Molecular anatomy of placode development in Xenopus laevis

Shh signaling directs dorsal ventral patterning in the regenerating X. tropicalis spinal cord

Tissue development and regeneration rely on the deployment of embryonic signals to drive progenitor activity and thus generate complex cell diversity and organization. One such signal is Sonic Hedgehog (Shh), which establishes the dorsal-ventral (D/V) axis of the spinal cord during embryogenesis. However, the existence of this D/V axis and its dependence on Shh signaling during regeneration varies by species. Here we investigate the function of Shh signaling in patterning the D/V axis during spinal cord regeneration in Xenopus tropicalis tadpoles. We find that neural progenitor markers Msx1/2, Nkx6.1, and Nkx2.2 are confined to dorsal, intermediate and ventral spatial domains, respectively, in both the uninjured and regenerating spinal cord. These domains are altered by perturbation of Shh signaling. Additionally, we find that these D/V domains are more sensitive to Shh perturbation during regeneration than uninjured tissue. The renewed sensitivity of these neural progenitor cells to Shh signals represents a regeneration specific response and raises questions about how responsiveness to developmental patterning cues is regulated in mature and regenerating tissues.

Marcks and Marcks-like 1 proteins promote spinal cord development and regeneration in Xenopus

Marcks and Marcksl1 are abundant proteins that shuttle between the cytoplasm and membrane to modulate multiple cellular processes, including cytoskeletal dynamics, proliferation, and secretion. Here, we performed loss- and gain-of-function experiments in Xenopus laevis to reveal the novel roles of these proteins in spinal cord development and regeneration. We show that Marcks and Marcksl1 have partly redundant functions and are required for normal neurite formation and proliferation of neuro-glial progenitors during embryonic spinal cord development and for its regeneration during tadpole stages. Rescue experiments in Marcks and Marcksl1 loss-of-function animals further suggested that some of the functions of Marcks and Marcksl1 in the spinal cord are mediated by phospholipid signaling. Taken together, these findings identify Marcks and Marcksl1 as critical new players in spinal cord development and regeneration and suggest new pathways to be targeted for therapeutic stimulation of spinal cord regeneration in human patients.

Shh signaling directs dorsal ventral patterning in the regenerating X. tropicalis spinal cord

Tissue development and regeneration rely on the deployment of embryonic signals to drive progenitor activity and thus generate complex cell diversity and organization. One such signal is Sonic Hedgehog (Shh), which establishes the dorsal-ventral (D/V) axis of the spinal cord during embryogenesis. However, the existence of this D/V axis and its dependence on Shh signaling during regeneration varies by species. Here we investigate the function of Shh signaling in patterning the D/V axis during spinal cord regeneration in Xenopus tropicalis tadpoles. We find that neural progenitor markers Msx1/2, Nkx6.1, and Nkx2.2 are confined to dorsal, intermediate and ventral spatial domains, respectively, in both the uninjured and regenerating spinal cord. These domains are altered by perturbation of Shh signaling. Additionally, we find that these D/V domains are more sensitive to Shh perturbation during regeneration than uninjured tissue. The renewed sensitivity of these neural progenitor cells to Shh signals represents a regeneration specific response and raises questions about how responsiveness to developmental patterning cues is regulated in mature and regenerating tissues.

Using Xenopus to discover new candidate genes involved in BOR and other congenital hearing loss syndromes

Hearing in infants is essential for brain development, acquisition of verbal language skills, and development of social interactions. Therefore, it is important to diagnose hearing loss soon after birth so that interventions can be provided as early as possible. Most newborns in the United States are screened for hearing deficits and commercially available next-generation sequencing hearing loss panels often can identify the causative gene, which may also identify congenital defects in other organs. One of the most prevalent autosomal dominant congenital hearing loss syndromes is branchio-oto-renal syndrome (BOR), which also presents with defects in craniofacial structures and the kidney. Currently, mutations in three genes, SIX1, SIX5, and EYA1, are known to be causative in about half of the BOR patients that have been tested. To uncover new candidate genes that could be added to congenital hearing loss genetic screens, we have combined the power of Drosophila mutants and protein biochemical assays with the embryological advantages of Xenopus, a key aquatic animal model with a high level of genomic similarity to human, to identify potential Six1 transcriptional targets and interacting proteins that play a role during otic development. We review our transcriptomic, yeast 2-hybrid, and proteomic approaches that have revealed a large number of new candidates. We also discuss how we have begun to identify how Six1 and co-factors interact to direct developmental events necessary for normal otic development.

Comparing the effects of three neonicotinoids on embryogenesis of the South African clawed frog Xenopus laevis

Neonicotinoids (NEOs) are widely used insecticides that are ubiquitous in agricultural use. Since NEOs are found in natural waters as well as in tap water and human urine in regions where NEOs are widely used, NEOs pose a potential hazard to non-target organisms such as animals and humans. Some of the commonly detected NEOs are imidacloprid (IMD), thiamethoxam (TMX), and its metabolite clothianidin (CLO). Although previously published scientific information, including an assessment of the environmental risks, particularly for bees, had resulted in a ban on the outdoor use of these three NEOs in the EU - their use is now only permitted in closed greenhouses - these NEOs continue to be used in agriculture in many other parts of the world. Therefore, a detailed study and comparison of the effects of NEOs on the embryonic development of non-target organisms is needed to further define the risk profiles. Embryos of the South African clawed frog Xenopus laevis, a well-established aquatic model, were exposed to different concentrations of IMD, TMX, or CLO (0.1-100 mg/L) to study and compare the possible effects of a single contaminant in natural water bodies on early embryogenesis. The results included a reduced body length, a smaller orbital space, impaired cranial cartilage and nerves, and an altered heart structure and function. At the molecular level, NEO exposure partially resulted in an altered expression of tissue-specific factors, which are involved in eye, cranial placode, and heart development. Our results suggest that the NEOs studied negatively affect the embryonic development of the non-target organism X. laevis. Since pesticides, especially NEOs, pollute the environment worldwide, it is suggested that they are strictly controlled and monitored in the areas where they are used. In addition, the question arises as to whether pesticide metabolites also pose a risk to the environment and need to be investigated further so that they can be taken into account when registering ingredients.

In vitro modeling of cranial placode differentiation: Recent advances, challenges, and perspectives

Cranial placodes are transient ectodermal thickenings that contribute to a diverse array of organs in the vertebrate head. They develop from a common territory, the pre-placodal region that over time segregates along the antero-posterior axis into individual placodal domains: the adenohypophyseal, olfactory, lens, trigeminal, otic, and epibranchial placodes. These placodes terminally differentiate into the anterior pituitary, the lens, and contribute to sensory organs including the olfactory epithelium, and inner ear, as well as several cranial ganglia. To study cranial placodes and their derivatives and generate cells for therapeutic purposes, several groups have turned to in vitro derivation of placodal cells from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs). In this review, we summarize the signaling cues and mechanisms involved in cranial placode induction, specification, and differentiation in vivo, and discuss how this knowledge has informed protocols to derive cranial placodes in vitro. We also discuss the benefits and limitations of these protocols, and the potential of in vitro cranial placode modeling in regenerative medicine to treat cranial placode-related pathologies.

