Transcription factor Ap-2α is necessary for development of embryonic melanophores, autonomic neurons and pharyngeal skeleton in zebrafish

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underlies facial shape variation, yet how those networks in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest, even during the late migratory phase, results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both TFAP2 family members dysregulates numerous midface GRN components involved in midface morphogenesis, patterning and differentiation. Notably, Alx1, Alx3 and Alx4 (ALX) transcript levels are reduced, whereas ChIP-seq analyses suggest TFAP2 family members directly and positively regulate ALX gene expression. Tfap2a, Tfap2b and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a zebrafish mutants present with abnormal alx3 expression patterns, Tfap2a binds ALX loci and tfap2a-alx3 genetic interactions are observed. Together, these data demonstrate TFAP2 paralogs regulate vertebrate midfacial development in part by activating expression of ALX transcription factor genes.

On the evolutionary origins and regionalization of the neural crest

The neural crest is a vertebrate-specific embryonic stem cell population that gives rise to a vast array of cell types throughout the animal body plan. These cells are first born at the edges of the central nervous system, from which they migrate extensively and differentiate into multiple cellular derivatives. Given the unique set of structures these cells comprise, the origin of the neural crest is thought to have important implications for the evolution and diversification of the vertebrate clade. In jawed vertebrates, neural crest cells exist as distinct subpopulations along the anterior-posterior axis. These subpopulations differ in terms of their respective differentiation potential and cellular derivatives. Thus, the modern neural crest is characterized as multipotent, migratory, and regionally segregated throughout the embryo. Here, we retrace the evolutionary origins of the neural crest, from the appearance of conserved regulatory circuitry in basal chordates to the emergence of neural crest subpopulations in higher vertebrates. Finally, we discuss a stepwise trajectory by which these cells may have arisen and diversified throughout vertebrate evolution.

TFAP2 paralogs facilitate chromatin access for MITF at pigmentation and cell proliferation genes

In developing melanocytes and in melanoma cells, multiple paralogs of the Activating-enhancer-binding Protein 2 family of transcription factors (TFAP2) contribute to expression of genes encoding pigmentation regulators, but their interaction with Microphthalmia transcription factor (MITF), a master regulator of these cells, is unclear. Supporting the model that TFAP2 facilitates MITF's ability to activate expression of pigmentation genes, single-cell seq analysis of zebrafish embryos revealed that pigmentation genes are only expressed in the subset of mitfa-expressing cells that also express tfap2 paralogs. To test this model in SK-MEL-28 melanoma cells we deleted the two TFAP2 paralogs with highest expression, TFAP2A and TFAP2C, creating TFAP2 knockout (TFAP2-KO) cells. We then assessed gene expression, chromatin accessibility, binding of TFAP2A and of MITF, and the chromatin marks H3K27Ac and H3K27Me3 which are characteristic of active enhancers and silenced chromatin, respectively. Integrated analyses of these datasets indicate TFAP2 paralogs directly activate enhancers near genes enriched for roles in pigmentation and proliferation, and directly repress enhancers near genes enriched for roles in cell adhesion. Consistently, compared to WT cells, TFAP2-KO cells proliferate less and adhere to one another more. TFAP2 paralogs and MITF co-operatively activate a subset of enhancers, with the former necessary for MITF binding and chromatin accessibility. By contrast, TFAP2 paralogs and MITF do not appear to co-operatively inhibit enhancers. These studies reveal a mechanism by which TFAP2 profoundly influences the set of genes activated by MITF, and thereby the phenotype of pigment cells and melanoma cells.

