T-Box Genes in Drosophila Mesoderm Development

New insights into mesoderm and endoderm development, and the nature of the onychophoran blastopore

BACKGROUND

Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior-posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy.

RESULTS

In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes.

CONCLUSION

We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.

Transcriptome-wide analysis of mRNA N6 -methyladenosine modification in the embryonic development of Spodoptera frugiperda

N6 -methyladenosine (m6 A) RNA is the most abundant modification of mRNA, and has been demonstrated in regulating various post-transcriptional processes. Many studies have shown that m6 A methylation plays key roles in sex determination, neuronal functions, and embryonic development in Drosophila and mammals. Here, we analyzed transcriptome-wide profile of m6 A modification in the embryonic development of the destructive agricultural pest Spodoptera frugiperda. We found that the 2 key mRNA m6 A methyltransferases SfrMETTL3 and SfrMETTL14 have high homologies with other insects and mammals, suggesting that SfrMETTL3 and SfrMETTL14 may have conserved function among different species. From methylated RNA immunoprecipitation sequencing analysis, we obtained 46 869 m6 A peaks representing 8 587 transcripts in the 2-h embryos after oviposition, and 41 389 m6 A peaks representing 9 230 transcripts in the 24-h embryos. In addition, 5 995 m6 A peaks were differentially expressed including 3 752 upregulated and 2243 downregulated peaks. Functional analysis with Gene Ontology and Kyoto Encyclopedia of Genes and Genomes suggested that differentially expressed m6 A peak-modified genes were enriched in cell and organ development between the 2- and 24-h embryos. By conjoint analysis of methylated RNA immunoprecipitation-seq and RNA-seq data, we found that RNA m6 A methylation may regulate the transcriptional levels of genes related to tissue and organ development from 2- to 24-h embryos. Our study reveals the role of RNA m6 A epigenetic regulation in the embryonic development of S. frugiperda, and provides new insights for the embryonic development of insects.

The Tbx6 Transcription Factor Dorsocross Mediates Dpp Signaling to Regulate Drosophila Thorax Closure

Movement and fusion of separate cell populations are critical for several developmental processes, such as neural tube closure in vertebrates or embryonic dorsal closure and pupal thorax closure in Drosophila. Fusion failure results in an opening or groove on the body surface. Drosophila pupal thorax closure is an established model to investigate the mechanism of tissue closure. Here, we report the identification of T-box transcription factor genes Dorsocross (Doc) as Decapentaplegic (Dpp) targets in the leading edge cells of the notum in the late third instar larval and early pupal stages. Reduction of Doc in the notum region results in a thorax closure defect, similar to that in dpp loss-of-function flies. Nine genes are identified as potential downstream targets of Doc in regulating thorax closure by molecular and genetic screens. Our results reveal a novel function of Doc in Drosophila development. The candidate target genes provide new clues for unravelling the mechanism of collective cell movement.

Cytoskeletal prestress: The cellular hallmark in mechanobiology and mechanomedicine

Increasing evidence demonstrates that mechanical forces, in addition to soluble molecules, impact cell and tissue functions in physiology and diseases. How living cells integrate mechanical signals to perform appropriate biological functions is an area of intense investigation. Here, we review the evidence of the central role of cytoskeletal prestress in mechanotransduction and mechanobiology. Elevating cytoskeletal prestress increases cell stiffness and reinforces cell stiffening, facilitates long-range cytoplasmic mechanotransduction via integrins, enables direct chromatin stretching and rapid gene expression, spurs embryonic development and stem cell differentiation, and boosts immune cell activation and killing of tumor cells whereas lowering cytoskeletal prestress maintains embryonic stem cell pluripotency, promotes tumorigenesis and metastasis of stem cell-like malignant tumor-repopulating cells, and elevates drug delivery efficiency of soft-tumor-cell-derived microparticles. The overwhelming evidence suggests that the cytoskeletal prestress is the governing principle and the cellular hallmark in mechanobiology. The application of mechanobiology to medicine (mechanomedicine) is rapidly emerging and may help advance human health and improve diagnostics, treatment, and therapeutics of diseases.

TBX2/3 is required for regeneration of dorsal-ventral and medial-lateral polarity in planarians

The molecular mechanisms responsible for axis establishment during non-embryonic processes remain elusive. The planarian flatworm is an ideal model organism to study body axis polarization and patterning in vivo. Here, we identified a homolog of the TBX2/3 in the planarian Dugesia japonica. RNA interference (RNAi) knockdown of TBX2/3 results in the ectopic formation of protrusions in the midline of the dorsal surface which shows an abnormal expression of midline and ventral cell markers. Additionally, the TBX2/3 RNAi animals also show the duplication of expression of the boundary marker at the lateral edge. Furthermore, TBX2/3 is expressed in muscle cells and co-expressed with bmp4. Inhibition of bone morphogenetic protein (BMP) signaling reduces the expression of TBX2/3 at the midline. These results suggest that TBX2/3 RNAi results in phenotypic characters caused by inhibition of the BMP signal, indicating that TBX2/3 is required for DV and ML patterning, and might be a downstream gene of BMP signaling.

