Direct activation of Shroom3 transcription by Pitx proteins drives epithelial morphogenesis in the developing gut

Shroom3 facilitates optic fissure closure via tissue alignment and reestablishment of apical-basal polarity during epithelial fusion

Optic cup morphogenesis is a complex process involving cellular behaviors such as epithelial folding, cell shape changes, proliferation, and tissue fusion. Disruptions to these processes can lead to an ocular coloboma, a congenital defect where the optic fissure fails to close. This study investigates the role of Shroom3, a protein implicated in epithelial morphogenesis, in mouse embryos during optic cup development. It was observed that Shroom3 is apically localized in the neural retina and retinal pigmented epithelium, and its deficiency leads to a both a conventional coloboma phenotype characterized by a gap in pigmented tissue as well as a unique type of coloboma where an ectopic ventral fold of neural tissue is present. Increased apical areas of both neural retina and retinal pigmented epithelial cells are present in the absence of Shroom3 leading to a greater apical surface area and disruption of optic fissure alignment. Neural retina specific gene ablation revealed that Shroom3 function in the RPE is likely sufficient to facilitate tissue alignment and permit fusion. However, the fusion process is ultimately disturbed due to a failure of the neural tissue to reestablish apical-basal polarity. Furthermore, it is demonstrated that Shroom3 deficiency also affects other epithelial fusion events in the embryo that rely on polarity reestablishment, such as lens vesicle separation, eyelid formation, and secondary palate closure. These findings highlight the importance of Shroom3 during optic cup morphogenesis, aid our understanding of optic fissure closure and coloboma formation, and implicates a role for Shroom3 in regulating apical-basal polarity.

Common cis-regulatory variation modifies the penetrance of pathogenic SHROOM3 variants in craniofacial microsomia

Pathogenic coding variants have been identified in thousands of genes, yet the mechanisms underlying the incomplete penetrance in individuals carrying these variants are poorly understood. In this study, in a cohort of 2009 craniofacial microsomia (CFM) patients of Chinese ancestry and 2625 Han Chinese controls, we identified multiple predicted pathogenic coding variants in SHROOM3 in both CFM patients and healthy individuals. We found that the penetrance of CFM correlates with specific haplotype combinations containing likely pathogenic-coding SHROOM3 variants and CFM-associated expression quantitative trait loci (eQTLs) of SHROOM3 expression. Further investigations implicate specific eQTL combinations, such as rs1001322 or rs344131, in combination with other significant CFM-associated eQTLs, which we term combined eQTL phenotype modifiers (CePMods). We additionally show that rs344131, located within a regulatory enhancer region of SHROOM3, demonstrates allele-specific effects on enhancer activity and thus impacts expression levels of the associated SHROOM3 allele harboring any rare coding variant. Our findings also suggest that CePMods may serve as pathogenic determinants, even in the absence of rare deleterious coding variants in SHROOM3 This highlights the critical role of allelic expression in determining the penetrance and severity of craniofacial abnormalities, including microtia and facial asymmetry. Additionally, using quantitative phenotyping, we demonstrate that both microtia and facial asymmetry are present in two separate Shroom3 mouse models, the severity of which is dependent on gene dosage. Our study establishes SHROOM3 as a likely pathogenic gene for CFM and demonstrates eQTLs as determinants of modified penetrance in the manifestation of the disease in individuals carrying likely pathogenic rare coding variants.

Apical constriction requires patterned apical surface remodeling to synchronize cellular deformation

Apical constriction is a basic mechanism for epithelial morphogenesis, making columnar cells into wedge shape and bending a flat cell sheet. It has long been thought that an apically localized myosin generates a contractile force and drives the cell deformation. However, when we tested the increased apical surface contractility in a cellular Potts model simulation, the constriction increased pressure inside the cell and pushed its lateral surface outward, making the cells adopt a drop shape instead of the expected wedge shape. To keep the lateral surface straight, we considered an alternative model in which the cell shape was determined by cell membrane elasticity and endocytosis, and the increased pressure is balanced among the cells. The cellular Potts model simulation succeeded in reproducing the apical constriction, and it also suggested that a too strong apical surface tension might prevent the tissue invagination.

The Critical Role of the Shroom Family Proteins in Morphogenesis, Organogenesis and Disease

The Shroom (Shrm) family of actin-binding proteins has a unique and highly conserved Apx/Shrm Domain 2 (ASD2) motif. Shroom protein directs the subcellular localization of Rho-associated kinase (ROCK), which remodels the actomyosin cytoskeleton and changes cellular morphology via its ability to phosphorylate and activate non-muscle myosin II. Therefore, the Shrm-ROCK complex is critical for the cellular shape and the development of many tissues, including the neural tube, eye, intestines, heart, and vasculature system. Importantly, the structure and expression of Shrm proteins are also associated with neural tube defects, chronic kidney disease, metastasis of carcinoma, and X-link mental retardation. Therefore, a better understanding of Shrm-mediated signaling transduction pathways is essential for the development of new therapeutic strategies to minimize damage resulting in abnormal Shrm proteins. This paper provides a comprehensive overview of the various Shrm proteins and their roles in morphogenesis and disease.

The Good and the Bad of SHROOM3 in Kidney Development and Disease: A Narrative Review

Purpose of review

Multiple large-scale genome-wide association meta-analyses studies have reliably identified an association between genetic variants within the SHROOM3 gene and chronic kidney disease. This association extends to alterations in known markers of kidney disease including baseline estimated glomerular filtration rate, urinary albumin-to-creatinine ratio, and blood urea nitrogen. Yet, an understanding of the molecular mechanisms behind the association of SHROOM3 and kidney disease remains poorly communicated. We conducted a narrative review to summarize the current state of literature regarding the genetic and molecular relationships between SHROOM3 and kidney development and disease.

Sources of information

PubMed, PubMed Central, SCOPUS, and Web of Science databases, as well as review of references from relevant studies and independent Google Scholar searches to fill gaps in knowledge.

Methods

A comprehensive narrative review was conducted to explore the molecular mechanisms underlying SHROOM3 and kidney development, function, and disease.

Key findings

SHROOM3 is a unique protein, as it is the only member of the SHROOM group of proteins that regulates actin dynamics through apical constriction and apicobasal cell elongation. It holds a dichotomous role in the kidney, as subtle alterations in SHROOM3 expression and function can be both pathological and protective toward kidney disease. Genome-wide association studies have identified genetic variants near the transcription start site of the SHROOM3 gene associated with chronic kidney disease. SHROOM3 also appears to protect the glomerular structure and function in conditions such as focal segmental glomerulosclerosis. However, little is known about the exact mechanisms by which this protection occurs, which is why SHROOM3 binding partners remain an opportunity for further investigation.

