Decoding Dishevelled-Mediated Wnt Signaling in Vertebrate Early Development

The dishevelled associated activator of morphogenesis protein 2 (Daam2) regulates neural tube closure

BACKGROUND

The Wnt signaling pathway is highly conserved in metazoans and regulates a large array of cellular processes including motility, polarity and fate determination, and stem cell homeostasis. Modulation of the actin cytoskeleton via the non-canonical Wnt pathway regulate cell polarity and cell migration that are required for proper vertebrate gastrulation and subsequent neurulation. However, the mechanism(s) of how the non-canonical pathway mediates actin cytoskeleton modulation is not fully understood.

RESULTS

Herein, we characterize the role of the Formin-homology protein; dishevelled associated activator of morphogenesis 2 (Daam2) protein in the Wnt signaling pathway. Co-immunoprecipitation assays confirm the binding of Daam2 to dishevelled2 (Dvl2) as well as the domains within these proteins required for interaction; additionally, the interaction between Daam2 and Dvl2 was Wnt-regulated. Sub-cellular localization studies reveal Daam2 is cytoplasmic and regulates the cellular actin cytoskeleton by modulating actin filament formation. During Xenopus development, a knockdown or loss of Daam2 specifically produces neural tube closure defects indicative of a role in non-canonical signaling. Additionally, our studies did not identify any role for Daam2 in canonical Wnt signaling in mammalian culture cells or the Xenopus embryo.

CONCLUSIONS

Our studies together identify Daam2 as a component of the non-canonical Wnt pathway and Daam2 is a regulator of neural tube morphogenesis during vertebrate development.

Structural and Functional Insights into Dishevelled-Mediated Wnt Signaling

Dishevelled (DVL) proteins precisely control Wnt signaling pathways with many effectors. While substantial research has advanced our understanding of DVL's role in Wnt pathways, key questions regarding its regulatory mechanisms and interactions remain unresolved. Herein, we present the recent advances and perspectives on how DVL regulates signaling. The experimentally determined conserved domain structures of DVL in conjunction with AlphaFold-predicted structures are used to understand the DVL's role in Wnt signaling regulation. We also summarize the role of DVL in various diseases and provide insights into further directions for research on the DVL-mediated signaling mechanisms. These findings underscore the importance of DVL as a pharmaceutical target or biological marker in diseases, offering exciting potential for future biomedical applications.

Generation of Zebrafish Maternal Mutants via Oocyte-Specific Knockout System

Maternal mRNAs and proteins are produced during oogenesis by more than 60% of zebrafish genes. They are indispensable for fertilization and early embryogenesis. Generation and analysis of the maternal mutant is the most direct way to characterize the maternal function of the specific gene. However, due to the lethality of zygotic mutants, the maternal function of most genes in zebrafish remains elusive. Several methods have been developed to circumvent this obstacle, including mRNA rescue, germ-line replacement, oocyte microinjection in situ, mosaic mutation, and bacterial artificial chromosome (BAC)-mediated conditional rescue. Here, we provide an alternative approach to generate zebrafish maternal mutants rapidly and efficiently by introducing four tandem sgRNA expression cassettes into Tg(zpc:zcas9) embryos. This method is more technically feasible and cost- and time-effective than other established methods. Key features • This protocol can circumvent the lethality or infertility of the zygotic mutants to obtain maternal mutants of the target gene. • This protocol is time-saving (one fish generation). • Using this protocol, double-gene maternal mutants can be obtained in a single generation. • Stable lines can be established to continuously produce maternal mutant embryos for the gene of interest.

Formin Binding Protein 1 (FNBP1) regulates non-canonical Wnt signaling and vertebrate gastrulation

The Formin protein Daam1 is required for Wnt-induced cytoskeletal changes during gastrulation, though how it accomplishes this remains unresolved. Here we report the characterization of Formin Binding Protein 1 (FNBP1) as a binding partner of Daam1. The interaction of Daam1 with FNBP1 and its domains required for this interaction were delineated. Immunofluorescence studies showed FNBP1 co-localizes with Daam1, and is an integral component of the actin cytoskeletal complex that is responsive to Wnt stimulation. Specifically, FNBP1 can induce intracellular tubule-like structures and localize to focal adhesions suggesting a role for FNBP1 in cell migration. Functional FNBP1 studies in Xenopus embryos uncover a critical role for FNBP1 in regulating vertebrate gastrulation. Additionally, suboptimal doses of Daam1 and FNBP1 synergize to produce severe gastrulation defects, indicating FNBP1 and Daam1 may function within the same signaling pathway. These results together show FNBP1 is an integral component of Daam1-regulated non-canonical Wnt signaling required for vertebrate gastrulation.