Tfap2a is a novel gatekeeper of nephron differentiation during kidney development

Renal functional units known as nephrons undergo patterning events during development that create a segmental array of cellular compartments with discrete physiological identities. Here, from a forward genetic screen using zebrafish, we report the discovery that transcription factor AP-2 alpha (tfap2a) coordinates a gene regulatory network that activates the terminal differentiation program of distal segments in the pronephros. We found that tfap2a acts downstream of Iroquois homeobox 3b (irx3b), a distal lineage transcription factor, to operate a circuit consisting of tfap2b, irx1a and genes encoding solute transporters that dictate the specialized metabolic functions of distal nephron segments. Interestingly, this regulatory node is distinct from other checkpoints of differentiation, such as polarity establishment and ciliogenesis. Thus, our studies reveal insights into the genetic control of differentiation, where tfap2a is essential for regulating a suite of segment transporter traits at the final tier of zebrafish pronephros ontogeny. These findings have relevance for understanding renal birth defects, as well as efforts to recapitulate nephrogenesis in vivo to facilitate drug discovery and regenerative therapies.

The gene regulatory basis of genetic compensation during neural crest induction

The neural crest (NC) is a vertebrate-specific cell type that contributes to a wide range of different tissues across all three germ layers. The gene regulatory network (GRN) responsible for the formation of neural crest is conserved across vertebrates. Central to the induction of the NC GRN are AP-2 and SoxE transcription factors. NC induction robustness is ensured through the ability of some of these transcription factors to compensate loss of function of gene family members. However the gene regulatory events underlying compensation are poorly understood. We have used gene knockout and RNA sequencing strategies to dissect NC induction and compensation in zebrafish. We genetically ablate the NC using double mutants of tfap2a;tfap2c or remove specific subsets of the NC with sox10 and mitfa knockouts and characterise genome-wide gene expression levels across multiple time points. We find that compensation through a single wild-type allele of tfap2c is capable of maintaining early NC induction and differentiation in the absence of tfap2a function, but many target genes have abnormal expression levels and therefore show sensitivity to the reduced tfap2 dosage. This separation of morphological and molecular phenotypes identifies a core set of genes required for early NC development. We also identify the 15 somites stage as the peak of the molecular phenotype which strongly diminishes at 24 hpf even as the morphological phenotype becomes more apparent. Using gene knockouts, we associate previously uncharacterised genes with pigment cell development and establish a role for maternal Hippo signalling in melanocyte differentiation. This work extends and refines the NC GRN while also uncovering the transcriptional basis of genetic compensation via paralogues.

Embryonic development is an intricate process that requires genes to be active at the right time and place. Organisms have evolved mechanisms that ensure faithful execution of developmental programmes even if genes fail to function. For example, in a process called genetic compensation, one or more genes become activated in response to loss of function of another. In this work we use the zebrafish model to investigate how two related genes, tfap2a and tfap2c, interact to ensure establishment of the neural crest, a vertebrate-specific cell type that contributes to many different tissues. Losing tfap2a activity causes mild morphological defects and losing tfap2c has no visible effect. Yet when both are inactive, embryos are severely abnormal due to lack of neural crest-derived tissues. Here we show that loss of tfap2a triggers upregulation of tfap2c which prevents the loss of neural crest tissue. However, the genes normally regulated by tfap2a respond differently to tfap2c allowing us to identify the first tier of the Ap2 network and new players in neural crest biology. Our work demonstrates that the expression signature of partial, but morphologically sufficient, genetic compensation provides an opportunity to dissect gene regulatory networks.

Latin American contributions to the neural crest field

The neural crest (NC) is one of the most fascinating structures during embryonic development. Unique to vertebrate embryos, these cells give rise to important components of the craniofacial skeleton, such as the jaws and skull, as well as melanocytes and ganglia of the peripheral nervous system. Worldwide, several groups have been studying NC development and specifically in the Latin America (LA) they have been growing in numbers since the 1990s. It is important for the world to recognize the contributions of LA researchers on the knowledge of NC development, as it can stimulate networking and improvement in the field. We developed a database of LA publications on NC development using ORCID and PUBMED as search engines. We thoroughly describe all of the contributions from LA, collected in five major topics on NC development mechanisms: i) induction and specification; ii) migration; iii) differentiation; iv) adult NC; and, v) neurocristopathies. Further analysis was done to correlate each LA country with topics and animal models, and to access collaboration between LA countries. We observed that some LA countries have made important contributions to the comprehension of NC development. Interestingly, some LA countries have a topic and an animal model as their strength; in addition, collaboration between LA countries is almost inexistent. This review will help LA NC research to be acknowledged, and to facilitate networking between students and researchers worldwide.