Limitations

Our search was limited to English articles. No structured assessment of study quality was performed, and selection bias of included articles may have occurred. As we discuss future directions and opportunities, this narrative review reflects the academic views of the authors.

CellNeighborEX: deciphering neighbor-dependent gene expression from spatial transcriptomics data

Cells have evolved their communication methods to sense their microenvironments and send biological signals. In addition to communication using ligands and receptors, cells use diverse channels including gap junctions to communicate with their immediate neighbors. Current approaches, however, cannot effectively capture the influence of various microenvironments. Here, we propose a novel approach to investigate cell neighbor-dependent gene expression (CellNeighborEX) in spatial transcriptomics (ST) data. To categorize cells based on their microenvironment, CellNeighborEX uses direct cell location or the mixture of transcriptome from multiple cells depending on ST technologies. For each cell type, CellNeighborEX identifies diverse gene sets associated with partnering cell types, providing further insight. We found that cells express different genes depending on their neighboring cell types in various tissues including mouse embryos, brain, and liver cancer. Those genes are associated with critical biological processes such as development or metastases. We further validated that gene expression is induced by neighboring partners via spatial visualization. The neighbor-dependent gene expression suggests new potential genes involved in cell-cell interactions beyond what ligand-receptor co-expression can discover.

Somitic mesoderm morphogenesis is necessary for neural tube closure during Xenopus development

Neural tube closure is a fundamental process during vertebrate embryogenesis, which leads to the formation of the central nervous system. Defective neural tube closure leads to neural tube defects which are some of the most common human birth defects. While the intrinsic morphogenetic events shaping the neuroepithelium have been studied extensively, how tissues mechanically coupled with the neural plate influence neural tube closure remains poorly understood. Here, using Xenopus laevis embryos, live imaging in combination with loss of function experiments and morphometric analysis of fixed samples we explore the reciprocal mechanical communication between the neural plate and the somitic mesoderm and its impact on tissue morphogenesis. We show that although somitic mesoderm convergent extension occurs independently from neural plate morphogenesis neural tube closure depends on somitic mesoderm morphogenesis. Specifically, impaired somitic mesoderm remodelling results in defective apical constriction within the neuroepithelium and failure of neural tube closure. Last, our data reveal that mild abnormalities in somitic mesoderm and neural plate morphogenesis have a synergistic effect during neurulation, leading to severe neural tube closure defects. Overall, our data reveal that defective morphogenesis of tissues mechanically coupled with the neural plate can not only drastically exacerbate mild neural tube defects that may arise from abnormalities within the neural tissue but can also elicit neural tube defects even when the neural plate is itself free of inherent defects.

Optogenetic control of apical constriction induces synthetic morphogenesis in mammalian tissues

The emerging field of synthetic developmental biology proposes bottom-up approaches to examine the contribution of each cellular process to complex morphogenesis. However, the shortage of tools to manipulate three-dimensional (3D) shapes of mammalian tissues hinders the progress of the field. Here we report the development of OptoShroom3, an optogenetic tool that achieves fast spatiotemporal control of apical constriction in mammalian epithelia. Activation of OptoShroom3 through illumination in an epithelial Madin-Darby Canine Kidney (MDCK) cell sheet reduces the apical surface of the stimulated cells and causes displacements in the adjacent regions. Light-induced apical constriction provokes the folding of epithelial cell colonies on soft gels. Its application to murine and human neural organoids leads to thickening of neuroepithelia, apical lumen reduction in optic vesicles, and flattening in neuroectodermal tissues. These results show that spatiotemporal control of apical constriction can trigger several types of 3D deformation depending on the initial tissue context.

Discovery of new therapeutic targets in ovarian cancer through identifying significantly non-mutated genes

BACKGROUND

Mutated and non-mutated genes interact to drive cancer growth and metastasis. While research has focused on understanding the impact of mutated genes on cancer biology, understanding non-mutated genes that are essential to tumor development could lead to new therapeutic strategies. The recent advent of high-throughput whole genome sequencing being applied to many different samples has made it possible to calculate if genes are significantly non-mutated in a specific cancer patient cohort.

METHODS

We carried out random mutagenesis simulations of the human genome approximating the regions sequenced in the publicly available Cancer Growth Atlas Project for ovarian cancer (TCGA-OV). Simulated mutations were compared to the observed mutations in the TCGA-OV cohort and genes with the largest deviations from simulation were identified. Pathway analysis was performed on the non-mutated genes to better understand their biological function. We then compared gene expression, methylation and copy number distributions of non-mutated and mutated genes in cell lines and patient data from the TCGA-OV project. To directly test if non-mutated genes can affect cell proliferation, we carried out proof-of-concept RNAi silencing experiments of a panel of nine selected non-mutated genes in three ovarian cancer cell lines and one primary ovarian epithelial cell line.

RESULTS

We identified a set of genes that were mutated less than expected (non-mutated genes) and mutated more than expected (mutated genes). Pathway analysis revealed that non-mutated genes interact in cancer associated pathways. We found that non-mutated genes are expressed significantly more than mutated genes while also having lower methylation and higher copy number states indicating that they could be functionally important. RNAi silencing of the panel of non-mutated genes resulted in a greater significant reduction of cell viability in the cancer cell lines than in the non-cancer cell line. Finally, as a test case, silencing ANKLE2, a significantly non-mutated gene, affected the morphology, reduced migration, and increased the chemotherapeutic response of SKOV3 cells.

CONCLUSION

We show that we can identify significantly non-mutated genes in a large ovarian cancer cohort that are well-expressed in patient and cell line data and whose RNAi-induced silencing reduces viability in three ovarian cancer cell lines. Targeting non-mutated genes that are important for tumor growth and metastasis is a promising approach to expand cancer therapeutic options.