Mapping oto-pharyngeal development in a human inner ear organoid model

Inner ear development requires the coordination of cell types from distinct epithelial, mesenchymal and neuronal lineages. Although we have learned much from animal models, many details about human inner ear development remain elusive. We recently developed an in vitro model of human inner ear organogenesis using pluripotent stem cells in a 3D culture, fostering the growth of a sensorineural circuit, including hair cells and neurons. Despite previously characterizing some cell types, many remain undefined. This study aimed to chart the in vitro development timeline of the inner ear organoid to understand the mechanisms at play. Using single-cell RNA sequencing at ten stages during the first 36 days of differentiation, we tracked the evolution from pluripotency to various ear cell types after exposure to specific signaling modulators. Our findings showcase gene expression that influences differentiation, identifying a plethora of ectodermal and mesenchymal cell types. We also discern aspects of the organoid model consistent with in vivo development, while highlighting potential discrepancies. Our study establishes the Inner Ear Organoid Developmental Atlas (IODA), offering deeper insights into human biology and improving inner ear tissue differentiation.

Gene expression mapping of the neuroectoderm across phyla - conservation and divergence of early brain anlagen between insects and vertebrates

Gene expression has been employed for homologizing body regions across bilateria. The molecular comparison of vertebrate and fly brains has led to a number of disputed homology hypotheses. Data from the fly Drosophila melanogaster have recently been complemented by extensive data from the red flour beetle Tribolium castaneum with its more insect-typical development. In this review, we revisit the molecular mapping of the neuroectoderm of insects and vertebrates to reconsider homology hypotheses. We claim that the protocerebrum is non-segmental and homologous to the vertebrate fore- and midbrain. The boundary between antennal and ocular regions correspond to the vertebrate mid-hindbrain boundary while the deutocerebrum represents the anterior-most ganglion with serial homology to the trunk. The insect head placode is shares common embryonic origin with the vertebrate adenohypophyseal placode. Intriguingly, vertebrate eyes develop from a different region compared to the insect compound eyes calling organ homology into question. Finally, we suggest a molecular re-definition of the classic concepts of archi- and prosocerebrum.

Paracrine regulation of neural crest EMT by placodal MMP28

Epithelial-mesenchymal transition (EMT) is an early event in cell dissemination from epithelial tissues. EMT endows cells with migratory, and sometimes invasive, capabilities and is thus a key process in embryo morphogenesis and cancer progression. So far, matrix metalloproteinases (MMPs) have not been considered as key players in EMT but rather studied for their role in matrix remodelling in later events such as cell migration per se. Here, we used Xenopus neural crest cells to assess the role of MMP28 in EMT and migration in vivo. We show that a catalytically active MMP28, expressed by neighbouring placodal cells, is required for neural crest EMT and cell migration. We provide strong evidence indicating that MMP28 is imported in the nucleus of neural crest cells where it is required for normal Twist expression. Our data demonstrate that MMP28 can act as an upstream regulator of EMT in vivo raising the possibility that other MMPs might have similar early roles in various EMT-related contexts such as cancer, fibrosis, and wound healing.

Early Steps towards Hearing: Placodes and Sensory Development

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

Characteristic tetraspanin expression patterns mark various tissues during early Xenopus development

The tetraspanins (Tspans) constitute a family of cell surface proteins with four transmembrane domains. Tspans have been found on the plasma membrane and on exosomes of various organelles. Reports on the function of Tspans during the early development of Xenopus have mainly focused on the expression of uroplakins in gametes. Although the roles of extracellular vesicles (EVs) including exosomes have been actively analyzed in cancer research, the contribution of EVs to early development is not well understood. This is because the diffusivity of EVs is not compatible with a very strict developmental process. In this study, we analyzed members of the Tspan family in early development of Xenopus. Expression was prominent in specific organs such as the notochord, eye, cranial neural crest cells (CNCs), trunk neural crest cells, placodes, and somites. We overexpressed several combinations of Tspans in CNCs in vitro and in vivo. Changing the partner changed the distribution of fluorescent-labeled Tspans. Therefore, it is suggested that expression of multiple Tspans in a particular tissue might produce heterogeneity of intercellular communication, which has not yet been recognized.

Human stem cell models to study placode development, function and pathology

Placodes are embryonic structures originating from the rostral ectoderm that give rise to highly diverse organs and tissues, comprising the anterior pituitary gland, paired sense organs and cranial sensory ganglia. Their development, including the underlying gene regulatory networks and signalling pathways, have been for the most part characterised in animal models. In this Review, we describe how placode development can be recapitulated by the differentiation of human pluripotent stem cells towards placode progenitors and their derivatives, highlighting the value of this highly scalable platform as an optimal in vitro tool to study the development of human placodes, and identify human-specific mechanisms in their development, function and pathology.

Feedback Regulation of Signaling Pathways for Precise Pre-Placodal Ectoderm Formation in Vertebrate Embryos

Intracellular signaling pathways are essential to establish embryonic patterning, including embryonic axis formation. Ectodermal patterning is also governed by a series of morphogens. Four ectodermal regions are thought to be controlled by morphogen gradients, but some perturbations are expected to occur during dynamic morphogenetic movement. Therefore, a mechanism to define areas precisely and reproducibly in embryos, including feedback regulation of signaling pathways, is necessary. In this review, we outline ectoderm pattern formation and signaling pathways involved in the establishment of the pre-placodal ectoderm (PPE). We also provide an example of feedback regulation of signaling pathways for robust formation of the PPE, showing the importance of this regulation.

MicroRNAs in neural crest development and neurocristopathies

The neural crest (NC) is a vertebrate-specific migratory population of multipotent stem cells that originate during late gastrulation in the region between the neural and non-neural ectoderm. This population of cells give rise to a range of derivatives, such as melanocytes, neurons, chondrocytes, chromaffin cells, and osteoblasts. Because of this, failure of NC development can cause a variety of pathologies, often syndromic, that are globally called neurocristopathies. Many genes are known to be involved in NC development, but not all of them have been identified. In recent years, attention has moved from protein-coding genes to non-coding genes, such as microRNAs (miRNA). There is increasing evidence that these non-coding RNAs are playing roles during embryogenesis by regulating the expression of protein-coding genes. In this review, we give an introduction to miRNAs in general and then focus on some miRNAs that may be involved in NC development and neurocristopathies. This new direction of research will give geneticists, clinicians, and molecular biologists more tools to help patients affected by neurocristopathies, as well as broadening our understanding of NC biology.

