Ectodermal Wnt signaling regulates abdominal myogenesis during ventral body wall development

Ventral body wall closure: Mechanistic insights from mouse models and translation to human pathology

The ventral body wall (VBW) that encloses the thoracic and abdominal cavities arises by extensive cell movements and morphogenetic changes during embryonic development. These morphogenetic processes include embryonic folding generating the primary body wall; the initial ventral cover of the embryo, followed by directed mesodermal cell migrations, contributing to the secondary body wall. Clinical anomalies in VBW development affect approximately 1 in 3000 live births. However, the cell interactions and critical cellular behaviors that control VBW development remain little understood. Here, we describe the embryonic origins of the VBW, the cellular and morphogenetic processes, and key genes, that are essential for VBW development. We also provide a clinical overview of VBW anomalies, together with environmental and genetic influences, and discuss the insight gained from over 70 mouse models that exhibit VBW defects, and their relevance, with respect to human pathology. In doing so we propose a phenotypic framework for researchers in the field which takes into account the clinical picture. We also highlight cases where there is a current paucity of mouse models for particular clinical defects and key gaps in knowledge about embryonic VBW development that need to be addressed to further understand mechanisms of human VBW pathologies.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Exomphalos with intestinal fistulation: Case series and systematic review for clinical characterization, management and embryopathogenesis

INTRODUCTION

Exomphalos with intestinal fistulation (EIF) is a rare variant of exomphalos with intestine opening to the surface of an intact sac, and may result in a diagnostic challenge. We report 3 new cases and conducted a systematic review of the literature, to characterize its association with the type of exomphalos and vitellointestinal duct (VI) as well as to evaluate its management and outcomes.

METHODS

A literature search from PubMed using keywords pertaining to exomphalos and fistulation was used to identify all unique cases reported between 1950 and 2020, in addition to the case series reported here, to establish the clinical presentation, histological findings, management and outcomes.

RESULTS

We found a total of 28 cases of EIF, of which 25 had been reported in 70 years from 19 reports. There was a male predominance (4-to-1 ratio). The majority presented as an exomphalos minor (n = 23, 82%) with a prolapsing patent VI duct (n = 16, 57%), most had evidence of Meckel's diverticulum (n = 25, 89%). All but one case were managed by fistula excision with or without ileal resection and anastomosis, followed by primary closure of the abdominal wall defect. All patients, except one with Trisomy 13 who received only palliative care, underwent surgery. Post-operative complications occurred in 7 patients (25%). Congenital anomalies were present in 12 (43%) and none had Beckwith-Wiedemann syndrome. Mortality occurred in 4 patients (14%) between 3 and 17 days. One EIF with exomphalos major failed early conservative treatment due to sac disintegration and sepsis, requiring staged closure, but had a good outcome.

DISCUSSION

EIF is a rare entity usually associated with exomphalos minor and vitelline duct involvement. EIF presentation is variable but primary surgery is the mainstay of treatment with generally good outcomes. Common features of EIF suggest a different embryopathogenesis to other forms of exomphalos.

The Fgf8 subfamily (Fgf8, Fgf17 and Fgf18) is required for closure of the embryonic ventral body wall

The closure of the embryonic ventral body wall in amniotes is an important morphogenetic event and is essential for life. Defects in human ventral wall closure are a major class of birth defect and a significant health burden. Despite this, very little is understood about how the ventral body wall is formed. Here, we show that fibroblast growth factor (FGF) ligands FGF8, FGF17 and FGF18 are essential for this process. Conditional mouse mutants for these genes display subtle migratory defects in the abdominal muscles of the ventral body wall and an enlarged umbilical ring, through which the internal organs are extruded. By refining where and when these genes are required using different Cre lines, we show that Fgf8 and Fgf17 are required in the presomitic mesoderm, whereas Fgf18 is required in the somites. This study identifies complex and multifactorial origins of ventral wall defects and has important implications for understanding their origins during embryonic development.

Development of the amniote ventrolateral body wall

In vertebrates, the trunk consists of the musculoskeletal structures of the back and the ventrolateral body wall, which together enclose the internal organs of the circulatory, digestive, respiratory and urogenital systems. This review gives an overview on the development of the thoracic and abdominal wall during amniote embryogenesis. Specifically, I briefly summarize relevant historical concepts and the present knowledge on the early embryonic development of ribs, sternum, intercostal muscles and abdominal muscles with respect to anatomical bauplan, origin and specification of precursor cells, initial steps of pattern formation, and cellular and molecular regulation of morphogenesis.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Abrogation of TGF-beta signalling in TAGLN expressing cells recapitulates Pentalogy of Cantrell in the mouse

