Loss of abdominal muscle in Pitx2 mutants associated with altered axial specification of lateral plate mesoderm

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.

Development of ribs and intercostal muscles in the chicken embryo

In the thorax of higher vertebrates, ribs and intercostal muscles play a decisive role in stability and respiratory movements of the body wall. They are derivatives of the somites, the ribs originating in the sclerotome and the intercostal muscles originating in the myotome. During thorax development, ribs and intercostal muscles extend into the lateral plate mesoderm and eventually contact the sternum during ventral closure. Here, we give a detailed description of the morphogenesis of ribs and thoracic muscles in the chicken embryo (Gallus gallus). Using Alcian blue staining as well as Sox9 and Desmin whole‐mount immunohistochemistry, we monitor synchronously the development of rib cartilage and intercostal muscle anlagen. We show that the muscle anlagen precede the rib anlagen during ventrolateral extension, which is in line with the inductive role of the myotome in rib differentiation. Our studies furthermore reveal the temporary formation of a previously unknown eighth rib in the chicken embryonic thorax.

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.

Connecting muscle development, birth defects, and evolution: An essential role for muscle connective tissue

Skeletal muscle powers all movement of the vertebrate body and is distributed in multiple regions that have evolved distinct functions. Axial muscles are ancestral muscles essential for support and locomotion of the whole body. The evolution of the head was accompanied by development of cranial muscles essential for eye movement, feeding, vocalization, and facial expression. With the evolution of paired fins and limbs and their associated muscles, vertebrates gained increased locomotor agility, populated the land, and acquired fine motor skills. Finally, unique muscles with specialized functions have evolved in some groups, and the diaphragm which solely evolved in mammals to increase respiratory capacity is one such example. The function of all these muscles requires their integration with the other components of the musculoskeletal system: muscle connective tissue (MCT), tendons, bones as well as nerves and vasculature. MCT is muscle's closest anatomical and functional partner. Not only is MCT critical in the adult for muscle structure and function, but recently MCT in the embryo has been found to be crucial for muscle development. In this review, we examine the important role of the MCT in axial, head, limb, and diaphragm muscles for regulating normal muscle development, discuss how defects in MCT-muscle interactions during development underlie the etiology of a range of birth defects, and explore how changes in MCT development or communication with muscle may have led to the modification and acquisition of new muscles during vertebrate evolution.

Requirement of Pitx2 for skeletal muscle homeostasis

Skeletal muscle is generated by the successive incorporation of primary (embryonic), secondary (fetal), and tertiary (adult) fibers into muscle. Conditional excision of Pitx2 function by an MCKCre driver resulted in animals with histological and ultrastructural defects in P30 muscles and fibers, respectively. Mutant muscle showed severe reduction in mitochondria and FoxO3-mediated mitophagy. Both oxidative and glycolytic energy metabolism were reduced. Conditional excision was limited to fetal muscle fibers after the G1-G0 transition and resulted in altered MHC, Rac1, MEF2a, and alpha-tubulin expression within these fibers. The onset of excision, monitored by a nuclear reporter gene, was observed as early as E16. Muscle at this stage was already severely malformed, but appeared to recover by P30 by the expansion of adjoining larger fibers. Our studies demonstrate that the homeodomain transcription factor Pitx2 has a postmitotic role in maintaining skeletal muscle integrity and energy homeostasis in fetal muscle fibers.

Mice doubly deficient in Six4 and Six5 show ventral body wall defects reproducing human omphalocele

Omphalocele is a human congenital anomaly in ventral body wall closure and may be caused by impaired formation of the primary abdominal wall (PAW) and/or defects in abdominal muscle development. Here, we report that mice doubly deficient in homeobox genes Six4 and Six5 showed the same ventral body wall closure defects as those seen in human omphalocele. SIX4 and SIX5 were localized in surface ectodermal cells and somatic mesoderm-derived mesenchymal and coelomic epithelial cells (CECs) in the PAW. Six4^-/-;Six5^-/- fetuses exhibited a large omphalocele with protrusion of both the liver and intestine, or a small omphalocele with protrusion of the intestine, with complete penetrance. The umbilical ring of Six4^-/-;Six5^-/- embryos was shifted anteriorly and its lateral size was larger than that of normal embryos at the E11.5 stage, before the onset of myoblast migration into the PAW. The proliferation rates of surface ectodermal cells in the left and right PAW and somatic mesoderm-derived cells in the right PAW were lower in Six4^-/-;Six5^-/- embryos than those of wild-type embryos at E10.5. The transition from CECs of the PAW to rounded mesothelial progenitor cells was impaired and the inner coelomic surface of the PAW was relatively smooth in Six4^-/-;Six5^-/- embryos at E11.25. Furthermore, Six4 overexpression in CECs of the PAW promoted ingression of CECs. Taken together, our results suggest that Six4 and Six5 are required for growth and morphological change of the PAW, and the impairment of these processes is linked to the abnormal positioning and expansion of the umbilical ring, which results in omphalocele.