Mechanical control of tissue shape: Cell-extrinsic and -intrinsic mechanisms join forces to regulate morphogenesis

During vertebrate development, cells must proliferate, move, and differentiate to form complex shapes. Elucidating the mechanisms underlying the molecular and cellular processes involved in tissue morphogenesis is essential to understanding developmental programmes. Mechanical stimuli act as a major contributor of morphogenetic processes and impact on cell behaviours to regulate tissue shape and size. Specifically, cell extrinsic physical forces are translated into biochemical signals within cells, through the process of mechanotransduction, activating multiple mechanosensitive pathways and defining cell behaviours. Physical forces generated by tissue mechanics and the extracellular matrix are crucial to orchestrate tissue patterning and cell fate specification. At the cell scale, the actomyosin network generates the cellular tension behind the tissue mechanics involved in building tissue. Thus, understanding the role of physical forces during morphogenetic processes requires the consideration of the contribution of cell intrinsic and cell extrinsic influences. The recent development of multidisciplinary approaches, as well as major advances in genetics, microscopy, and force-probing tools, have been key to push this field forward. With this review, we aim to discuss recent work on how tissue shape can be controlled by mechanical forces by focusing specifically on vertebrate organogenesis. We consider the influences of mechanical forces by discussing the cell-intrinsic forces (such as cell tension and proliferation) and cell-extrinsic forces (such as substrate stiffness and flow forces). We review recently described processes supporting the role of intratissue force generation and propagation in the context of shape emergence. Lastly, we discuss the emerging role of tissue-scale changes in tissue material properties, extrinsic forces, and shear stress on shape establishment.

Programmed and self-organized flow of information during morphogenesis

How the shape of embryos and organs emerges during development is a fundamental question that has fascinated scientists for centuries. Tissue dynamics arise from a small set of cell behaviours, including shape changes, cell contact remodelling, cell migration, cell division and cell extrusion. These behaviours require control over cell mechanics, namely active stresses associated with protrusive, contractile and adhesive forces, and hydrostatic pressure, as well as material properties of cells that dictate how cells respond to active stresses. In this Review, we address how cell mechanics and the associated cell behaviours are robustly organized in space and time during tissue morphogenesis. We first outline how not only gene expression and the resulting biochemical cues, but also mechanics and geometry act as sources of morphogenetic information to ultimately define the time and length scales of the cell behaviours driving morphogenesis. Next, we present two idealized modes of how this information flows - how it is read out and translated into a biological effect - during morphogenesis. The first, akin to a programme, follows deterministic rules and is hierarchical. The second follows the principles of self-organization, which rests on statistical rules characterizing the system's composition and configuration, local interactions and feedback. We discuss the contribution of these two modes to the mechanisms of four very general classes of tissue deformation, namely tissue folding and invagination, tissue flow and extension, tissue hollowing and, finally, tissue branching. Overall, we suggest a conceptual framework for understanding morphogenetic information that encapsulates genetics and biochemistry as well as mechanics and geometry as information modules, and the interplay of deterministic and self-organized mechanisms of their deployment, thereby diverging considerably from the traditional notion that shape is fully encoded and determined by genes.

SHROOM3 is downstream of the planar cell polarity pathway and loss-of-function results in congenital heart defects

Congenital heart disease (CHD) is the most common birth defect, and the leading cause of death due to birth defects, yet causative molecular mechanisms remain mostly unknown. We previously implicated a novel CHD candidate gene, SHROOM3, in a patient with CHD. Using a Shroom3 gene trap knockout mouse (Shroom3^gt/gt) we demonstrate that SHROOM3 is downstream of the noncanonical Wnt planar cell polarity signaling pathway (PCP) and loss-of-function causes cardiac defects. We demonstrate Shroom3 expression within cardiomyocytes of the ventricles and interventricular septum from E10.5 onward, as well as within cardiac neural crest cells and second heart field cells that populate the cardiac outflow tract. We demonstrate that Shroom3^gt/gt mice exhibit variable penetrance of a spectrum of CHDs that include ventricular septal defects, double outlet right ventricle, and thin left ventricular myocardium. This CHD spectrum phenocopies what is observed with disrupted PCP. We show that during cardiac development SHROOM3 interacts physically and genetically with, and is downstream of, key PCP signaling component Dishevelled 2. Within Shroom3^gt/gt hearts we demonstrate disrupted terminal PCP components, actomyosin cytoskeleton, cardiomyocyte polarity, organization, proliferation and morphology. Together, these data demonstrate SHROOM3 functions during cardiac development as an actomyosin cytoskeleton effector downstream of PCP signaling, revealing SHROOM3's novel role in cardiac development and CHD.

Convergent extension in the amphibian, Xenopus laevis

This review is a comprehensive analysis of the cell biology and biomechanics of Convergent Extension in Xenopus.

Folic acid modifies the shape of epithelial cells during morphogenesis via a Folr1 and MLCK dependent mechanism

Folic acid supplementation can prevent neural tube defects, but the specific molecular mechanisms by which it does have not been elucidated. During neural plate morphogenesis, epithelial cell apical constriction cooperates with other events to drive tissue-bending, and when defective, can result in neural tube defects. A Rho-kinase deficient binding mutant of the apical constriction regulating protein, Shroom3 (Shroom3^R1838C), is one of only a handful of mouse mutant lines with neural tube defects that can be rescued by folic acid supplementation. This provided a unique opportunity to probe the functional rescue of a protein linked to neural tube development by folic acid. Utilizing an epithelial cell culture model of apical constriction, it was observed that treatment with exogenous folic acid, as well as co-expression of the folic acid receptor Folr1, can rescue the function of the Rho-kinase binding deficient mutant of Shroom3 in vitro It was also determined that the rescuing ability of folic acid is RhoA and Rho-kinase independent but myosin light chain kinase (MLCK) and Src-kinase dependent. Inhibition of Rho-kinase-dependent apical constriction in chick embryo neural epithelium was also observed to be rescued by exogenous folic acid and that treatment with folic acid is accompanied by elevated activated myosin light chain and MLCK. Furthermore, doubly heterozygous mouse embryos lacking one copy each of Shroom3 and Folr1 exhibit a low rate of neural tube defects and also have lower levels of activated myosin light chain and MLCK. These studies suggest a novel mechanism by which folic acid modifies epithelial cell shape during morphogenesis, shedding light onto how folic acid may prevent neural tube defects.

The RhoGEF protein Plekhg5 regulates apical constriction of bottle cells during gastrulation

Apical constriction regulates epithelial morphogenesis during embryonic development, but how this process is controlled is not understood completely. Here, we identify a Rho guanine nucleotide exchange factor (GEF) gene plekhg5 as an essential regulator of apical constriction of bottle cells during Xenopus gastrulation. plekhg5 is expressed in the blastopore lip and its expression is sufficient to induce ectopic bottle cells in epithelia of different germ layers in a Rho-dependent manner. This activity is not shared by arhgef3, which encodes another organizer-specific RhoGEF. Plekhg5 protein is localized in the apical cell cortex via its pleckstrin homology domain, and the GEF activity enhances its apical recruitment. Plekhg5 induces apical actomyosin accumulation and cell elongation. Knockdown of plekhg5 inhibits activin-induced bottle cell formation and endogenous blastopore lip formation in gastrulating frog embryos. Apical accumulation of actomyosin, apical constriction and bottle cell formation fail to occur in these embryos. Taken together, our data indicate that transcriptional regulation of plekhg5 expression at the blastopore lip determines bottle cell morphology via local polarized activation of Rho by Plekhg5, which stimulates apical actomyosin activity to induce apical constriction.