TFII-I and AP2α Co-Occupy the Promoters of Key Regulatory Genes Associated with Craniofacial Development

OBJECTIVES

The aim of this study is to define the candidate target genes for TFII-I and AP2α regulation in neural crest progenitor cells.

DESIGN

The GTF2I and GTF2IRD1 genes encoding the TFII-I family of transcription factors are prime candidates for the Williams-Beuren syndrome, a complex multisystem disorder characterized by craniofacial, skeletal, and neurocognitive deficiencies. AP2α, a product of the TFAP2A gene, is a master regulator of neural crest cell lineage. Mutations in TFAP2A cause branchio-oculo-facial syndrome characterized by dysmorphic facial features and orofacial clefts. In this study, we examined the genome-wide promoter occupancy of TFII-I and AP2α in neural crest progenitor cells derived from in vitro-differentiated human embryonic stem cells.

RESULTS

Our study revealed that TFII-I and AP2α co-occupy a selective set of genes that control the specification of neural crest cells.

CONCLUSIONS

The data suggest that TFII-I and AP2α may coordinately control the expression of genes encoding chromatin-modifying proteins, epigenetic enzymes, transcription factors, and signaling proteins.

AP-2α and AP-2β cooperatively orchestrate homeobox gene expression during branchial arch patterning

The evolution of a hinged moveable jaw with variable morphology is considered a major factor behind the successful expansion of the vertebrates. DLX homeobox transcription factors are crucial for establishing the positional code that patterns the mandible, maxilla and intervening hinge domain, but how the genes encoding these proteins are regulated remains unclear. Herein, we demonstrate that the concerted action of the AP-2α and AP-2β transcription factors within the mouse neural crest is essential for jaw patterning. In the absence of these two proteins, the hinge domain is lost and there are alterations in the size and patterning of the jaws correlating with dysregulation of homeobox gene expression, with reduced levels of Emx, Msx and Dlx paralogs accompanied by an expansion of Six1 expression. Moreover, detailed analysis of morphological features and gene expression changes indicate significant overlap with various compound Dlx gene mutants. Together, these findings reveal that the AP-2 genes have a major function in mammalian neural crest development, influencing patterning of the craniofacial skeleton via the DLX code, an effect that has implications for vertebrate facial evolution, as well as for human craniofacial disorders.