Repressive Interactions Between Transcription Factors Separate Different Embryonic Ectodermal Domains

The embryonic ectoderm is composed of four domains: neural plate, neural crest, pre-placodal region (PPR) and epidermis. Their formation is initiated during early gastrulation by dorsal-ventral and anterior-posterior gradients of signaling factors that first divide the embryonic ectoderm into neural and non-neural domains. Next, the neural crest and PPR domains arise, either via differential competence of the neural and non-neural ectoderm (binary competence model) or via interactions between the neural and non-neural ectoderm tissues to produce an intermediate neural border zone (NB) (border state model) that subsequently separates into neural crest and PPR. Many previous gain- and loss-of-function experiments demonstrate that numerous TFs are expressed in initially overlapping zones that gradually resolve into patterns that by late neurula stages are characteristic of each of the four domains. Several of these studies suggested that this is accomplished by a combination of repressive TF interactions and competence to respond to local signals. In this study, we ectopically expressed TFs that at neural plate stages are characteristic of one domain in a different domain to test whether they act cell autonomously as repressors. We found that almost all tested TFs caused reduced expression of the other TFs. At gastrulation these effects were strictly within the lineage-labeled cells, indicating that the effects were cell autonomous, i.e., due to TF interactions within individual cells. Analysis of previously published single cell RNAseq datasets showed that at the end of gastrulation, and continuing to neural tube closure stages, many ectodermal cells express TFs characteristic of more than one neural plate stage domain, indicating that different TFs have the opportunity to interact within the same cell. At neurula stages repression was observed both in the lineage-labeled cells and in adjacent cells not bearing detectable lineage label, suggesting that cell-to-cell signaling has begun to contribute to the separation of the domains. Together, these observations directly demonstrate previous suggestions in the literature that the segregation of embryonic ectodermal domains initially involves cell autonomous, repressive TF interactions within an individual cell followed by the subsequent advent of non-cell autonomous signaling to neighbors.

Systematic mapping of rRNA 2'-O methylation during frog development and involvement of the methyltransferase Fibrillarin in eye and craniofacial development in Xenopus laevis

Ribosomes are essential nanomachines responsible for protein production. Although ribosomes are present in every living cell, ribosome biogenesis dysfunction diseases, called ribosomopathies, impact particular tissues specifically. Here, we evaluate the importance of the box C/D snoRNA-associated ribosomal RNA methyltransferase fibrillarin (Fbl) in the early embryonic development of Xenopus laevis. We report that in developing embryos, the neural plate, neural crest cells (NCCs), and NCC derivatives are rich in fbl transcripts. Fbl knockdown leads to striking morphological defects affecting the eyes and craniofacial skeleton, due to lack of NCC survival caused by massive p53-dependent apoptosis. Fbl is required for efficient pre-rRNA processing and 18S rRNA production, which explains the early developmental defects. Using RiboMethSeq, we systematically reinvestigated ribosomal RNA 2’-O methylation in X. laevis, confirming all 89 previously mapped sites and identifying 15 novel putative positions in 18S and 28S rRNA. Twenty-three positions, including 10 of the new ones, were validated orthogonally by low dNTP primer extension. Bioinformatic screening of the X. laevis transcriptome revealed candidate box C/D snoRNAs for all methylated positions. Mapping of 2’-O methylation at six developmental stages in individual embryos indicated a trend towards reduced methylation at specific positions during development. We conclude that fibrillarin knockdown in early Xenopus embryos causes reduced production of functional ribosomal subunits, thus impairing NCC formation and migration.

Ribosomes are essential nanomachines responsible for protein production in all cells. Ribosomopathies are diseases caused by improper ribosome formation due to mutations in ribosomal proteins or ribosome assembly factors. Such diseases primarily affect the brain and blood, and it is unclear how malfunctioning of a process as general as ribosome formation can lead to tissue-specific diseases. Here we have examined how fibrillarin, an enzyme which modifies ribosomal RNA by adding methyl groups at specific sites, affects early embryonic development in the frog Xenopus laevis. We have revealed its importance in the maturation of cells forming an embryonic structure called the neural crest. Fibrillarin depletion leads to reduced eye size and abnormal head shape, reminiscent of other conditions such as Treacher Collins syndrome. Molecularly, the observed phenotypes are explainable by increased p53-dependent programmed cell death triggered by inhibition of certain pre-rRNA processing steps. Our systematic investigation of the ribosomal RNA 2’-O methylation repertoire across development has further revealed hypomodification at a late stage of development, which might play a role in late developmental transitions involving differential translation by compositionally different ribosomes.

Collective durotaxis along a self-generated stiffness gradient in vivo

Collective cell migration underlies morphogenesis, wound healing and cancer invasion^1,2. Most directed migration in vivo has been attributed to chemotaxis, whereby cells follow a chemical gradient^3-5. Cells can also follow a stiffness gradient in vitro, a process called durotaxis^3,4,6-8, but evidence for durotaxis in vivo is lacking⁶. Here we show that in Xenopus laevis the neural crest-an embryonic cell population-self-generates a stiffness gradient in the adjacent placodal tissue, and follows this gradient by durotaxis. The gradient moves with the neural crest, which is continually pursuing a retreating region of high substrate stiffness. Mechanistically, the neural crest induces the gradient due to N-cadherin interactions with the placodes and senses the gradient through cell-matrix adhesions, resulting in polarized Rac activity and actomyosin contractility, which coordinates durotaxis. Durotaxis synergizes with chemotaxis, cooperatively polarizing actomyosin machinery of the cell group to prompt efficient directional collective cell migration in vivo. These results show that durotaxis and dynamic stiffness gradients exist in vivo, and gradients of chemical and mechanical signals cooperate to achieve efficient directional cell migration.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Development and evolution of the vestibular apparatuses of the inner ear

The vertebrate inner ear is a labyrinthine sensory organ responsible for perceiving sound and body motion. While a great deal of research has been invested in understanding the auditory system, a growing body of work has begun to delineate the complex developmental program behind the apparatuses of the inner ear involved with vestibular function. These animal studies have helped identify genes involved in inner ear development and model syndromes known to include vestibular dysfunction, paving the way for generating treatments for people suffering from these disorders. This review will provide an overview of known inner ear anatomy and function and summarize the exciting discoveries behind inner ear development and the evolution of its vestibular apparatuses.

Responses of Epibranchial Placodes to Disruptions of the FGF and BMP Signaling Pathways in Embryonic Mice