Nucleocytoplasmic distribution of S6K1 depends on the density and motility of MCF-7 cells in vitro

Background: The ribosomal protein S6 kinase 1 (S6K1) is one of the main components of the mTOR/S6K signal transduction pathway, which controls cellular metabolism, autophagy, growth, and proliferation. Overexpression of S6K1 was detected in tumors of different origin including breast cancer, and correlated with the worse disease outcome. In addition, significant accumulation of S6K1 was found in the nuclei of breast carcinoma cells suggesting the implication of kinase nuclear substrates in tumor progression. However, this aspect of S6K1 functioning is still poorly understood. The main aim of the present work was to study the subcellular localization of S6K1 in breast cancer cells with the focus on cell migration. Methods: Multicellular spheroids of MCF-7 cells were generated using agarose-coated Petri dishes. Cell migration was induced by spheroids seeding onto adhesive growth surface and subsequent cultivation for 24 to 72 hours. The subcellular localization of S6K1 was studied in human normal breast and cancer tissue samples, 2D and 3D MCF-7 cell cultures using immunofluorescence analysis and confocal microscopy. Results: Analysis of histological sections of human breast tissue samples revealed predominantly nuclear localization of S6K1 in breast malignant cells and its mainly cytoplasmic localization in conditionally normal cells. In vitro studies of MCF-7 cells demonstrated that the subcellular localization of S6K1 depends on the cell density in the monolayer culture. S6K1 relocalization from the cytoplasm into the nucleus was detected in MCF-7 cells migrating from multicellular spheroids onto growth surface. Immunofluorescence analysis of S6K1 and immunocoprecipitation assay revealed the colocalization and interaction between S6K1 and transcription factor TBR2 (T-box brain protein 2) in MCF-7 cells. Conclusions: Subcellular localization of S6K1 depends on the density and locomotor activity of the MCF-7 cells.

Nucleocytoplasmic distribution of S6K1 depends on the density and motility of MCF-7 cells in vitro

Background: The ribosomal protein S6 kinase 1 (S6K1) is one of the main components of the mTOR/S6K signal transduction pathway, which controls cellular metabolism, autophagy, growth, and proliferation. Overexpression of S6K1 was detected in tumors of different origin including breast cancer, which was associated with a worse disease outcome. In addition, significant accumulation of S6K1 was found in the nuclei of breast carcinoma cells suggesting the implication of kinase nuclear substrates in tumor progression. However, this aspect of S6K1 functioning is poorly understood. The main aim of the present work was to study the subcellular localization of S6K1 in breast cancer cells with focus on cell migration. Methods: Multicellular spheroids of MCF-7 cells were generated using agarose-coated Petri dishes. Cell migration was initiated by spheroids seeding onto growth surface and subsequent cultivation for 24 and 72 hours. S6K1 subcellular localization was studied in human breast cancer and normal tissue, 2D and 3D MCF-7 cell culture using immunofluorescence analysis and confocal microscopy. Results: Analysis of histological sections of human breast cancer and normal tissue revealed predominantly nuclear localization of S6K1 in breast malignant cells and mainly cytoplasmic one in conditionally normal cells. In vitro studies of MCF-7 cells showed that the subcellular localization of S6K1 depends on the cell density in the monolayer culture. S6K1 relocalization from the cytoplasm into the nucleus was detected in MCF-7 cells migrating from multicellular spheroids onto growth surface. Immunofluorescence analysis of S6K1 and immunocoprecipitation assay revealed the colocalization and interaction between S6K1 and transcription factor TBR2 (T-box brain protein 2) in MCF-7 cells. Bioinformatical analysis revealed existence of several phosphorylation sites in TBR2 for S6K1 suggesting that TBR2 can be a target for phosphorylation and regulation by S6K1. Conclusions: Subcellular localization of S6K1 depends on the density and locomotor activity of the MCF-7 cells.

Diversification of heart progenitor cells by EGF signaling and differential modulation of ETS protein activity

For coordinated circulation, vertebrate and invertebrate hearts require stereotyped arrangements of diverse cell populations. This study explores the process of cardiac cell diversification in the Drosophila heart, focusing on the two major cardioblast subpopulations: generic working myocardial cells and inflow valve-forming ostial cardioblasts. By screening a large collection of randomly induced mutants, we identified several genes involved in cardiac patterning. Further analysis revealed an unexpected, specific requirement of EGF signaling for the specification of generic cardioblasts and a subset of pericardial cells. We demonstrate that the Tbx20 ortholog Midline acts as a direct target of the EGFR effector Pointed to repress ostial fates. Furthermore, we identified Edl/Mae, an antagonist of the ETS factor Pointed, as a novel cardiac regulator crucial for ostial cardioblast specification. Combining these findings, we propose a regulatory model in which the balance between activation of Pointed and its inhibition by Edl controls cardioblast subtype-specific gene expression.

Organs contain many different kinds of cells, each specialised to perform a particular role. The fruit fly heart, for example, has two types of muscle cells: generic heart muscle cells and ostial heart muscle cells. The generic cells contract to force blood around the body, whilst the ostial cells form openings that allow blood to enter the heart. Though both types of cells carry the same genetic information, each uses a different combination of active genes to perform their role.

During development, the cells must decide whether to become generic or ostial. They obtain signals from other cells in and near the developing heart, and respond by turning genes on or off. The response uses proteins called transcription factors, which bind to regulatory portions of specific genes.

The sequence of signals and transcription factors that control the fate of developing heart muscle cells was not known. So Schwarz et al. examined the process using a technique called a mutagenesis screen. This involved triggering random genetic mutations and looking for flies with defects in their heart muscle cells. Matching the defects to the mutations revealed genes responsible for heart development.

Schwarz et al. found that for cells to develop into generic heart muscle cells, a signal called epidermal growth factor (EGF) switches on a transcription factor called Pointed in the cells. Pointed then turns on another transcription factor that switches off the genes for ostial cells. Conversely, ostial heart muscle cells develop when a protein called ‘ETS-domain lacking’ (Edl) interferes with Pointed, allowing the ostial genes to remain on. The balance between Pointed and Edl controls which type of heart cell each cell will become.

Many cells in other tissues in fruit flies also produce the Pointed and Edl proteins and respond to EGF signals. This means that this system may help to decide the fate of cells in other organs. The EGF signaling system is also present in other animals, including humans. Future work could reveal whether the same molecular decision making happens in our own hearts.