Principles of Actomyosin Regulation In Vivo

The actomyosin cytoskeleton is responsible for most force-driven processes in cells and tissues. How it assembles into the necessary structures at the right time and place is an important question. Here, we focus on molecular mechanisms of actomyosin regulation recently elucidated in animal models, and highlight several common principles that emerge. The architecture of the actomyosin network - an important determinant of its function - results from actin polymerization, crosslinking and turnover, localized myosin activation, and contractility-driven self-organization. Spatiotemporal regulation is achieved by tissue-specific expression and subcellular localization of Rho GTPase regulators. Subcellular anchor points of actomyosin structures control the outcome of their contraction, and molecular feedback mechanisms dictate whether they are transient, cyclic, or persistent.

Transcriptional regulation of cell shape during organ morphogenesis

The emerging field of transcriptional regulation of cell shape changes aims to address the critical question of how gene expression programs produce a change in cell shape. Together with cell growth, division, and death, changes in cell shape are essential for organ morphogenesis. Whereas most studies of cell shape focus on posttranslational events involved in protein organization and distribution, cell shape changes can be genetically programmed. This review highlights the essential role of transcriptional regulation of cell shape during morphogenesis of the heart, lungs, gastrointestinal tract, and kidneys. We emphasize the evolutionary conservation of these processes across different model organisms and discuss perspectives on open questions and research avenues that may provide mechanistic insights toward understanding birth defects.

The left-right asymmetry of liver lobation is generated by Pitx2c-mediated asymmetries in the hepatic diverticulum

Internal organs exhibit left-right asymmetric sizes, shapes and anatomical positions, but how these different lateralities develop is poorly understood. Here we use the experimentally tractable Xenopus model to uncover the morphogenetic events that drive the left-right asymmetrical lobation of the liver. On the right side of the early hepatic diverticulum, endoderm cells become columnar and apically constricted, forming an expanded epithelial surface and, ultimately, an enlarged right liver lobe. In contrast, the cells on the left side become rounder, and rearrange into a compact, stratified architecture that produces a smaller left lobe. Side-specific gain- and loss-of-function studies reveal that asymmetric expression of the left-right determinant Pitx2c elicits distinct epithelial morphogenesis events in the left side of the diverticulum. Surprisingly, the cellular events induced by Pitx2c during liver development are opposite those induced in other digestive organs, suggesting divergent cellular mechanisms underlie the formation of different lateralities.

Pitx1 regulates cement gland development in Xenopus laevis through activation of transcriptional targets and inhibition of BMP signaling

The cement gland in Xenopus laevis has long been used as a model to study the interplay of cell signaling and transcription factors during embryogenesis. It has been shown that an intermediate level of Bone Morphogenetic Protein (BMP) signaling is essential for cement gland formation. In addition, several transcription factors have been linked to cement gland development. One of these, the homeodomain-containing protein Pitx1, can generate ectopic cement gland formation; however, the mechanisms underlying this process remain obscure. We report here, for the first time, a requirement for Pitx proteins in cement gland formation, in vivo: knockdown of both pitx1 and the closely related pitx2c inhibit endogenous cement gland formation. Pitx1 transcriptionally activates cement gland differentiation genes through both direct and indirect mechanisms, and functions as a transcriptional activator to inhibit BMP signaling. This inhibition, required for the expression of pitx genes, is partially mediated by Pitx1-dependent follistatin expression. Complete suppression of BMP signaling inhibits induction of cement gland markers by Pitx1; furthermore, we find that Pitx1 physically interacts with Smad1, an intracellular transducer of BMP signaling. We propose a model of cement gland formation in which Pitx1 limits local BMP signaling within the cement gland primordium, and recruits Smad1 to activate direct downstream targets.

Cadherin-mediated cell-cell interactions in normal and cancer cells

Adherens junctions (AJs) are molecular complexes that mediate cell-cell adhesive interactions and play pivotal roles in maintenance of tissue organization in adult organisms and at various stages of development. AJs consist of cadherin adhesion receptors, providing homophilic ligation with cadherins on adjacent cells, and members of the catenin protein family: p120, β- and α-catenin. α-catenin's linkage with the actin cytoskeleton defines the linear or punctate organization of AJs in different cell types. Myosin II-dependent tension drives vinculin recruitment by α-catenin and stabilizes the linkage of the cadherin/catenin complex to F-actin. Neoplastic transformation leads to prominent changes in the organization, regulation and stability of AJs. Epithelial-mesenchymal transition (EMT) whereby epithelial cells lose stable cell-cell adhesion, and reorganize their cytoskeleton to acquire migratory activity, plays the central role in cancer cell invasion and metastasis. Recent data demonstrated that a partial EMT resulting in a hybrid epithelial/mesenchymal phenotype with retention of E-cadherin is essential for cancer cell dissemination. E-cadherin and E-cadherin-based AJs are required for collective invasion and migration, survival in circulation, and metastatic outgrowth.

Stomach curvature is generated by left-right asymmetric gut morphogenesis

Left-right (LR) asymmetry is a fundamental feature of internal anatomy, yet the emergence of morphological asymmetry remains one of the least understood phases of organogenesis. Asymmetric rotation of the intestine is directed by forces outside the gut, but the morphogenetic events that generate anatomical asymmetry in other regions of the digestive tract remain unknown. Here, we show in mouse and Xenopus that the mechanisms that drive the curvature of the stomach are intrinsic to the gut tube itself. The left wall of the primitive stomach expands more than the right wall, as the left epithelium becomes more polarized and undergoes radial rearrangement. These asymmetries exist across several species, and are dependent on LR patterning genes, including Foxj1, Nodal and Pitx2 Our findings have implications for how LR patterning manifests distinct types of morphological asymmetries in different contexts.

La-related protein 6 controls ciliated cell differentiation

Background

La-related protein 6 (LARP6) is an evolutionally conserved RNA-binding protein. Vertebrate LARP6 binds the 5′ stem-loop found in mRNAs encoding type I collagen to regulate their translation, but other target mRNAs and additional functions for LARP6 are unknown. The aim of this study was to elucidate an additional function of LARP6 and to evaluate the importance of its function during development.

Methods

To uncover the role of LARP6 in development, we utilized Morpholino Oligos to deplete LARP6 protein in Xenopus embryos. Then, embryonic phenotypes and ciliary structures of LAPR6 morphants were examined. To identify the molecular mechanism underlying ciliogenesis regulated by LARP6, we tested the expression level of cilia-related genes, which play important roles in ciliogenesis, by RT-PCR or whole mount in situ hybridization (WISH).