Reversion induced LIM domain protein (RIL) is a Daam1-interacting protein and regulator of the actin cytoskeleton during non-canonical Wnt signaling

The Daam1 protein regulates Wnt-induced cytoskeletal changes during vertebrate gastrulation though its full mode of action and binding partners remain unresolved. Here we identify Reversion Induced LIM domain protein (RIL) as a new interacting protein of Daam1. Interaction studies uncover binding of RIL to the C-terminal actin-nucleating portion of Daam1 in a Wnt-responsive manner. Immunofluorescence studies showed subcellular localization of RIL to actin fibers and co-localization with Daam1 at the plasma membrane. RIL gain- and loss-of-function approaches in Xenopus produced severe gastrulation defects in injected embryos. Additionally, a simultaneous loss of Daam1 and RIL synergized to produce severe gastrulation defects indicating RIL and Daam1 may function in the same signaling pathway. RIL further synergizes with another novel Daam1-interacting protein, Formin Binding Protein 1 (FNBP1), to regulate gastrulation. Our studies altogether show RIL mediates Daam1-regulated non-canonical Wnt signaling that is required for vertebrate gastrulation.

Canonical and Non-Canonical Wnt Signaling Generates Molecular and Cellular Asymmetries to Establish Embryonic Axes

The formation of embryonic axes is a critical step during animal development, which contributes to establishing the basic body plan in each particular organism. Wnt signaling pathways play pivotal roles in this fundamental process. Canonical Wnt signaling that is dependent on β-catenin regulates the patterning of dorsoventral, anteroposterior, and left-right axes. Non-canonical Wnt signaling that is independent of β-catenin modulates cytoskeletal organization to coordinate cell polarity changes and asymmetric cell movements. It is now well documented that components of these Wnt pathways biochemically and functionally interact to mediate cell-cell communications and instruct cellular polarization in breaking the embryonic symmetry. The dysfunction of Wnt signaling disrupts embryonic axis specification and proper tissue morphogenesis, and mutations of Wnt pathway genes are associated with birth defects in humans. This review discusses the regulatory roles of Wnt pathway components in embryonic axis formation by focusing on vertebrate models. It highlights current progress in decoding conserved mechanisms underlying the establishment of asymmetry along the three primary body axes. By providing an in-depth analysis of canonical and non-canonical pathways in regulating cell fates and cellular behaviors, this work offers insights into the intricate processes that contribute to setting up the basic body plan in vertebrate embryos.

Ischemia-reperfusion injury: molecular mechanisms and therapeutic targets

Ischemia-reperfusion (I/R) injury paradoxically occurs during reperfusion following ischemia, exacerbating the initial tissue damage. The limited understanding of the intricate mechanisms underlying I/R injury hinders the development of effective therapeutic interventions. The Wnt signaling pathway exhibits extensive crosstalk with various other pathways, forming a network system of signaling pathways involved in I/R injury. This review article elucidates the underlying mechanisms involved in Wnt signaling, as well as the complex interplay between Wnt and other pathways, including Notch, phosphatidylinositol 3-kinase/protein kinase B, transforming growth factor-β, nuclear factor kappa, bone morphogenetic protein, N-methyl-D-aspartic acid receptor-Ca2+-Activin A, Hippo-Yes-associated protein, toll-like receptor 4/toll-interleukine-1 receptor domain-containing adapter-inducing interferon-β, and hepatocyte growth factor/mesenchymal-epithelial transition factor. In particular, we delve into their respective contributions to key pathological processes, including apoptosis, the inflammatory response, oxidative stress, extracellular matrix remodeling, angiogenesis, cell hypertrophy, fibrosis, ferroptosis, neurogenesis, and blood-brain barrier damage during I/R injury. Our comprehensive analysis of the mechanisms involved in Wnt signaling during I/R reveals that activation of the canonical Wnt pathway promotes organ recovery, while activation of the non-canonical Wnt pathways exacerbates injury. Moreover, we explore novel therapeutic approaches based on these mechanistic findings, incorporating evidence from animal experiments, current standards, and clinical trials. The objective of this review is to provide deeper insights into the roles of Wnt and its crosstalk signaling pathways in I/R-mediated processes and organ dysfunction, to facilitate the development of innovative therapeutic agents for I/R injury.

Apatinib induces zebrafish hepatotoxicity by inhibiting Wnt signaling and accumulation of oxidative stress

Apatinib, a small-molecule VEGFR2-tyrosine kinase inhibitor, has shown potent anticancer activity in various clinical cancer treatments, but also different adverse reactions. Therefore, it is necessary to study its potential toxicity and working mechanism. We used zebrafish to investigate the effects of apatinib on the development of embryos. Zebrafish exposed to 2.5, 5, and 10 μM apatinib showed adverse effects such as decreased liver area, pericardial oedema, slow yolk absorption, bladder atrophy, and body length shortening. At the same time, it leads to abnormal liver tissue structure, liver function and related gene expression. Furthermore, after exposure to apatinib, oxidative stress levels were significantly elevated but liver developmental toxicity was effectively ameliorated with oxidative stress inhibitor treatment. Apatinib induces down-regulation of key target genes of Wnt signaling pathway in zebrafish, and it is found that Wnt activator can significantly rescue liver developmental defects. These results suggest that apatinib may induce zebrafish hepatotoxicity by inhibiting the Wnt signaling pathway and up-regulating oxidative stress, helping to strengthen our understanding of rational clinical application of apatinib.