Pentalogy of Cantrell (PC) is a rare multi-organ congenital anomaly that impedes ventral body wall closure and results in diaphragmatic hernia, intra- and pericardial defects. The underlying cellular and molecular changes that lead to these severe developmental defects have remained unknown largely due to the lack of representative animal models. Here we provide in depth characterization of a mouse model with conditional ablation of TGFβRII in Transgelin (Tagln) expressing cells. We show that Tagln is transiently expressed in a variety of cells that participate in the embryonic development and patterning of ventral structures. Genetic ablation of TGFβRII in these cells leads to ventral midline closure defect, diaphragmatic hernia, dilated cardiac outflow tract and aberrant cardiac septation, providing a reliable model to study the morphological changes leading to PC. We show that myogenisis in the diaphragm is independent of TGFβ and the diaphragmatic hernia arises from fibroblast-specific migration defect. In the dorsal body wall Tagln expression is initiated after the closure process, revealing a remarkable difference between ventral and dorsal body walls development. Our study demonstrates the use of micro-CT scanning to obtain a 3-dimensional high-resolution overview of embryonic anomalies and provides the first mechanistic insight into the development of PC.

Exposure to sub-lethal dose of a combination insecticide during early embryogenesis influences the normal patterning of mesoderm resulting in incomplete closure of ventral body wall of chicks of domestic hen

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Chlorpyrifos and cypermethrin treatment induced developmental anomalies in chicks.

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Protrusion of visceral organs and microphthalmia were the major anomalies observed.

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Treated embryos were conspicuous with incomplete ventral body wall and sternum.

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Altered expression pattern of E-cadherin, Shh, bmp4, Wnt11 and Pitx2 were recorded.

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Impairment of major regulators of development is suspected to induce VBWD.

Pesticide exposure to the non target groups especially during embryonic development has quite often resulted in congenital malformations. A commercially available combination insecticide (Ci, 50% chlorpyrifos and 5% cypermethrin) is known to induce ventral body wall defects (VBWDs) wherein abdominal viscera protrude out of the ventral body wall. Herein, an attempt was made to understand the mechanistic insight into Ci induced VBWDs. For this, before incubation, the chick embryos were dosed with the test chemical and then at different developmental stages of incubation, they were monitored for the changes in the expression of certain genes, which are indispensable for the ventral body wall closure since they regulate the cell fate, proliferation and survival. Concurrently, histopathological changes during the embryonic development were examined to corroborate the above observations. The results of mRNA profiling revealed a significant downregulation of Shh on day 4 and upregulation on day 10, while bmp4, Pitx2, E-cadherin, Wnt11, Wnt6, Pxn, MyoD1, Caspase-3, AHR, Cyp3A4, showed a significant upregulation on day 4 and/or on day 10. N-cadherin, fgf8, bmp1 showed no significant changes. The possible means by which these skewed expression patterns of regulatory molecules culminated into the VBWD are discussed.

Transgelin-expressing myofibroblasts orchestrate ventral midline closure through TGFβ signalling

Ventral body wall (VBW) defects are among the most common congenital malformations, yet their embryonic origin and underlying molecular mechanisms remain poorly characterised. Transforming growth factor beta (TGFβ) signalling is essential for VBW closure, but the responding cells are not known. Here, we identify in mouse a population of migratory myofibroblasts at the leading edge of the closing VBW that express the actin-binding protein transgelin (TAGLN) and TGFβ receptor (TGFβR). These cells respond to a temporally regulated TGFβ2 gradient originating from the epithelium of the primary body wall. Targeted elimination of TGFβR2 in TAGLN⁺ cells impairs midline closure and prevents the correct subsequent patterning of the musculature and skeletal components. Remarkably, deletion of Tgfbr2 in myogenic or chondrogenic progenitor cells does not manifest in midline defects. Our results indicate a pivotal significance of VBW myofibroblasts in orchestrating ventral midline closure by mediating the response to the TGFβ gradient. Altogether, our data enable us to distinguish highly regulated epithelial-mesenchymal signalling and successive cellular migration events in VBW closure that explain early morphological changes underlying the development of congenital VBW defects.

Summary: A population of migratory myofibroblasts at the leading edge of the closing ventral body wall expresses cytoskeletal components and TGFβR2 and responds to an epithelial TGFβ2 morphogen gradient to drive midline closure.