Results

We knocked down LARP6 in Xenopus embryos and found neural tube closure defects. LARP6 mutant, which compromises the collagen synthesis, could rescue these defects. Neural tube closure defects are coincident with lack of cilia, antenna-like cellular organelles with motility- or sensory-related functions, in the neural tube. The absence of cilia at the epidermis was also observed in LARP6 morphants, and this defect was due to the absence of basal bodies which are formed from centrioles and required for ciliary assembly. In the process of multi-ciliated cell (MCC) differentiation, mcidas, which activates the transcription of genes required for centriole formation during ciliogenesis, could partially restore MCCs in LARP6 morphants. In addition, LARP6 likely controls the expression of mcidas in a Notch-independent manner.

Conclusions

La-related protein 6 is involved in ciliated cell differentiation during development by controlling the expression of cilia-related genes including mcidas. This LARP6 function involves a mechanism that is distinct from its established role in binding to collagen mRNAs and regulating their translation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-017-0047-7) contains supplementary material, which is available to authorized users.

Frogs as integrative models for understanding digestive organ development and evolution

The digestive system comprises numerous cells, tissues and organs that are essential for the proper assimilation of nutrients and energy. Many aspects of digestive organ function are highly conserved among vertebrates, yet the final anatomical configuration of the gut varies widely between species, especially those with different diets. Improved understanding of the complex molecular and cellular events that orchestrate digestive organ development is pertinent to many areas of biology and medicine, including the regeneration or replacement of diseased organs, the etiology of digestive organ birth defects, and the evolution of specialized features of digestive anatomy. In this review, we highlight specific examples of how investigations using Xenopus laevis frog embryos have revealed insight into the molecular and cellular dynamics of digestive organ patterning and morphogenesis that would have been difficult to obtain in other animal models. Additionally, we discuss recent studies of gut development in non-model frog species with unique feeding strategies, such as Lepidobatrachus laevis and Eleutherodactylous coqui, which are beginning to provide glimpses of the evolutionary mechanisms that may generate morphological variation in the digestive tract. The unparalleled experimental versatility of frog embryos make them excellent, integrative models for studying digestive organ development across multiple disciplines.

TGF-β Signaling Regulates the Differentiation of Motile Cilia

The cilium is a small cellular organelle with motility- and/or sensory-related functions that plays a crucial role during developmental and homeostatic processes. Although many molecules or signal transduction pathways that control cilia assembly have been reported, the mechanisms of ciliary length control have remained enigmatic. Here, we report that Smad2-dependent transforming growth factor β (TGF-β) signaling impacts the length of motile cilia at the Xenopus left-right (LR) organizer, the gastrocoel roof plate (GRP), as well as at the neural tube and the epidermis. Blocking TGF-β signaling resulted in the absence of the transition zone protein B9D1/MSKR-1 from cilia in multi-ciliated cells (MCCs) of the epidermis. Interestingly, this TGF-β activity is not mediated by Mcidas, Foxj1, and RFX2, the known major regulators of ciliogenesis. These data indicate that TGF-β signaling is crucial for the function of the transition zone, which in turn may affect the regulation of cilia length.

Xenopus as a Model for GI/Pancreas Disease

Diseases affecting endodermal organs like the pancreas, lung and gastrointestinal (GI) tract have a substantial impact on human welfare. Since many of these are congenital defects that arise as a result of defects during development broad efforts are focused on understanding the development of these organs so as to better identify risk factors, disease mechanisms and therapeutic targets. Studies implementing model systems, like the amphibian Xenopus, have contributed immensely to our understanding of signaling (e.g. Wnt, FGF, BMP, RA) pathways and gene regulation (e.g. hhex, ptf1a, ngn3) that underlie normal development as well as disease progression. Recent advances in genome engineering further enhance the capabilities of the Xenopus model system for pursuing biomedical research, and will undoubtedly result in a boom of new information underlying disease mechanisms ultimately leading to advancements in diagnosis and therapy.

Shroom3 contributes to the maintenance of the glomerular filtration barrier integrity

Genome-wide association studies (GWAS) identify regions of the genome correlated with disease risk but are restricted in their ability to identify the underlying causative mechanism(s). Thus, GWAS are useful "roadmaps" that require functional analysis to establish the genetic and mechanistic structure of a particular locus. Unfortunately, direct functional testing in humans is limited, demonstrating the need for complementary approaches. Here we used an integrated approach combining zebrafish, rat, and human data to interrogate the function of an established GWAS locus (SHROOM3) lacking prior functional support for chronic kidney disease (CKD). Congenic mapping and sequence analysis in rats suggested Shroom3 was a strong positional candidate gene. Transferring a 6.1-Mb region containing the wild-type Shroom3 gene significantly improved the kidney glomerular function in FHH (fawn-hooded hypertensive) rat. The wild-type Shroom3 allele, but not the FHH Shroom3 allele, rescued glomerular defects induced by knockdown of endogenous shroom3 in zebrafish, suggesting that the FHH Shroom3 allele is defective and likely contributes to renal injury in the FHH rat. We also show for the first time that variants disrupting the actin-binding domain of SHROOM3 may cause podocyte effacement and impairment of the glomerular filtration barrier.

Apical constriction: themes and variations on a cellular mechanism driving morphogenesis

Apical constriction is a cell shape change that promotes tissue remodeling in a variety of homeostatic and developmental contexts, including gastrulation in many organisms and neural tube formation in vertebrates. In recent years, progress has been made towards understanding how the distinct cell biological processes that together drive apical constriction are coordinated. These processes include the contraction of actin-myosin networks, which generates force, and the attachment of actin networks to cell-cell junctions, which allows forces to be transmitted between cells. Different cell types regulate contractility and adhesion in unique ways, resulting in apical constriction with varying dynamics and subcellular organizations, as well as a variety of resulting tissue shape changes. Understanding both the common themes and the variations in apical constriction mechanisms promises to provide insight into the mechanics that underlie tissue morphogenesis.

Dynamic contacts: rearranging adherens junctions to drive epithelial remodelling

Epithelial cells display dynamic behaviours, such as rearrangement, movement and shape changes, particularly during embryonic development and in equivalent processes in adults. Accumulating evidence suggests that the remodelling of cell junctions, especially adherens junctions (AJs), has major roles in controlling these behaviours. AJs comprise cadherin adhesion receptors and cytoplasmic proteins that associate with them, including catenins and actin filaments, and exhibit various forms, such as linear or punctate. Remodelling of AJs induces epithelial reshaping in various ways, including by planar-polarized apical constriction that is driven by the contraction of AJ-associated actomyosin and that occurs during neural plate bending and germband extension. RHO GTPases and their effectors regulate actin polymerization and actomyosin contraction at AJs during the epithelial reshaping processes.