CTNNB1 in neurodevelopmental disorders

CTNNB1 is the gene that encodes β-catenin which acts as a key player in the Wnt signaling pathway and regulates cellular homeostasis. Most CTNNB1-related studies have been mainly focused on its role in cancer. Recently, CTNNB1 has also been found involved in neurodevelopmental disorders (NDDs), such as intellectual disability, autism, and schizophrenia. Mutations of CTNNB1 lead to the dysfunction of the Wnt signaling pathway that regulates gene transcription and further disturbs synaptic plasticity, neuronal apoptosis, and neurogenesis. In this review, we discuss a wide range of aspects of CTNNB1 and its physiological and pathological functions in the brain. We also provide an overview of the most recent research regarding CTNNB1 expression and its function in NDDs. We propose that CTNNB1 would be one of the top high-risk genes for NDDs. It could also be a potential therapeutic target for the treatment of NDDs.

Planar cell polarity regulators in asymmetric organogenesis during development and disease

The phenomenon of planar cell polarity is critically required for a myriad of morphogenetic processes in metazoan and is accurately controlled by several conserved modules. Six "core" proteins, including Frizzled, Flamingo (Celsr), Van Gogh (Vangl), Dishevelled, Prickle, and Diego (Ankrd6), are major components of the Wnt/planar cell polarity pathway. The Fat/Dchs protocadherins and the Scrib polarity complex also function to instruct cellular polarization. In vertebrates, all these pathways are essential for tissue and organ morphogenesis, such as neural tube closure, left-right symmetry breaking, heart and gut morphogenesis, lung and kidney branching, stereociliary bundle orientation, and proximal-distal limb elongation. Mutations in planar polarity genes are closely linked to various congenital diseases. Striking advances have been made in deciphering their contribution to the establishment of spatially oriented pattern in developing organs and the maintenance of tissue homeostasis. The challenge remains to clarify the complex interplay of different polarity pathways in organogenesis and the link of cell polarity to cell fate specification. Interdisciplinary approaches are also important to understand the roles of mechanical forces in coupling cellular polarization and differentiation. This review outlines current advances on planar polarity regulators in asymmetric organ formation, with the aim to identify questions that deserve further investigation.

Wnt/planar cell polarity signaling controls morphogenetic movements of gastrulation and neural tube closure

Gastrulation and neurulation are successive morphogenetic processes that play key roles in shaping the basic embryonic body plan. Importantly, they operate through common cellular and molecular mechanisms to set up the three spatially organized germ layers and to close the neural tube. During gastrulation and neurulation, convergent extension movements driven by cell intercalation and oriented cell division generate major forces to narrow the germ layers along the mediolateral axis and elongate the embryo in the anteroposterior direction. Apical constriction also makes an important contribution to promote the formation of the blastopore and the bending of the neural plate. Planar cell polarity proteins are major regulators of asymmetric cell behaviors and critically involved in a wide variety of developmental processes, from gastrulation and neurulation to organogenesis. Mutations of planar cell polarity genes can lead to general defects in the morphogenesis of different organs and the co-existence of distinct congenital diseases, such as spina bifida, hearing deficits, kidney diseases, and limb elongation defects. This review outlines our current understanding of non-canonical Wnt signaling, commonly known as Wnt/planar cell polarity signaling, in regulating morphogenetic movements of gastrulation and neural tube closure during development and disease. It also attempts to identify unanswered questions that deserve further investigations.

Circumventing Zygotic Lethality to Generate Maternal Mutants in Zebrafish

Maternal gene products accumulated during oogenesis are essential for supporting early developmental processes in both invertebrates and vertebrates. Therefore, understanding their regulatory functions should provide insights into the maternal control of embryogenesis. The CRISPR/Cas9 genome editing technology has provided a powerful tool for creating genetic mutations to study gene functions and developing disease models to identify new therapeutics. However, many maternal genes are also essential after zygotic genome activation; as a result, loss of their zygotic functions often leads to lethality or sterility, thus preventing the generation of maternal mutants by classical crossing between zygotic homozygous mutant adult animals. Although several approaches, such as the rescue of mutant phenotypes through an injection of the wild-type mRNA, germ-line replacement, and the generation of genetically mosaic females, have been developed to overcome this difficulty, they are often technically challenging and time-consuming or inappropriate for many genes that are essential for late developmental events or for germ-line formation. Recently, a method based on the oocyte transgenic expression of CRISPR/Cas9 and guide RNAs has been designed to eliminate maternal gene products in zebrafish. This approach introduces several tandem guide RNA expression cassettes and a GFP reporter into transgenic embryos expressing Cas9 to create biallelic mutations and inactivate genes of interest specifically in the developing oocytes. It is particularly accessible and allows for the elimination of maternal gene products in one fish generation. By further improving its efficiency, this method can be used for the systematic characterization of maternal-effect genes.