Placodes are ectodermal thickenings of the embryonic vertebrate head. Their descendants contribute to sensory organ development, but also give rise to sensory neurons of the cranial nerves. In mammals, the signaling pathways which regulate the morphogenesis and neurogenesis of epibranchial placodes, localized dorsocaudally to the pharyngeal clefts, are poorly understood. Therefore, we performed mouse whole embryo culture experiments to assess the impact of pan-fibroblast growth factor receptor (FGFR) inhibitors, anti-FGFR3 neutralizing antibodies or the pan-bone morphogenetic protein receptor (BMPR) inhibitor LDN193189 on epibranchial development. We demonstrate that each of the three paired epibranchial placodes is regulated by a unique combination of FGF and/or bone morphogenetic protein (BMP) signaling. Thus, neurogenesis depends on fibroblast growth factor (FGF) signals, albeit to different degrees, in all epibranchial placodes (EP), whereas only EP1 and EP3 significantly rely on neurogenic BMP signals. Furthermore, individual epibranchial placodes vary in the extent to which FGF and/or BMP signals (1) have access to certain receptor subtypes, (2) affect the production of Neurogenin (Ngn)2⁺ and/or Ngn1⁺ neuroblasts, and (3) regulate either neurogenesis alone or together with structural maintenance. In EP2 and EP3, all FGF-dependent production of Ngn2⁺ neuroblasts is mediated via FGFR3 whereas, in EP1, it depends on FGFR1 and FGFR3. Differently, production of FGF-dependent Ngn1⁺ neuroblasts almost completely depends on FGFR3 in EP1 and EP2, but not in EP3. Finally, FGF signals turned out to be responsible for the maintenance of both placodal thickening and neurogenesis in all epibranchial placodes, whereas administration of the pan-BMPR inhibitor, apart from its negative neurogenic effects in EP1 and EP3, causes only decreases in the thickness of EP3. Experimentally applied inhibitors most probably not only blocked receptors in the epibranchial placodes, but also endodermal receptors in the pharyngeal pouches, which act as epibranchial signaling centers. While high doses of pan-FGFR inhibitors impaired the development of all pharyngeal pouches, high doses of the pan-BMPR inhibitor negatively affected only the pharyngeal pouches 3 and 4. In combination with partly concordant, partly divergent findings in other vertebrate classes our observations open up new approaches for research into the complex regulation of neurogenic placode development.

Eya1 protein distribution during embryonic development of Xenopus laevis

Eya1 and other Eya proteins are important regulators of progenitor proliferation, cell differentiation and morphogenesis in all three germ layers. At present, most of our knowledge of Eya1 distribution is based on in situ hybridization for Eya1 mRNA. However, to begin to dissect the mechanisms underlying Eya1 functions, we need a better understanding of the spatiotemporal distribution of Eya1 proteins during embryonic development, their subcellular localization and their levels of expression in various tissues. Here we report the localization of Eya1 protein throughout embryonic development from neural plate stages to tadpole stages of Xenopus laevis using a specific antibody for Xenopus Eya1. Our study confirms the expression of Eya1 protein in cranial placodes, placodally derived sensory primordia (olfactory epithelium, otic vesicle, lateral line primordia) and cranial ganglia, as well as in somites, secondary heart field and pharyngeal endoderm. In addition, we report here a novel expression of Eya1 proteins in scattered epidermal cells in Xenopus. Our findings also reveal that, while being predominantly expressed in nuclei in most expression domains, Eya1 protein is also localized to the cytoplasm, in particular in the early preplacodal ectoderm, some placode-derived ganglia and a subset of epidermal cells. While some cytoplasmic roles of Eya1 have been previously described in other contexts, the functions of cytoplasmic Eya1 in the preplacodal ectoderm, cranial ganglia and epidermal cells remain to be investigated.

Sobp modulates the transcriptional activation of Six1 target genes and is required during craniofacial development

Branchio-oto-renal syndrome (BOR) is a disorder characterized by hearing loss, and craniofacial and/or renal defects. Variants in the transcription factor Six1 and its co-factor Eya1, both of which are required for otic development, are linked to BOR. We previously identified Sobp as a potential Six1 co-factor, and SOBP variants in mouse and humans cause otic phenotypes; therefore, we asked whether Sobp interacts with Six1 and thereby may contribute to BOR. Co-immunoprecipitation and immunofluorescence experiments demonstrate that Sobp binds to and colocalizes with Six1 in the cell nucleus. Luciferase assays show that Sobp interferes with the transcriptional activation of Six1+Eya1 target genes. Experiments in Xenopus embryos that either knock down or increase expression of Sobp show that it is required for formation of ectodermal domains at neural plate stages. In addition, altering Sobp levels disrupts otic vesicle development and causes craniofacial cartilage defects. Expression of Xenopus Sobp containing the human variant disrupts the pre-placodal ectoderm similar to full-length Sobp, but other changes are distinct. These results indicate that Sobp modifies Six1 function and is required for vertebrate craniofacial development, and identify Sobp as a potential candidate gene for BOR.

Aberrant early growth of individual trigeminal sensory and motor axons in a series of mouse genetic models of 22q11.2 deletion syndrome

We identified divergent modes of initial axon growth that prefigure disrupted differentiation of the trigeminal nerve (CN V), a cranial nerve essential for suckling, feeding and swallowing (S/F/S), a key innate behavior compromised in multiple genetic developmental disorders including DiGeorge/22q11.2 Deletion Syndrome (22q11.2 DS). We combined rapid in vivo labeling of single CN V axons in LgDel^+/− mouse embryos, a genomically accurate 22q11.2DS model, and 3D imaging to identify and quantify phenotypes that could not be resolved using existing methods. We assessed these phenotypes in three 22q11.2-related genotypes to determine whether individual CN V motor and sensory axons wander, branch and sprout aberrantly in register with altered anterior–posterior hindbrain patterning and gross morphological disruption of CN V seen in LgDel^+/−. In the additional 22q11.2-related genotypes: Tbx1^+/−, Ranbp1^−/−, Ranbp1^+/− and LgDel^+/−:Raldh2^+/−; axon phenotypes are seen when hindbrain patterning and CN V gross morphology is altered, but not when it is normal or restored toward WT. This disordered growth of CN V sensory and motor axons, whose appropriate targeting is critical for optimal S/F/S, may be an early, critical determinant of imprecise innervation leading to inefficient oropharyngeal function associated with 22q11.2 deletion from birth onward.

Mutations in SIX1 Associated with Branchio-oto-Renal Syndrome (BOR) Differentially Affect Otic Expression of Putative Target Genes

Several single-nucleotide mutations in SIX1 underlie branchio-otic/branchio-oto-renal (BOR) syndrome, but the clinical literature has not been able to correlate different variants with specific phenotypes. We previously assessed whether variants in either the cofactor binding domain (V17E, R110W) or the DNA binding domain (W122R, Y129C) might differentially affect early embryonic gene expression, and found that each variant had a different combination of effects on neural crest and placode gene expression. Since the otic vesicle gives rise to the inner ear, which is consistently affected in BOR, herein we focused on whether the variants differentially affected the otic expression of genes previously found to be likely Six1 targets. We found that V17E, which does not bind Eya cofactors, was as effective as wild-type Six1 in reducing most otic target genes, whereas R110W, W122R and Y129C, which bind Eya, were significantly less effective. Notably, V17E reduced the otic expression of prdm1, whereas R110W, W122R and Y129C expanded it. Since each mutant has defective transcriptional activity but differs in their ability to interact with Eya cofactors, we propose that altered cofactor interactions at the mutated sites differentially interfere with their ability to drive otic gene expression, and these differences may contribute to patient phenotype variability.