Structure of a highly conserved domain of Rock1 required for Shroom-mediated regulation of cell morphology

Rho-associated coiled coil containing protein kinase (Rho-kinase or Rock) is a well-defined determinant of actin organization and dynamics in most animal cells characterized to date. One of the primary effectors of Rock is non-muscle myosin II. Activation of Rock results in increased contractility of myosin II and subsequent changes in actin architecture and cell morphology. The regulation of Rock is thought to occur via autoinhibition of the kinase domain via intramolecular interactions between the N-terminus and the C-terminus of the kinase. This autoinhibited state can be relieved via proteolytic cleavage, binding of lipids to a Pleckstrin Homology domain near the C-terminus, or binding of GTP-bound RhoA to the central coiled-coil region of Rock. Recent work has identified the Shroom family of proteins as an additional regulator of Rock either at the level of cellular distribution or catalytic activity or both. The Shroom-Rock complex is conserved in most animals and is essential for the formation of the neural tube, eye, and gut in vertebrates. To address the mechanism by which Shroom and Rock interact, we have solved the structure of the coiled-coil region of Rock that binds to Shroom proteins. Consistent with other observations, the Shroom binding domain is a parallel coiled-coil dimer. Using biochemical approaches, we have identified a large patch of residues that contribute to Shrm binding. Their orientation suggests that there may be two independent Shrm binding sites on opposing faces of the coiled-coil region of Rock. Finally, we show that the binding surface is essential for Rock colocalization with Shroom and for Shroom-mediated changes in cell morphology.

The maternal-to-zygotic transition targets actin to promote robustness during morphogenesis

Robustness is a property built into biological systems to ensure stereotypical outcomes despite fluctuating inputs from gene dosage, biochemical noise, and the environment. During development, robustness safeguards embryos against structural and functional defects. Yet, our understanding of how robustness is achieved in embryos is limited. While much attention has been paid to the role of gene and signaling networks in promoting robust cell fate determination, little has been done to rigorously assay how mechanical processes like morphogenesis are designed to buffer against variable conditions. Here we show that the cell shape changes that drive morphogenesis can be made robust by mechanisms targeting the actin cytoskeleton. We identified two novel members of the Vinculin/α-Catenin Superfamily that work together to promote robustness during Drosophila cellularization, the dramatic tissue-building event that generates the primary epithelium of the embryo. We find that zygotically-expressed Serendipity-α (Sry-α) and maternally-loaded Spitting Image (Spt) share a redundant, actin-regulating activity during cellularization. Spt alone is sufficient for cellularization at an optimal temperature, but both Spt plus Sry-α are required at high temperature and when actin assembly is compromised by genetic perturbation. Our results offer a clear example of how the maternal and zygotic genomes interact to promote the robustness of early developmental events. Specifically, the Spt and Sry-α collaboration is informative when it comes to genes that show both a maternal and zygotic requirement during a given morphogenetic process. For the cellularization of Drosophilids, Sry-α and its expression profile may represent a genetic adaptive trait with the sole purpose of making this extreme event more reliable. Since all morphogenesis depends on cytoskeletal remodeling, both in embryos and adults, we suggest that robustness-promoting mechanisms aimed at actin could be effective at all life stages.

Every embryo develops under its own unique set of circumstances, with variable inputs coming from mother, father, and the environment. To then ensure a reliable outcome, mechanisms are built into development to buffer against challenges like genetic deficiency, maternal fever, alcohol exposure, etc. This buffering, called “robustness”, can be overwhelmed, ending in miscarriage, pre-mature birth, and structural and functional birth defects. Thus, we need to understand how robustness arises in order to define an embryo's susceptibilities to genetic background and environment; and to ultimately promote healthy reproduction. In this work we provide new insight into how morphogenesis, the process of tissue building in embryos, is made more robust. First, we show that early gene expression by the embryo, or zygote, supplements the stockpile of proteins already supplied by the mother to ensure the robustness of early morphogenesis. Specifically, our data suggests that a specific gene, sry-α, and its expression by the embryo at the maternal-to-zygotic transition, is a genetic adaptation with the sole function of making the first tissue building event in the fruit fly more robust. In addition, we show that the robustness of this morphogenetic event is promoted by mechanisms regulating the actin cytoskeleton.

The Xenopus homeobox gene pitx3 impinges upon somitogenesis and laterality

Pitx3 has been identified as the causative locus in a developmental eye mutation associated with mammalian anterior segment dysgenesis, congenital cataracts, and aphakia. In recent studies of frog eye development we discovered that pitx3 expresses symmetrically in the somites and lateral plate mesoderm and asymmetrically during cardiac and gut looping. We report that disruption of pitx3 activity on one side of an embryo relative to the other, either by over- or underexpression of pitx3, elicits a crooked dorsal axis in embryos that is a consequence of a retarded progression through somitogenesis. Unlike in amniotes, Xenopus somites form as cohorts of presomitic cells that rotate perpendicular to the dorsal axis. Since no vertebral anomalies have been reported in mouse and human Pitx3 mutants, we attempt to distinguish whether the segmentation clock is uniquely affected in frog or if the pitx3 perturbation inhibits the cellular changes that are necessary to rotation of presomitic cells. In Xenopus, pitx3 appears to inhibit the rotation of presomitic cell cohorts and to be necessary to the bilaterally symmetric expression of pitx2 in somites.

Shroom3 is required downstream of FGF signalling to mediate proneuromast assembly in zebrafish

During development, morphogenetic processes require a precise coordination of cell differentiation, cell shape changes and, often, cell migration. Yet, how pattern information is used to orchestrate these different processes is still unclear. During lateral line (LL) morphogenesis, a group of cells simultaneously migrate and assemble radially organized cell clusters, termed rosettes, that prefigure LL sensory organs. This process is controlled by Fibroblast growth factor (FGF) signalling, which induces cell fate changes, cell migration and cell shape changes. However, the exact molecular mechanisms induced by FGF activation that mediate these changes on a cellular level are not known. Here, we focus on the mechanisms by which FGFs control apical constriction and rosette assembly. We show that apical constriction in the LL primordium requires the activity of non-muscle myosin. We demonstrate further that shroom3, a well-known regulator of non-muscle myosin activity, is expressed in the LL primordium and that its expression requires FGF signalling. Using gain- and loss-of-function experiments, we demonstrate that Shroom3 is the main organizer of cell shape changes during rosette assembly, probably by coordinating Rho kinase recruitment and non-muscle myosin activation. In order to quantify morphogenesis in the LL primordium in an unbiased manner, we developed a unique trainable 'rosette detector'. We thus propose a model in which Shroom3 drives rosette assembly in the LL downstream of FGF in a Rho kinase- and non-muscle myosin-dependent manner. In conclusion, we uncovered the first mechanistic link between patterning and morphogenesis during LL sensory organ formation.