Lysosomal degradation of the maternal dorsal determinant Hwa safeguards dorsal body axis formation

The Spemann and Mangold Organizer (SMO) is of fundamental importance for dorsal ventral body axis formation during vertebrate embryogenesis. Maternal Huluwa (Hwa) has been identified as the dorsal determinant that is both necessary and sufficient for SMO formation. However, it remains unclear how Hwa is regulated. Here, we report that the E3 ubiquitin ligase zinc and ring finger 3 (ZNRF3) is essential for restricting the spatial activity of Hwa and therefore correct SMO formation in Xenopus laevis. ZNRF3 interacts with and ubiquitinates Hwa, thereby regulating its lysosomal trafficking and protein stability. Perturbation of ZNRF3 leads to the accumulation of Hwa and induction of an ectopic axis in embryos. Ectopic expression of ZNRF3 promotes Hwa degradation and dampens the axis-inducing activity of Hwa. Thus, our findings identify a substrate of ZNRF3, but also highlight the importance of the regulation of Hwa temporospatial activity in body axis formation in vertebrate embryos.

Regulation of Dishevelled protein activity and stability by post-translational modifications and autophagy

As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) plays essential roles in development processes and adult tissue homeostasis in multicellular organisms, and its deregulation results in human development disorders and other diseases. Dvl integrates and relays complex Wnt signals by acting as a branch-point of β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. It dynamically interacts with multiple proteins to modulate Wnt signaling, while its activity and stability are tightly controlled by other proteins. This Review summarizes the current understanding of regulation of Dvl activity, localization, and stability by post-translational modifications, aggregation, and autophagy, and the impacts on Dvl function in both Wnt signaling and biological processes.

A Time-Saving Strategy to Generate Double Maternal Mutants by an Oocyte-Specific Conditional Knockout System in Zebrafish

Simple Summary

Maternally supplied mRNAs and proteins, termed maternal factors, are produced by over 14,000 coding genes in zebrafish. They play exclusive roles in controlling the formation of oocytes and the development of early embryos. These maternal factors can also compensate for the loss of function of its corresponding zygotic gene products. Thus, eliminating both maternal and zygotic gene products is essential to elucidate the functions of more than half of zebrafish genes. However, it is always challenging to inactivate maternal factors, because traditional genetic methods are either technically demanding or time-consuming. Our recent work established a rapid conditional knockout method to generate maternal or maternal and zygotic mutants in one fish generation. Here, we further test the feasibility of this approach to knock out two maternal genes with functional redundancy simultaneously. As a proof of principle, we successfully generated double maternal mutant embryos for dvl2 and dvl3a genes in three months for the first time. The cell movement defects in mutant embryos obtained by this approach mimic the genuine mutant embryos generated after fifteen months of time-consuming screening following the previously reported mosaic strategy. Therefore, this method has the potential to speed up the functional study of paralogous maternal genes.

Abstract

Maternal products are those mRNAs and proteins deposited during oogenesis, which play critical roles in controlling oocyte formation, fertilization, and early embryonic development. However, loss-of-function studies for these maternal factors are still lacking, mainly because of the prolonged period of transgenerational screening and technical barriers that prevent the generation of maternal (M) and maternal and zygotic (MZ) mutant embryos. By the transgenic expression of multiple sgRNAs targeting a single gene of interest in the background of a transgenic line Tg(zpc:zcas9) with oocyte-specific cas9 expression, we have successfully obtained maternal or maternal–zygotic mutant for single genes in F1 embryos. In this work, we tandemly connected a maternal GFP marker and eight sgRNA expression units to target dvl2 and dvl3a simultaneously and introduced this construct to the genome of Tg(zpc:zcas9) by meganuclease I-Sce I. As expected, we confirmed the existence of Mdvl2;Mdvl3a embryos with strong defective convergence and extension movement during gastrulation among outcrossed GFP positive F1 offspring. The MZdvl2;MZdvl3a embryos were also obtained by crossing the mutant carrying mosaic F0 female with dvl2^+/−;dvl3a^−/− male fish. This proof-of-principle thus highlights the potential of this conditional knockout strategy to circumvent the current difficulty in the study of genes with multiple functionally redundant paralogs.