Molecular mechanisms of hearing loss in Nager syndrome

Nager syndrome is a rare human developmental disorder characterized by hypoplastic neural crest-derived craniofacial bones and limb defects. Mutations in SF3B4 gene, which encodes a component of the spliceosome, are a major cause for Nager. A review of the literature indicates that 45% of confirmed cases are also affected by conductive, sensorineural or mixed hearing loss. Conductive hearing loss is due to defective middle ear ossicles, which are neural crest derived, while sensorineural hearing loss typically results from defective inner ear or vestibulocochlear nerve, which are both derived from the otic placode. Animal model of Nager syndrome indicates that upon Sf3b4 knockdown cranial neural crest progenitors are depleted, which may account for the conductive hearing loss in these patients. To determine whether Sf3b4 plays a role in otic placode formation we analyzed the impact of Sf3b4 knockdown on otic development. Sf3b4-depleted Xenopus embryos exhibited reduced expression of several pan-placodal genes six1, dmrta1 and foxi4.1. We confirmed the dependence of placode genes expression on Sf3b4 function in animal cap explants expressing noggin, a BMP antagonist critical to induce placode fate in the ectoderm. Later in development, Sf3b4 morphant embryos had reduced expression of pax8, tbx2, otx2, bmp4 and wnt3a at the otic vesicle stage, and altered otic vesicle development. We propose that in addition to the neural crest, Sf3b4 is required for otic development, which may account for sensorineural hearing loss in Nager syndrome.

Retinoic acid production, regulation and containment through Zic1, Pitx2c and Cyp26c1 control cranial placode specification

All paired sensory organs arise from a common precursor domain called the pre-placodal region (PPR). In Xenopus, Zic1 non-cell autonomously regulates PPR formation by activating retinoic acid (RA) production. Here, we have identified two Zic1 targets, the RA catabolizing enzyme Cyp26c1 and the transcription factor Pitx2c, expressed in the vicinity of the PPR as being crucially required for maintaining low RA levels in a spatially restricted, PPR-adjacent domain. Morpholino- or CRISPR/Cas9-mediated Cyp26c1 knockdown abrogated PPR gene expression, yielding defective cranial placodes. Direct measurement of RA levels revealed that this is mediated by a mechanism involving excess RA accumulation. Furthermore, we show that pitx2c is activated by RA and required for Cyp26c1 expression in a domain-specific manner through induction of FGF8. We propose that Zic1 anteriorly establishes a program of RA containment and regulation through activation of Cyp26c1 and Pitx2c that cooperates to promote PPR specification in a spatially restricted domain.

Hoxb3 Regulates Jag1 Expression in Pharyngeal Epithelium and Affects Interaction With Neural Crest Cells

Craniofacial morphogenesis depends on proper migration of neural crest cells and their interactions with placodes and other cell types. Hox genes provide positional information and are important in patterning the neural crest and pharyngeal arches (PAs) for coordinated formation of craniofacial structures. Hox genes are expressed in the surface ectoderm and epibranchial placodes, their roles in the pharyngeal epithelium and their downstream targets in regulating PA morphogenesis have not been established. We altered the Hox code in the pharyngeal region of the Hoxb3^Tg/+ mutant, in which Hoxb3 is driven to ectopically expressed in Hoxb2 domain in the second pharyngeal arch (PA2). In the transgenic mutant, ectopic Hoxb3 expression was restricted to the surface ectoderm, including the proximal epibranchial placodal region and the distal pharyngeal epithelium. The Hoxb3^Tg/+ mutants displayed hypoplasia of PA2, multiple neural crest-derived facial skeletal and nerve defects. Interestingly, we found that in the Hoxb3^Tg/+ mutant, expression of the Notch ligand Jag1 was specifically up-regulated in the ectodermal pharyngeal epithelial cells of PA2. By molecular experiments, we demonstrated that Hoxb3 could bind to an upstream genomic site S2 and directly regulate Jag1 expression. In the Hoxb3^Tg/+ mutant, elevated expression of Jag1 in the pharyngeal epithelium led to abnormal cellular interaction and deficiency of neural crest cells migrating into PA2. In summary, we showed that Hoxb3 regulates Jag1 expression and proposed a model of pharyngeal epithelium and neural crest interaction during pharyngeal arch development.

Dynamic expression of MMP28 during cranial morphogenesis

Matrix metalloproteinases (MMPs) are a large family of proteases comprising 24 members in vertebrates. They are well known for their extracellular matrix remodelling activity. MMP28 is the latest member of the family to be discovered. It is a secreted MMP involved in wound healing, immune system maturation, cell survival and migration. MMP28 is also expressed during embryogenesis in human and mouse. Here, we describe the detailed expression profile of MMP28 in Xenopus laevis embryos. We show that MMP28 is expressed maternally and accumulates at neurula and tail bud stages specifically in the cranial placode territories adjacent to migrating neural crest cells. As a secreted MMP, MMP28 may be required in neural crest-placode interactions. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Evolutionary and Developmental Associations of Neural Crest and Placodes in the Vertebrate Head: Insights From Jawless Vertebrates

Neural crest and placodes are key innovations of the vertebrate clade. These cells arise within the dorsal ectoderm of all vertebrate embryos and have the developmental potential to form many of the morphological novelties within the vertebrate head. Each cell population has its own distinct developmental features and generates unique cell types. However, it is essential that neural crest and placodes associate together throughout embryonic development to coordinate the emergence of several features in the head, including almost all of the cranial peripheral sensory nervous system and organs of special sense. Despite the significance of this developmental feat, its evolutionary origins have remained unclear, owing largely to the fact that there has been little comparative (evolutionary) work done on this topic between the jawed vertebrates and cyclostomes—the jawless lampreys and hagfishes. In this review, we briefly summarize the developmental mechanisms and genetics of neural crest and placodes in both jawed and jawless vertebrates. We then discuss recent studies on the role of neural crest and placodes—and their developmental association—in the head of lamprey embryos, and how comparisons with jawed vertebrates can provide insights into the causes and consequences of this event in early vertebrate evolution.

Cell fate decisions during the development of the peripheral nervous system in the vertebrate head

Sensory placodes and neural crest cells are among the key cell populations that facilitated the emergence and diversification of vertebrates throughout evolution. Together, they generate the sensory nervous system in the head: both form the cranial sensory ganglia, while placodal cells make major contributions to the sense organs-the eye, ear and olfactory epithelium. Both are instrumental for integrating craniofacial organs and have been key to drive the concentration of sensory structures in the vertebrate head allowing the emergence of active and predatory life forms. Whereas the gene regulatory networks that control neural crest cell development have been studied extensively, the signals and downstream transcriptional events that regulate placode formation and diversity are only beginning to be uncovered. Both cell populations are derived from the embryonic ectoderm, which also generates the central nervous system and the epidermis, and recent evidence suggests that their initial specification involves a common molecular mechanism before definitive neural, neural crest and placodal lineages are established. In this review, we will first discuss the transcriptional networks that pattern the embryonic ectoderm and establish these three cell fates with emphasis on sensory placodes. Second, we will focus on how sensory placode precursors diversify using the specification of otic-epibranchial progenitors and their segregation as an example.