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.

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.

Mapping the Chromatin State Dynamics in Myoblasts

Background

Genome-wide mapping reveals chromatin landscapes unique to cell states. Histone marks of regulatory genes involved in cell specification and organ development provide a powerful tool to map regulatory sequences. H3K4me3 marks promoter regions; H3K27me3 marks repressed regions, and Pol II presence indicates active transcription. The presence of both H3K4me3 and H3K27me3 characterize poised sequences, a common characteristic of genes involved in pattern formation during organogenesis.

Results

We used genome-wide profiling for H3K27me3, H3K4me3, and Pol II to map chromatin states in mouse embryonic day 12 forelimbs in wild type (control) and Pitx2-null mutant mice. We compared these data with previous gene expression studies from forelimb Lbx1⁺ migratory myoblasts and correlated Pitx2-dependent expression profiles and chromatin states. During forelimb development, several lineages including myoblast, osteoblast, neurons, angioblasts etc., require synchronized growth to form a functional limb. We identified 125 genes in the developing forelimb that are Pitx2-dependent. Genes involved in muscle specification and cytoskeleton architecture were positively regulated, while genes involved in axonal path finding were poised.

Conclusion

Our results have established histone modification profiles as a useful tool for identifying gene regulatory states in muscle development, and identified the role of Pitx2 in extending the time of myoblast progression, promoting formation of sarcomeric structures, and suppressing attachment of neuronal axons.

Ectodermal Wnt signaling regulates abdominal myogenesis during ventral body wall development

Defects of the ventral body wall are prevalent birth anomalies marked by deficiencies in body wall closure, hypoplasia of the abdominal musculature and multiple malformations across a gamut of organs. However, the mechanisms underlying ventral body wall defects remain elusive. Here, we investigated the role of Wnt signaling in ventral body wall development by inactivating Wls or β-catenin in murine abdominal ectoderm. The loss of Wls in the ventral epithelium, which blocks the secretion of Wnt proteins, resulted in dysgenesis of ventral musculature and genito-urinary tract during embryonic development. Molecular analyses revealed that the dermis and myogenic differentiation in the underlying mesenchymal progenitor cells was perturbed by the loss of ectodermal Wls. The activity of the Wnt-Pitx2 axis was impaired in the ventral mesenchyme of the mutant body wall, which partially accounted for the defects in ventral musculature formation. In contrast, epithelial depletion of β-catenin or Wnt5a did not resemble the body wall defects in the ectodermal Wls mutant. These findings indicate that ectodermal Wnt signaling instructs the underlying mesodermal specification and abdominal musculature formation during ventral body wall development, adding evidence to the theory that ectoderm-mesenchyme signaling is a potential unifying mechanism for the origin of ventral body wall defects.

Pitx2-mediated cardiac outflow tract remodeling

BACKGROUND

Heart morphogenesis involves sequential anatomical changes from a linear tube of a single channel peristaltic pump to a four-chamber structure with two channels controlled by one-way valves. The developing heart undergoes continuous remodeling, including septation.

RESULTS

Pitx2-null mice are characterized by cardiac septational defects of the atria, ventricles, and outflow tract. Pitx2-null mice also exhibited a short outflow tract, including unseptated conus and deformed endocardial cushions. Cushions were characterized with a jelly-like structure, rather than the distinct membrane-looking leaflets, indicating that endothelial mesenchymal transition was impaired in Pitx2(-/-) embryos. Mesoderm cells from the branchial arches and neural crest cells from the otic region contribute to the development of the endocardial cushions, and both were reduced in number. Members of the Fgf and Bmp families exhibited altered expression levels in the mutants.

CONCLUSIONS

We suggest that Pitx2 is involved in the cardiac outflow tract septation by promoting and/or maintaining the number and the remodeling process of the mesoderm progenitor cells. Pitx2 influences the expression of transcription factors and signaling molecules involved in the differentiation of the cushion mesenchyme during heart development.