Microtubular TRIM36 E3 Ubiquitin Ligase in Embryonic Development and Spermatogenesis

TRIM36 is a member of the tripartite motif (TRIM) family of RING-containing proteins, also known as Haprin, which was first discovered for its abundance in testis and found to be implicated in the spermatozoa acrosome reaction. TRIM36 is a microtubule-associated E3 ubiquitin ligase that plays a role in cytoskeletal organization, and according to data gathered in different species, coordinates growth speed and stability, acting on the microtubules’ plus end, and impacting on cell cycle progression. TRIM36 is also crucial for early developmental processes, in Xenopus, where it is needed for dorso-ventral axis formation, but also in humans as bi-allelic mutations in the TRIM36 gene cause a form of severe neural tube closure defect, called anencephaly. Here, we review TRIM36-related mechanisms implicated in such composite physiological and pathological processes.

BMP Signaling: Lighting up the Way for Embryonic Dorsoventral Patterning

One of the most significant events during early embryonic development is the establishment of a basic embryonic body plan, which is defined by anteroposterior, dorsoventral (DV), and left-right axes. It is well-known that the morphogen gradient created by BMP signaling activity is crucial for DV axis patterning across a diverse set of vertebrates. The regulation of BMP signaling during DV patterning has been strongly conserved across evolution. This is a remarkable regulatory and evolutionary feat, as the BMP gradient has been maintained despite the tremendous variation in embryonic size and shape across species. Interestingly, the embryonic DV axis exhibits robust stability, even in face of variations in BMP signaling. Multiple lines of genetic, molecular, and embryological evidence have suggested that numerous BMP signaling components and their attendant regulators act in concert to shape the developing DV axis. In this review, we summarize the current knowledge of the function and regulation of BMP signaling in DV patterning. Throughout, we focus specifically on popular model animals, such as Xenopus and zebrafish, highlighting the similarities and differences of the regulatory networks between species. We also review recent advances regarding the molecular nature of DV patterning, including the initiation of the DV axis, the formation of the BMP gradient, and the regulatory molecular mechanisms behind BMP signaling during the establishment of the DV axis. Collectively, this review will help clarify our current understanding of the molecular nature of DV axis formation.

Dynamic changes in the association between maternal mRNAs and endoplasmic reticulum during ascidian early embryogenesis

Axis formation is one of the most important events occurring at the beginning of animal development. In the ascidian egg, the antero-posterior axis is established at this time owing to a dynamic cytoplasmic movement called cytoplasmic and cortical reorganisation. During this movement, mitochondria, endoplasmic reticulum (ER), and maternal mRNAs (postplasmic/PEM RNAs) are translocated to the future posterior side. Although accumulating evidence indicates the crucial roles played by the asymmetrical localisation of these organelles and the translational regulation of postplasmic/PEM RNAs, the organisation of ER has not been described in sufficient detail to date owing to technical difficulties. In this study, we developed three different multiple staining protocols for visualising the ER in combination with mitochondria, microtubules, or mRNAs in whole-mount specimens. We defined the internally expanded “dense ER” using these protocols and described cisterna-like structures of the dense ER using focused ion beam-scanning electron microscopy. Most importantly, we described the dynamic changes in the colocalisation of postplasmic/PEM mRNAs and dense ER; for example, macho-1 mRNA was detached and excluded from the dense ER during the second phase of ooplasmic movements. These detailed descriptions of the association between maternal mRNA and ER can provide clues for understanding the translational regulation mechanisms underlying axis determination during ascidian early embryogenesis.

T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development

Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of β-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/β-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.

The Organizer and Its Signaling in Embryonic Development

Germ layer specification and axis formation are crucial events in embryonic development. The Spemann organizer regulates the early developmental processes by multiple regulatory mechanisms. This review focuses on the responsive signaling in organizer formation and how the organizer orchestrates the germ layer specification in vertebrates. Accumulated evidence indicates that the organizer influences embryonic development by dual signaling. Two parallel processes, the migration of the organizer’s cells, followed by the transcriptional activation/deactivation of target genes, and the diffusion of secreting molecules, collectively direct the early development. Finally, we take an in-depth look at active signaling that originates from the organizer and involves germ layer specification and patterning.

Temporal Transcriptome Analysis Reveals Dynamic Expression Profiles of Gametes and Embryonic Development in Japanese Flounder (Paralichthys olivaceus)

The maternal-to-zygotic transition (MZT) is a crucial event in embryo development. While the features of the MZT across species are shared, the stage of this transition is different among species. We characterized MZT in a flatfish species, Japanese flounder (Paralichthys olivaceus). In this study, we analyzed the 551.57 GB transcriptome data of two types of gametes (sperms and eggs) and 10 embryo developmental stages in Japanese flounder. We identified 2512 maternal factor-related genes and found that most of those maternal factor-related genes expression decreased at the low blastula (LB) stage and remained silent in the subsequent embryonic development period. Meanwhile, we verified that the zygotic genome transcription might occur at the 128-cell stage and large-scale transcription began at the LB stage, which indicates the LB stage is the major wave zygotic genome activation (ZGA) occurs. In addition, we indicated that the Wnt signaling pathway, playing a diverse role in embryonic development, was involved in the ZGA and the axis formation. The results reported the list of the maternal genes in Japanese flounder and defined the stage of MZT, contributing to the understanding of the details of MZT during Japanese flounder embryonic development.

Symmetry breaking of hPSCs in micropattern generates a polarized spinal cord-like organoid (pSCO) with dorsoventral organization

Brain organoid research is advancing, but generation of organoids with proper axis formation, which could lead to spatially ordered structures for complex brain structure and function, still remains a challenge. Axis formation and related spatial cell organization in the CNS are initiated by the symmetry breaking during the early embryo development. It has been demonstrated that the geometrically confined culture of human pluripotent stem cells (hPSCs) can be used to induce symmetry breaking and regionalized cell differentiation. In this study, we generated a polarized spinal cord organoid with a self-organized dorsoventral (DV) organization, using 2D cell patterning by geometric confinement. Initially, the application of caudalization signals to hPSCs promoted the regionalized cell differentiation along the radial axis and sprouting-like protrusion morphogenesis in cell colonies confined to ECM protein micropatterns. Detachment of colonies turned them into extended spinal cord-like organoids which maintained center- and edge-derived two poles. Further analyses including single cell RNA sequencing and spatial transcriptome analysis unveiled that these organoids contained rich repertoire of developing spinal cord cells and exhibited the spatially ordered DV domain formation along the long axis without external organizing signals. Modulation of BMP and Shh signaling can control the extent of DV coverage in organoids following the principles of embryo patterning. Our study provides a simple, and precisely controllable method to generate spatially-ordered organoids for understanding of biological principles of cell patterning and axis formation during neural development.