Structure of Shroom domain 2 reveals a three-segmented coiled-coil required for dimerization, Rock binding, and apical constriction

Shroom (Shrm) proteins are essential regulators of cell shape and tissue morpho-logy during animal development that function by interacting directly with the coiled-coil region of Rho kinase (Rock). The Shrm-Rock interaction is sufficient to direct Rock subcellular localization and the subsequent assembly of contractile actomyosin networks in defined subcellular locales. However, it is unclear how the Shrm-Rock interaction is regulated at the molecular level. To begin investigating this issue, we present the structure of Shrm domain 2 (SD2), which mediates the interaction with Rock and is required for Shrm function. SD2 is a unique three-segmented dimer with internal symmetry, and we identify conserved residues on the surface and within the dimerization interface that are required for the Rock-Shrm interaction and Shrm activity in vivo. We further show that these residues are critical in both vertebrate and invertebrate Shroom proteins, indicating that the Shrm-Rock signaling module has been functionally and molecularly conserved. The structure and biochemical analysis of Shrm SD2 indicate that it is distinct from other Rock activators such as RhoA and establishes a new paradigm for the Rock-mediated assembly of contractile actomyosin networks.

SHROOM3 is a novel candidate for heterotaxy identified by whole exome sequencing

Background

Heterotaxy-spectrum cardiovascular disorders are challenging for traditional genetic analyses because of clinical and genetic heterogeneity, variable expressivity, and non-penetrance. In this study, high-resolution SNP genotyping and exon-targeted array comparative genomic hybridization platforms were coupled to whole-exome sequencing to identify a novel disease candidate gene.

Results

SNP genotyping identified absence-of-heterozygosity regions in the heterotaxy proband on chromosomes 1, 4, 7, 13, 15, 18, consistent with parental consanguinity. Subsequently, whole-exome sequencing of the proband identified 26,065 coding variants, including 18 non-synonymous homozygous changes not present in dbSNP132 or 1000 Genomes. Of these 18, only 4 - one each in CXCL2, SHROOM3, CTSO, RXFP1 - were mapped to the absence-of-heterozygosity regions, each of which was flanked by more than 50 homozygous SNPs, confirming recessive segregation of mutant alleles. Sanger sequencing confirmed the SHROOM3 homozygous missense mutation and it was predicted as pathogenic by four bioinformatic tools. SHROOM3 has been identified as a central regulator of morphogenetic cell shape changes necessary for organogenesis and can physically bind ROCK2, a rho kinase protein required for left-right patterning. Screening 96 sporadic heterotaxy patients identified four additional patients with rare variants in SHROOM3.

Conclusions

Using whole exome sequencing, we identify a recessive missense mutation in SHROOM3 associated with heterotaxy syndrome and identify rare variants in subsequent screening of a heterotaxy cohort, suggesting SHROOM3 as a novel target for the control of left-right patterning. This study reveals the value of SNP genotyping coupled with high-throughput sequencing for identification of high yield candidates for rare disorders with genetic and phenotypic heterogeneity.

RA. Shroom3 and a Pitx2-N-cadherin pathway function cooperatively to generate asymmetric cell shape changes during gut morphogenesis

The cytoskeletal protein Shroom3 is a potent inducer of epithelial cell shape change and is required for lens and neural plate morphogenesis. Analysis of gut morphogenesis in Shroom3 deficient mouse embryos revealed that the direction of gut rotation is also disrupted. It was recently established that Pitx2-dependent, asymmetrical cellular behaviors in the dorsal mesentery (DM) of the early mid-gut, a structure connecting the gut-tube to the rest of the embryo, contribute to the direction of gut rotation in chicken embryos by influencing the direction of the dorsal mesenteric tilt. Asymmetric cell shapes in the DM epithelium are hypothesized to contribute to the tilt, however, it is unclear what lies downstream of Pitx2 to alter epithelial cell shape. The cells of the left DM epithelium in either Pitx2 or Shroom3 deficient embryos are shorter and wider than those in control embryos and resemble the shape of those on the right, demonstrating that like Pitx2, Shroom3 is required for cell shape asymmetry and the leftward DM tilt. Because N-cadherin expression is specific to the left side and is Pitx2 dependent, we determined whether Shroom3 and N-cadherin function together to regulate cell shape in the left DM epithelium. Analysis of mouse embryos lacking one allele of both Shroom3 and N-cadherin revealed that they possess shorter and wider left epithelial DM cells when compared with Shroom3 or N-cadherin heterozygous embryos. This indicates a genetic interaction. Together these data provide evidence that Shroom3 and N-cadherin function cooperatively downstream of Pitx2 to directly regulate cell shape changes necessary for early gut tube morphogenesis.

Cell dynamics in fetal intestinal epithelium: implications for intestinal growth and morphogenesis

The cellular mechanisms that drive growth and remodeling of the early intestinal epithelium are poorly understood. Current dogma suggests that the murine fetal intestinal epithelium is stratified, that villi are formed by an epithelial remodeling process involving the de novo formation of apical surface at secondary lumina, and that radial intercalation of the stratified cells constitutes a major intestinal lengthening mechanism. Here, we investigate cell polarity, cell cycle dynamics and cell shape in the fetal murine intestine between E12.5 and E14.5. We show that, contrary to previous assumptions, this epithelium is pseudostratified. Furthermore, epithelial nuclei exhibit interkinetic nuclear migration, a process wherein nuclei move in concert with the cell cycle, from the basal side (where DNA is synthesized) to the apical surface (where mitosis takes place); such nuclear movements were previously misinterpreted as the radial intercalation of cells. We further demonstrate that growth of epithelial girth between E12.5 and E14.5 is driven by microtubule- and actinomyosin-dependent apicobasal elongation, rather than by progressive epithelial stratification as was previously thought. Finally, we show that the actin-binding protein Shroom3 is crucial for the maintenance of the single-layered pseudostratified epithelium. In mice lacking Shroom3, the epithelium is disorganized and temporarily stratified during villus emergence. These results favor an alternative model of intestinal morphogenesis in which the epithelium remains single layered and apicobasally polarized throughout early intestinal development.