Six1 proteins with human branchio-oto-renal mutations differentially affect cranial gene expression and otic development

Single-nucleotide mutations in human SIX1 result in amino acid substitutions in either the protein-protein interaction domain or the homeodomain, and cause ∼4% of branchio-otic (BOS) and branchio-oto-renal (BOR) cases. The phenotypic variation between patients with the same mutation, even within affected members of the same family, make it difficult to functionally distinguish between the different SIX1 mutations. We made four of the BOS/BOR substitutions in the Xenopus Six1 protein (V17E, R110W, W122R, Y129C), which is 100% identical to human in both the protein-protein interaction domain and the homeodomain, and expressed them in embryos to determine whether they cause differential changes in early craniofacial gene expression, otic gene expression or otic morphology. We confirmed that, similar to the human mutants, all four mutant Xenopus Six1 proteins access the nucleus but are transcriptionally deficient. Analysis of craniofacial gene expression showed that each mutant causes specific, often different and highly variable disruptions in the size of the domains of neural border zone, neural crest and pre-placodal ectoderm genes. Each mutant also had differential effects on genes that pattern the otic vesicle. Assessment of the tadpole inner ear demonstrated that while the auditory and vestibular structures formed, the volume of the otic cartilaginous capsule, otoliths, lumen and a subset of the hair cell-containing sensory patches were reduced. This detailed description of the effects of BOS/BOR-associated SIX1 mutations in the embryo indicates that each causes subtle changes in gene expression in the embryonic ectoderm and otocyst, leading to inner ear morphological anomalies.

Notch signalling regulates epibranchial placode patterning and segregation

Epibranchial placodes are the geniculate, petrosal and nodose placodes that generate parts of cranial nerves VII, IX and X, respectively. How the three spatially separated placodes are derived from the common posterior placodal area is poorly understood. Here, we reveal that the broad posterior placode area is first patterned into a Vgll2⁺/Irx5⁺ rostral domain and a Sox2⁺/Fgf3⁺/Etv5⁺ caudal domain relative to the first pharyngeal cleft. This initial rostral and caudal patterning is then sequentially repeated along each pharyngeal cleft for each epibranchial placode. The caudal domains give rise to the neuronal and non-neuronal cells in the placode, whereas the rostral domains are previously unrecognized structures, serving as spacers between the final placodes. Notch signalling regulates the balance between the rostral and caudal domains: high levels of Notch signalling expand the caudal domain at the expense of the rostral domain, whereas loss of Notch signalling produces the converse phenotype. Collectively, these data unravel a new patterning principle for the early phases of epibranchial placode development and a role for Notch signalling in orchestrating epibranchial placode segregation and differentiation.

Neural crest and placode roles in formation and patterning of cranial sensory ganglia in lamprey

Vertebrates possess paired cranial sensory ganglia derived from two embryonic cell populations, neural crest and placodes. Cranial sensory ganglia arose prior to the divergence of jawed and jawless vertebrates, but the developmental mechanisms that facilitated their evolution are unknown. Using gene expression and cell lineage tracing experiments in embryos of the sea lamprey, Petromyzon marinus, we find that in the cranial ganglia we targeted, development consists of placode-derived neuron clusters in the core of ganglia, with neural crest cells mostly surrounding these neuronal clusters. To dissect functional roles of neural crest and placode cell associations in these developing cranial ganglia, we used CRISPR/Cas9 gene editing experiments to target genes critical for the development of each population. Genetic ablation of SoxE2 and FoxD-A in neural crest cells resulted in differentiated cranial sensory neurons with abnormal morphologies, whereas deletion of DlxB in cranial placodes resulted in near-total loss of cranial sensory neurons. Taken together, our cell-lineage, gene expression, and gene editing results suggest that cranial neural crest cells may not be required for cranial ganglia specification but are essential for shaping the morphology of these sensory structures. We propose that the association of neural crest and placodes in the head of early vertebrates was a key step in the organization of neurons and glia into paired sensory ganglia.

Deregulated NKL Homeobox Genes in B-Cell Lymphoma

Recently, we have described physiological expression patterns of NKL homeobox genes in early hematopoiesis and in subsequent lymphopoiesis. We identified nine genes which constitute the so-called NKL-code. Aberrant overexpression of code-members or ectopically activated non-code NKL homeobox genes are described in T-cell leukemia and in T- and B-cell lymphoma, highlighting their oncogenic role in lymphoid malignancies. Here, we introduce the NKL-code in normal hematopoiesis and focus on deregulated NKL homeobox genes in B-cell lymphoma, including HLX, MSX1 and NKX2-2 in Hodgkin lymphoma; HLX, NKX2-1 and NKX6-3 in diffuse large B-cell lymphoma; and NKX2-3 in splenic marginal zone lymphoma. Thus, the roles of various members of the NKL homeobox gene subclass are considered in normal and pathological hematopoiesis in detail.

The emerging role of cranial nerves in shaping craniofacial development

Organs and structures of the vertebrate head perform a plethora of tasks including visualization, digestion, vocalization/communication, auditory functions, and respiration in response to neuronal input. This input is primarily derived from afferent and efferent fibers of the cranial nerves (sensory and motor respectively) and efferent fibers of the cervical sympathetic trunk. Despite their essential contribution to the function and integration of processes necessary for survival, how organ innervation is established remains poorly understood. Furthermore, while it has been appreciated for some time that innervation of organs by cranial nerves is regulated in part by secreted factors and cell surface ligands expressed by those organs, whether nerves also regulate the development of facial organs is only beginning to be elucidated. This review will provide an overview of cranial nerve development in relation to the organs they innervate, and outline their known contributions to craniofacial development, thereby providing insight into how nerves may shape the organs they innervate during development. Throughout, the interaction between different cell and tissue types will be highlighted.

Six1 and Irx1 have reciprocal interactions during cranial placode and otic vesicle formation

The specialized sensory organs of the vertebrate head are derived from thickened patches of cells in the ectoderm called cranial sensory placodes. The developmental program that generates these placodes and the genes that are expressed during the process have been studied extensively in a number of animals, yet very little is known about how these genes regulate one another. We previously found via a microarray screen that Six1, a known transcriptional regulator of cranial placode fate, up-regulates Irx1 in ectodermal explants. In this study, we investigated the transcriptional relationship between Six1 and Irx1 and found that they reciprocally regulate each other throughout cranial placode and otic vesicle formation. Although Irx1 expression precedes that of Six1 in the neural border zone, its continued and appropriately patterned expression in the pre-placodal region (PPR) and otic vesicle requires Six1. At early PPR stages, Six1 expands the Irx1 domain, but this activity subsides over time and changes to a predominantly repressive effect. Likewise, Irx1 initially expands Six1 expression in the PPR, but later represses it. We also found that Irx1 and Sox11, a known direct target of Six1, reciprocally affect each other. This work demonstrates that the interactions between Six1 and Irx1 are continuous during PPR and placode development and their transcriptional effects on one another change over developmental time.