Epispadias and the associated embryopathies: genetic and developmental basis

The abnormalities in the urogenital organs are frequently observed as human developmental diseases. Among such diseases, the defects in the upper part of external genitalia are rather rare named epispadias. The cleft in the dorsal part of external genitalia often reaches to the urethra. In general, the urogenital abnormalities accompany defects in the adjacent tissues and organs. The ventral body wall and bladder can also be affected in the patients with dorsal defects of the external genitalia. Therefore, such multiple malformations are often classified as bladder exstrophy and epispadias complex (BEEC). Because of the lower frequency of such birth defects and their early embryonic development, animal models are required to analyze the pathogenic mechanisms and the functions of responsible genes. Mutant mouse analyses on various signal cascades for external genitalia and body wall development are increasingly performed. The genetic interactions between growth factors such as bone morphogenetic proteins (Bmp) and transcription factors such as Msx1/2 and Isl1 have been suggested to play roles for such organogenesis. The significance of epithelial-mesenchymal interaction (EMI) is suggested during development. In this review, we describe on such local interactions and developmental regulators. We also introduce some mutant mouse models displaying external genitalia-body wall abnormalities.

Control of stomach smooth muscle development and intestinal rotation by transcription factor BARX1

Diverse functions of the homeodomain transcription factor BARX1 include Wnt-dependent, non-cell autonomous specification of the stomach epithelium, tracheo-bronchial septation, and Wnt-independent expansion of the spleen primordium. Tight spatio-temporal regulation of Barx1 levels in the mesentery and stomach mesenchyme suggests additional roles. To determine these functions, we forced constitutive BARX1 expression in the Bapx1 expression domain, which includes the mesentery and intestinal mesenchyme, and also examined Barx1(-/)(-) embryos in further detail. Transgenic embryos invariably showed intestinal truncation and malrotation, in part reflecting abnormal left-right patterning. Ectopic BARX1 expression did not affect intestinal epithelium, but intestinal smooth muscle developed with features typical of the stomach wall. BARX1, which is normally restricted to the developing stomach, drives robust smooth muscle expansion in this organ by promoting proliferation of myogenic progenitors at the expense of other sub-epithelial cells. Undifferentiated embryonic stomach and intestinal mesenchyme showed modest differences in mRNA expression and BARX1 was sufficient to induce much of the stomach profile in intestinal cells. However, limited binding at cis-regulatory sites implies that BARX1 may act principally through other transcription factors. Genes expressed ectopically in BARX1(+) intestinal mesenchyme and reduced in Barx1(-/-) stomach mesenchyme include Isl1, Pitx1, Six2 and Pitx2, transcription factors known to control left-right patterning and influence smooth muscle development. The sum of evidence suggests that potent BARX1 functions in intestinal rotation and stomach myogenesis occur through this small group of intermediary transcription factors.

Mesenchymal Wnt signaling promotes formation of sternum and thoracic body wall

Midline defects account for approximately 5% of congenital abnormalities observed at birth. However, the molecular mechanisms underlying the formation of the ventral body wall are not well understood. Recent studies linked mutations in Porcupine-an O-acetyl transferase mediating Wnt ligand acylation-with defects in the thoracic body wall. We hypothesized that anomalous Wnt signaling is involved in the pathogenesis of defective closure of the thoracic body wall. We generated a mouse model wherein Wntless (Wls), which encodes a cargo receptor mediating secretion of Wnt ligands, was conditionally deleted from the developing mesenchyme using Dermo1Cre mice. Wls(f/f);Dermo1(Cre/+) embryos died during mid-gestation. At E13.5, skeletal defects were observed in the forelimbs, jaw, and rib cage. At E14.5, midline defects in the thoracic body wall began to emerge: the sternum failed to fuse and the heart protruded through the body wall at the midline (ectopia cordis). To determine the molecular mechanism underlying the phenotype observed in Wls(f/f);Dermo1(Cre/+) embryos, we tested whether Wnt/β-catenin signaling was operative in developing the embryonic ventral body wall using Axin2(LacZ) and BatGal reporter mice. While Wnt/β-catenin signaling activity was observed at the midline of the ventral body wall before sternal fusion, this pattern of activity was altered and scattered throughout the body wall after mesenchymal deletion of Wls. Mesenchymal cell migration was disrupted in Wls(f/f);Dermo1(Cre/+) thoracic body wall partially due to anomalous β-catenin independent Wnt signaling as determined by in vitro assays. Deletion of Lrp5 and Lrp6 receptors, which mediate Wnt/β-catenin signaling in the mesenchyme, partially recapitulated the phenotype observed in the chest midline of Wls(f/f);Dermo1(Cre/+) embryos supporting a role for Wnt/β-catenin signaling activity in the normal formation of the ventral body wall mesenchyme. We conclude that Wls-mediated secretion of Wnt ligands from the developing ventral body wall mesenchyme plays a critical role in fusion of the sternum and closure of the secondary body wall. Thus, impaired Wls activity in the ventral body wall mesenchyme is a mechanism underlying ectopia cordis and unfused sternum.