axenfeld-Rieger syndrome associated with brain, ocular, and dental phenotypes

Axenfeld-Rieger Syndrome (ARS) is a genetically heterogeneous birth defect characterized by malformation of the anterior segment of the eye associated with glaucoma. Mutation of the PITX2 homeobox gene has been identified as a cause of ARS. We report a novel Arg5Trp missense mutation in the PITX2 homeodomain, which is associated with brain abnormalities. One patient had a small sella turcica likely to reflect hypoplasia of the pituitary gland and consistent with the critical role identified for Pitx2 in pituitary development in mice. Two patients had an enlarged cisterna magna, one with a malformed cerebellum, and two had executive skills deficits one in isolation and one in association with a below average intellectual capacity. The mutation caused a typical ARS ocular phenotype. All affected had iris hypoplasia, anterior iris to corneal adhesions, and corectopia. The ocular phenotype varied significantly in severity and showed some asymmetry. All affected also had redundant peri-umbilical skin, a hypoplastic maxilla, microdontia, and hypodontia missing between 20 and 27 teeth with an unusual pattern of tooth loss. Dental phenotypes were documented as they are often poorly characterized in ARS patients. All affected individuals showed an absence of first permanent molars with variable absence of other rarely absent teeth: the permanent upper central incisors, maxillary and mandibular first and second molars, and the mandibular canines. Based on the distinctive dental anomalies, we suggest that the dental phenotype can assist in predicting the presence of a PITX2 mutation and the possibility of brain abnormalities.

The splanchnic mesodermal plate directs spleen and pancreatic laterality, and is regulated by Bapx1/Nkx3.2

The mechanism by which left-right (LR) information is interpreted by organ primordia during asymmetric morphogenesis is largely unknown. We show that spleen and pancreatic laterality is dependent on a specialised, columnar mesodermal-derived cell layer referred to here as the splanchnic mesodermal plate (SMP). At early embryonic stages, the SMP is bilateral, surrounding the midline-located stomach and dorsal pancreatic bud. Under control of the LR asymmetry pathway, the left SMP is maintained and grows laterally. Mice carrying the dominant hemimelia (Dh) mutation lack the SMP. Significantly, the mice are asplenic and the pancreas remains positioned along the embryonic midline. In the absence of Fgf10 expression, the spleno-pancreatic mesenchyme and surrounding SMP grow laterally but contain no endodermal component, showing that leftward growth is autonomous and independent of endoderm. In the Bapx1(-/-) mutants, the SMP is defective. Normally, the SMP is a source for both Fgf9 and Fgf10; however, in the Bapx1 mutant, Fgf10 expression is downregulated and the dorsal pancreas remains at the midline. We conclude that the SMP is an organiser responsible for the leftward growth of the spleno-pancreatic region and that Bapx1 regulates SMP functions required for pancreatic laterality.

Left-right asymmetry: all hands to the pump

How do embryos establish differences between their left and right sides? Previous studies have implicated various signaling molecules and directional beating of cilia. Now, a new player enters the scene: the proton-potassium pump.

Deciduous tooth crown size and asymmetry in strabismic children

OBJECTIVES

To explore deciduous tooth crown dimensions in strabismic children and the relationship between the type of strabismus and tooth crown mesio-distal (M-D) and labio-lingual (L-L) size asymmetries.

MATERIAL

Dental casts at mixed dentition of 2159 Collaborative Perinatal Study black and white children were measured, 123 of them strabismic at 1 year of age, age ranging from 6 to 12 years.

METHODS

Directional and fluctuating asymmetries in antimeric teeth were explored in various types of strabismus having unilateral, bilateral or alternating expression. ANOVA and T-square test were used for size comparisons and calculated asymmetries were explored by comparing the variances and Pearson correlations.

RESULTS

Strabismus was associated with significant M-D size increase of deciduous maxillary canines in black boys and white girls, black girls had size reduction in their mandibular canine, but white boys were unaffected. Right side size dominance was found in the strabismic children in the lower second deciduous molar M-D dimensions and in the children with alternating strabismus in their upper deciduous canine M-D dimensions. Children with unilateral strabismus had random fluctuating dental asymmetry in their upper deciduous second molar L-L dimensions when compared with healthy normals. Higher left-right correlations were found in lower second deciduous molar dimensions in strabismic girls when compared with that in controls and in strabismic boys, suggesting better developmental canalization in female.

CONCLUSIONS

Asymmetries in the head area, such as promoted here in strabismic children, may have associations with asymmetries in the dentition, focusing the embryonal origins and timing of developmental processes.

Regulation of gut and heart left-right asymmetry by context-dependent interactions between xenopus lefty and BMP4 signaling

The Lefty subfamily of TGFbeta signaling molecules has been implicated in early development in mouse, zebrafish, and chick. Here, we show that Xenopus lefty (Xlefty) is expressed both bilaterally in symmetric midline domains and unilaterally in left lateral plate mesoderm and anterior dorsal endoderm. To examine the roles of Xlefty in left-right development, we created a system for scoring gut asymmetry and examined the effects of unilateral Xlefty misexpression on gut development, heart development, and Xnr-1 and XPitx2 expression. In contrast to the unilateral effects of Vg1, Activin, Nodal, or BMPs, targeted expression of Xlefty in either the left or the right side of Xenopus embryos randomized the direction of heart looping, gut coiling, and left-right positioning of the gut and downregulated the asymmetric expression of Xnr-1 and XPitx2. It is currently thought that Lefty proteins act as feedback inhibitors of Nodal signaling. However, this would not explain the effects of right-sided Xlefty misexpression. Here, we show that Xlefty interacts with the signaling pathways of other members of the TGFbeta family during left-right development. Results from coexpression of Xlefty and Vg1 indicate that Xlefty can nullify the effects of Vg1 ectopic expression and that Xlefty is downstream of left-sided Vg1 signaling. Results from coexpression of Xlefty and XBMP4 indicate that XLefty and XBMP4 interact both synergistically and antagonistically in a context-dependent manner. We propose a model in which interactions of Xlefty with multiple members of the TGFbeta family enhance the differences between the right-sided BMP/ALK2/Smad pathway and the left-sided Vg1/anti-BMP/Nodal pathway, leading to left-right morphogenesis of the gut and heart.