A gene regulatory network underlying the formation of pre-placodal ectoderm in Xenopus laevis

BACKGROUND

The neural plate border ectoderm gives rise to key developmental structures during embryogenesis, including the neural crest and the preplacodal ectoderm. Many sensory organs and ganglia of vertebrates develop from cranial placodes, which themselves arise from preplacodal ectoderm, defined by expression of transcription factor Six1 and its coactivator Eya1. Here we elucidate the gene regulatory network underlying the specification of the preplacodal ectoderm in Xenopus, and the functional interactions among transcription factors that give rise to this structure.

RESULTS

To elucidate the gene regulatory network upstream of preplacodal ectoderm formation, we use gain- and loss-of-function studies to explore the role of early ectodermal transcription factors for establishing the preplacodal ectoderm and adjacent ectodermal territories, and the role of Six1 and Eya1 in feedback regulation of these transcription factors. Our findings suggest that transcription factors with expression restricted to ventral (non-neural) ectoderm (AP2, Msx1, FoxI1, Vent2, Dlx3, GATA2) and those restricted to dorsal (neural) ectoderm (Pax3, Hairy2b, Zic1) are required for specification of both preplacodal ectoderm and neural crest in a context-dependent fashion and are cross-regulated by Eya1 and Six1.

CONCLUSION

These findings allow us to elucidate a detailed gene regulatory network at the neural plate border upstream of preplacodal ectoderm formation based on functional interactions between ectodermal transcription factors. We propose a new model to explain the formation of immediately juxtaposed preplacodal ectoderm and neural crest territories at the neural plate border, uniting previous models.

Tbx2 is required for the suppression of mesendoderm during early Xenopus development

BACKGROUND

T-box family proteins are DNA-binding transcriptional regulators that play crucial roles during germ layer formation in the early vertebrate embryo. Well-characterized members of this family, including the transcriptional activators Brachyury and VegT, are essential for the proper formation of mesoderm and endoderm, respectively. To date, T-box proteins have not been shown to play a role in the promotion of the third primary germ layer, ectoderm.

RESULTS

Here, we report that the T-box factor Tbx2 is both sufficient and necessary for ectodermal differentiation in the frog Xenopus laevis. Tbx2 is expressed zygotically in the presumptive ectoderm, during blastula and gastrula stages. Ectopic expression of Tbx2 represses mesoderm and endoderm, while loss of Tbx2 leads to inappropriate expression of mesoderm- and endoderm-specific genes in the region fated to give rise to ectoderm. Misexpression of Tbx2 also promotes neural tissue in animal cap explants, suggesting that Tbx2 plays a role in both the establishment of ectodermal fate and its dorsoventral patterning.

CONCLUSIONS

Our studies demonstrate that Tbx2 functions as a transcriptional repressor during germ layer formation, and suggest that this activity is mediated in part through repression of target genes that are stimulated, in the mesendoderm, by transactivating T-box proteins. Taken together, our results point to a critical role for Tbx2 in limiting the potency of blastula-stage progenitor cells during vertebrate germ layer differentiation. Developmental Dynamics 247:903-913, 2018. © 2018 Wiley Periodicals, Inc.

Three-dimensional reconstruction of the cranial and anterior spinal nerves in early tadpoles of Xenopus laevis (Pipidae, Anura)

Xenopus laevis is one of the most widely used model organism in neurobiology. It is therefore surprising, that no detailed and complete description of the cranial nerves exists for this species. Using classical histological sectioning in combination with fluorescent whole mount antibody staining and micro-computed tomography we prepared a detailed innervation map and a freely-rotatable three-dimensional (3D) model of the cranial nerves and anterior-most spinal nerves of early X. laevis tadpoles. Our results confirm earlier descriptions of the pre-otic cranial nerves and present the first detailed description of the post-otic cranial nerves. Tracing the innervation, we found two previously undescribed head muscles (the processo-articularis and diaphragmatico-branchialis muscles) in X. laevis. Data on the cranial nerve morphology of tadpoles are scarce, and only one other species (Discoglossus pictus) has been described in great detail. A comparison of Xenopus and Discoglossus reveals a relatively conserved pattern of the post-otic and a more variable morphology of the pre-otic cranial nerves. Furthermore, the innervation map and the 3D models presented here can serve as an easily accessible basis to identify alterations of the innervation produced by experimental studies such as genetic gain- and loss of function experiments.

Generation of a biotinylatable Sox2 mouse model to identify Sox2 complexes in vivo

Sox2 is a Sry-box containing family member of related transcription factors sharing homology in their DNA binding domain. Sox2 is important during different stages of development, and previously we showed that Sox2 plays an important role in branching morphogenesis and epithelial cell differentiation in lung development. The transcriptional activity of Sox2 depends on its interaction with other proteins, leading to ‘complex-specific’ DNA binding and transcriptional regulation. In this study, we generated a mouse model containing a biotinylatable-tag targeted at the translational start site of the endogenous Sox2 gene (bioSox2). This tag was biotinylated by the bacterial birA protein and the resulting bioSox2 protein was used to identify associating partners of Sox2 at different phases of lung development in vivo (the Sox2 interactome). Homozygous bioSox2 mice are viable and fertile irrespective of the biotinylation of the bio tag, indicating that the bioSox2 gene is normally expressed and the protein is functional in all tissues. This suggests that partners of Sox2 are most likely able to associate with the bioSox2 protein. BioSox2 complexes were isolated with high affinity using streptavidin beads and analysed by MALDI-ToF mass spectrometry analysis. Several of the identified binding partners are already shown to have a respiratory phenotype. Two of these partners, Wdr5 and Tcf3, were validated to confirm their association in Sox2 complexes. This bioSox2 mouse model will be a valuable tool for isolating in vivo Sox2 complexes from different tissues.

Cloning and spatiotemporal expression of Xenopus laevis Apolipoprotein CI

Apolipoprotein CI (ApoCI) belongs to the Apolipoprotein superfamily, members of which are involved in lipid transport, uptake and homeostasis. Excessive ApoCI has been implicated in atherosclerosis and Alzheimer's disease in humans. In this study we report the isolation of Xenopus laevis apoCI and describe the expression pattern of this gene during early development, using reverse transcription polymerase chain reaction and whole mount in situ hybridization. Xenopus apoCI is enriched in the dorsal ectoderm during gastrulation, and is subsequently expressed in sensory placodes, neural tube and cranial neural crest. These data suggest as yet uncharacterized roles for ApoCI during early vertebrate embryogenesis.

Regulation of continuous but complex expression pattern of Six1 during early sensory development

BACKGROUND

In vertebrates, cranial sensory placodes give rise to neurosensory and endocrine structures, such as the olfactory epithelium, inner ear, and anterior pituitary. We report here the establishment of a transgenic mouse line that expresses Cre recombinase under the control of Six1-21, a major placodal enhancer of the homeobox gene Six1.