Evolutionary crossroads in developmental biology: the tunicates

The tunicates, or urochordates, constitute a large group of marine animals whose recent common ancestry with vertebrates is reflected in the tadpole-like larvae of most tunicates. Their diversity and key phylogenetic position are enhanced, from a research viewpoint, by anatomically simple and transparent embryos, compact rapidly evolving genomes, and the availability of powerful experimental and computational tools with which to study these organisms. Tunicates are thus a powerful system for exploring chordate evolution and how extreme variation in genome sequence and gene regulatory network architecture is compatible with the preservation of an ancestral chordate body plan.

Shroom2 regulates contractility to control endothelial morphogenesis

The intrinsic contractile, migratory, and adhesive properties of endothelial cells are central determinants in the formation of vascular networks seen in vertebrate organisms. Because Shroom2 (Shrm2) is expressed within the endothelium, is localized to cortical actin and cell-cell adhesions, and contains a conserved Rho kinase (Rock) binding domain, we hypothesized that Shrm2 may participate in the regulation of endothelial cell behavior during vascular morphogenesis. Consistent with this hypothesis, depletion of Shrm2 results in elevated branching and sprouting angiogenic behavior of endothelial cells. This is recapitulated in human umbilical vein endothelial cells and in a vasculogenesis assay in which differentiated embryonic stem cells depleted for Shrm2 form a more highly branched endothelial network. Further analyses indicate that the altered behavior observed following Shrm2 depletion is due to aberrant cell contractility, as evidenced by decreased stress fiber organization and collagen contraction with an increase in cellular migration. Because Shrm2 directly interacts with Rock, and Shrm2 knockdown results in the loss of Rock and activated myosin II from sites of cell-cell adhesion, we conclude that Shrm2 facilitates the formation of a contractile network within endothelial cells, the loss of which leads to an increase in endothelial sprouting, migration, and angiogenesis.

Tension and epithelial morphogenesis in Drosophila early embryos

During morphogenesis, tissues are shaped by cell behaviors such as apical cell constriction and cell intercalation, which are the result of cell intrinsic forces, but are also shaped passively by forces acting on the cells. The latter extrinsic forces can be produced either within the deforming tissue by the tissue-scale integration of intrinsic forces, or outside the tissue by other tissue movements or by fluid flows. Here we review the intrinsic and extrinsic forces that sculpt the epithelium of early Drosophila embryos, focusing on three conserved morphogenetic processes: tissue internalization, axis extension, and segment boundary formation. Finally, we look at how the actomyosin cytoskeleton forms force-generating structures that power these three morphogenetic events at the cell and the tissue scales.

Transcriptome analyses based on genetic screens for Pax3 myogenic targets in the mouse embryo

Background

Pax3 is a key upstream regulator of the onset of myogenesis, controlling progenitor cell survival and behaviour as well as entry into the myogenic programme. It functions in the dermomyotome of the somite from which skeletal muscle derives and in progenitor cell populations that migrate from the somite such as those of the limbs. Few Pax3 target genes have been identified. Identifying genes that lie genetically downstream of Pax3 is therefore an important endeavour in elucidating the myogenic gene regulatory network.

Results

We have undertaken a screen in the mouse embryo which employs a Pax3^GFP allele that permits isolation of Pax3 expressing cells by flow cytometry and a Pax3^PAX3-FKHR allele that encodes PAX3-FKHR in which the DNA binding domain of Pax3 is fused to the strong transcriptional activation domain of FKHR. This constitutes a gain of function allele that rescues the Pax3 mutant phenotype. Microarray comparisons were carried out between Pax3^GFP/+ and Pax3^{GFP/PAX3-FKHR} preparations from the hypaxial dermomyotome of somites at E9.5 and forelimb buds at E10.5. A further transcriptome comparison between Pax3-GFP positive and negative cells identified sequences specific to myogenic progenitors in the forelimb buds. Potential Pax3 targets, based on changes in transcript levels on the gain of function genetic background, were validated by analysis on loss or partial loss of function Pax3 mutant backgrounds. Sequences that are up- or down-regulated in the presence of PAX3-FKHR are classified as somite only, somite and limb or limb only. The latter should not contain sequences from Pax3 positive neural crest cells which do not invade the limbs. Verification by whole mount in situ hybridisation distinguishes myogenic markers. Presentation of potential Pax3 target genes focuses on signalling pathways and on transcriptional regulation.

Conclusions

Pax3 orchestrates many of the signalling pathways implicated in the activation or repression of myogenesis by regulating effectors and also, notably, inhibitors of these pathways. Important transcriptional regulators of myogenesis are candidate Pax3 targets. Myogenic determination genes, such as Myf5 are controlled positively, whereas the effect of Pax3 on genes encoding inhibitors of myogenesis provides a potential brake on differentiation. In the progenitor cell population, Pax7 and also Hdac5 which is a potential repressor of Foxc2, are subject to positive control by Pax3.

Specific isoforms of drosophila shroom define spatial requirements for the induction of apical constriction

Vertebrate Shroom proteins define cytoskeletal organization and cellular architecture by binding directly to F-actin and Rho-kinase and spatially regulating the activity of nonmuscle myosin II (myosin II). Here, we report characterization and gain-of-function analysis of Drosophila Shroom. The dShrm locus expresses at least two protein isoforms, dShrmA and dShrmB, which localize to adherens junctions and the apical membrane, respectively. dShrmA and dShrmB exhibit differing abilities to induce apical constriction that are based on their subcellular distribution and the subsequent assembly of spatially and organizationally distinct actomyosin networks that are dependent on the ability to engage Rho-kinase and the activity of myosin II. These data show that the differential subcellular distribution of naturally occurring isoforms of Shroom proteins can define both the position and organization of actomyosin networks in vivo. We further hypothesize that differentially positioned contractile arrays have distinct effects on cellular morphologies and behaviors.

Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

The mammalian definitive endoderm arises as a simple epithelial sheet. This sheet of cells will eventually produce the innermost tube that comprises the entire digestive tract from the esophagus to the colon as well as the epithelial component of the digestive and respiratory organs including the thymus, thyroid, lung, liver, gallbladder, and pancreas. Thus a wide array of tissue types are derived from the early endodermal sheet, and understanding the morphological and molecular mechanisms used to produce this tissue is integral to understanding the development of all these organs. The goal of this chapter is to summarize what is known about the morphological and molecular mechanisms used to produce this embryonic germ layer. Although this chapter mainly focuses on the mechanisms used to generate the murine endoderm, supportive or suggestive data from other species, including chick, frog (Xenopus laevis), and the Zebrafish (Danio rerio) are also examined.