RESULTS

In the new Cre-expressing line, mSix1-21-NLSCre, the earliest Cre-mediated recombination was induced at embryonic day 8.5 in the region overlapping with the otic-epibranchial progenitor domain (OEPD), a transient, common precursor domain for the otic and epibranchial placodes. Recombination was later observed in the OEPD-derived structures (the entire inner ear and the VIIth-Xth cranial sensory ganglia), olfactory epithelium, anterior pituitary, pharyngeal ectoderm and pouches. Other Six1-positive structures, such as salivary/lacrimal glands and limb buds, were also positive for recombination. Moreover, comparison with another mouse line expressing Cre under the control of the sensory neuron enhancer, Six1-8, indicated that the continuous and complex expression pattern of Six1 during sensory organ formation is pieced together by separate enhancers.

CONCLUSIONS

mSix1-21-NLSCre has several unique characteristics to make it suitable for analysis of cell lineage and gene function in sensory placodes as well as nonplacodal Six1-positive structures. Developmental Dynamics 247:250-261, 2018. © 2017 Wiley Periodicals, Inc.

Identification of novel cis-regulatory elements of Eya1 in Xenopus laevis using BAC recombineering

The multifunctional Eya1 protein plays important roles during the development of cranial sensory organs and ganglia, kidneys, hypaxial muscles and several other organs in vertebrates. Eya1 is encoded by a complex locus with candidate cis-regulatory elements distributed over a 329 kbp wide genomic region in Xenopus. Consequently, very little is currently known about how expression of Eya1 is controlled by upstream regulators. Here we use a library of Xenopus tropicalis genomic sequences in bacterial artificial chromosomes (BAC) to analyze the genomic region surrounding the Eya1 locus for enhancer activity. We used BAC recombineering to first create GFP reporter constructs, which were analysed for enhancer activity by injection into Xenopus laevis embryos. We then used a second round of BAC recombineering to create deletion constructs of these BAC reporters to localize enhancer activity more precisely. This double recombineering approach allowed us to probe a large genomic region for enhancer activity without assumptions on sequence conservation. Using this approach we were able to identify two novel cis-regulatory regions, which direct Eya1 expression to the somites, pharyngeal pouches, the preplacodal ectoderm (the common precursor region of many cranial sensory organs and ganglia), and other ectodermal domains.

Adenohypophysis placodal precursors exhibit distinctive features within the rostral preplacodal ectoderm

Placodes are discrete thickenings of the vertebrate cranial ectoderm that generate morpho-functionally distinct structures, such as the adenohypophysis, olfactory epithelium and lens. All placodes arise from a horseshoe-shaped preplacodal ectoderm in which the precursors of individual placodes are intermingled. However, fate-map studies indicated that cells positioned at the preplacodal midline give rise to only the adenohypophyseal placode, suggesting a unique organization of these precursors within the preplacode. To test this possibility, we combined embryological and molecular approaches in chick embryos to show that, at gastrula stage, adenohypophyseal precursors are clustered in the median preplacodal ectoderm, largely segregated from those of the adjacent olfactory placode. Median precursors are elongated, densely packed and, at neurula stage, express a molecular signature that distinguishes them from the remaining preplacodal cells. Olfactory placode precursors and midline neural cells can replace ablated adenohypophyseal precursors up to head-fold stage, although with a more plastic organization. We thus propose that adenohypophyseal placode precursors are unique within the preplacodal ectoderm possibly because they originate the only single placode and the only one with an endocrine character.

Six1 and Eya1 both promote and arrest neuronal differentiation by activating multiple Notch pathway genes

The transcription factor Six1 and its cofactor Eya1 are important regulators of neurogenesis in cranial placodes, activating genes promoting both a progenitor state, such as hes8, and neuronal differentiation, such as neurog1. Here, we use gain and loss of function studies in Xenopus laevis to elucidate how these genes function during placodal neurogenesis. We first establish that hes8 is activated by Notch signaling and represses neurog1 and neuronal differentiation, indicating that it mediates lateral inhibition. Using hes8 knockdown we demonstrate that hes8 is essential for limiting neuronal differentiation during normal placode development. We next show that Six1 and Eya1 cell autonomously activate both hes8 and neurog1 in a dose-dependent fashion, with increasing upregulation at higher doses, while neuronal differentiation is increasingly repressed. However, high doses of Six1 and Eya1 upregulate neurog1 only transiently, whereas low doses of Six1 and Eya1 ultimately promote both neurog1 expression and neuronal differentiation. Finally, we show that Six1 and Eya1 can activate hes8 and arrest neuronal differentiation even when Notch signaling is blocked. Our findings indicate that Six1 and Eya1 can both promote and arrest neuronal differentiation by activating the Notch pathway genes neurog1 and hes8, respectively, revealing a novel mechanism of Six1/Eya1 action during placodal neurogenesis.

Retinoic acid is required and Fgf, Wnt, and Bmp signaling inhibit posterior lateral line placode induction in zebrafish

The lateral line system is a mechanosensory systems present in aquatic animals. The anterior and posterior lateral lines develop from anterior and posterior lateral line placodes (aLLp and pLLp), respectively. Although signaling molecules required for the induction of other cranial placodes have been well studied, the molecular mechanisms underlying formation of the lateral line placodes are unknown. In this study we tested the requirement of multiple signaling pathways, such as Wnt, Bmp Fgf, and Retinoic Acid for aLLp and pLLp induction. We determined that aLLp specification requires Fgf signaling, whilst pLLp specification requires retinoic acid which inhibits Fgf signaling. pLLp induction is also independent of Wnt and Bmp activities, even though these pathways limit the boundaries of the pLLp. This is the first report that the aLLp and pLLp depend on different inductive mechanisms and that pLLp induction requires the inhibition of Fgf, Wnt and Bmp signaling.

Pax2/Pax8-defined subdomains and the occurrence of apoptosis in the posterior placodal area of mice

The present work aims to improve our understanding of the causes and functions of apoptosis during the morphogenesis of epibranchial placodes in mice. Schematic maps helped to compare the spatiotemporal sequence of apoptotic events with the protein expression patterns of general (Six1) and specific placodal markers (Pax2, Pax8). Our findings challenge the view that, in mammals, all three epibranchial placodes spring from the original posterior placodal area (PPA) of presomite or early somite embryos. Instead, close-meshed analysis of the Pax2/Pax8 expression patterns demonstrates the stepwise emergence of two subdomains which both belong to the gradually expanding PPA, and which largely give rise to the otic placode and epibranchial placode 1 (anterior subdomain), or to the caudal epibranchial placodes (posterior subdomain). Our observations reinforce previous doubts raised on the PPA progeny of early somite Xenopus embryos (Schlosser and Ahrens, Dev Biol 271:439-466, 2004). They also demonstrate that partly different Pax2/Pax8 codes accompany epibranchial placode development in Xenopus laevis and mice. In mice, interplacodal apoptosis assists in the establishment of the two PPA subdomains and, subsequently, of individualized placodes by predominantly eliminating Six1⁺ placodal precursor cells. Onset of interplacodal and intraplacodal large-scale apoptosis is almost always preceded and/or paralleled by Pax2/Pax8 expression minima in the very same region. Future work will demand the use of knock-out mice and whole embryo culture to experimentally test, whether the combined action of differentially expressed Pax2 and Pax8 genes exerts antiapoptotic effects in the mammalian PPA.