TFAP2 paralogs regulate melanocyte differentiation in parallel with MITF

Mutations in the gene encoding transcription factor TFAP2A result in pigmentation anomalies in model organisms and premature hair graying in humans. However, the pleiotropic functions of TFAP2A and its redundantly-acting paralogs have made the precise contribution of TFAP2-type activity to melanocyte differentiation unclear. Defining this contribution may help to explain why TFAP2A expression is reduced in advanced-stage melanoma compared to benign nevi. To identify genes with TFAP2A-dependent expression in melanocytes, we profile zebrafish tissue and mouse melanocytes deficient in Tfap2a, and find that expression of a small subset of genes underlying pigmentation phenotypes is TFAP2A-dependent, including Dct, Mc1r, Mlph, and Pmel. We then conduct TFAP2A ChIP-seq in mouse and human melanocytes and find that a much larger subset of pigmentation genes is associated with active regulatory elements bound by TFAP2A. These elements are also frequently bound by MITF, which is considered the "master regulator" of melanocyte development. For example, the promoter of TRPM1 is bound by both TFAP2A and MITF, and we show that the activity of a minimal TRPM1 promoter is lost upon deletion of the TFAP2A binding sites. However, the expression of Trpm1 is not TFAP2A-dependent, implying that additional TFAP2 paralogs function redundantly to drive melanocyte differentiation, which is consistent with previous results from zebrafish. Paralogs Tfap2a and Tfap2b are both expressed in mouse melanocytes, and we show that mouse embryos with Wnt1-Cre-mediated deletion of Tfap2a and Tfap2b in the neural crest almost completely lack melanocytes but retain neural crest-derived sensory ganglia. These results suggest that TFAP2 paralogs, like MITF, are also necessary for induction of the melanocyte lineage. Finally, we observe a genetic interaction between tfap2a and mitfa in zebrafish, but find that artificially elevating expression of tfap2a does not increase levels of melanin in mitfa hypomorphic or loss-of-function mutants. Collectively, these results show that TFAP2 paralogs, operating alongside lineage-specific transcription factors such as MITF, directly regulate effectors of terminal differentiation in melanocytes. In addition, they suggest that TFAP2A activity, like MITF activity, has the potential to modulate the phenotype of melanoma cells.

Epigenetic programming of Dnmt3a mediated by AP2α is required for granting preadipocyte the ability to differentiate

Adipogenesis has an important role in regulating energy homeostasis in mammals. 3T3-L1 preadipocytes have been widely used as an in vitro model for analyzing the molecular mechanism of adipogenesis. Previous reports indicated that the stage of contact inhibition (CI), through which the proliferating cells exit from the cell cycle, was required for granting preadipocyte the ability to differentiate. While this kind of the granting mechanism remains elusive. In the present study, we showed that DNA (cytosine-5) methyltransferase 3a (Dnmt3a) was upregulated at both the mRNA and protein level during the CI stage, and resulted in increasing promoter methylation of adipogenic genes. We further identified that the expression of Activator protein 2α (AP2α), a member of the transcription factor activator protein 2 (AP2) family, was highly correlated with the expression of Dnmt3a during the CI stage. In addition, we showed that AP2α transcriptionally upregulated Dnmt3a by directly binding to its proximal promoter region. Importantly, treatment of 3T3-L1 preadipocytes with AP2α-specific siRNAs inhibited the preadipocyte differentiation in a stage-dependent manner, supporting the conclusion that AP2α has an important role during the CI stage. Furthermore, overexpression of Dnmt3a partially rescued the impairment of adipogenesis induced by AP2α knockdown. Collectively, our findings reveal that AP2α is an essential regulator for granting preadipocyte the ability to differentiate through the upregulation of Dnmt3a expression during the CI stage.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

Aberrant CpG methylation of the TFAP2A gene constitutes a mechanism for loss of TFAP2A expression in human metastatic melanoma

Metastatic melanoma is a deadly treatment-resistant form of skin cancer whose global incidence is on the rise. During melanocyte transformation and melanoma progression the expression profile of many genes changes. Among these, a gene implicated in several steps of melanocyte development, TFAP2A, is frequently silenced; however, the molecular mechanism of TFAP2A silencing in human melanoma remains unknown. In this study, we measured TFAP2A mRNA expression in primary human melanocytes compared to 11 human melanoma samples by quantitative real-time RT-PCR. In addition, we assessed CpG DNA methylation of the TFAP2A promoter in these samples using bisulfite sequencing. Compared to primary melanocytes, which showed high TFAP2A mRNA expression and no promoter methylation, human melanoma samples showed decreased TFAP2A mRNA expression and increased promoter methylation. We further show that increased CpG methylation correlates with decreased TFAP2A mRNA expression. Using The Cancer Genome Atlas, we further identified TFAP2A as a gene displaying among the most decreased expression in stage 4 melanomas vs. non-stage 4 melanomas, and whose CpG methylation was frequently associated with lack of mRNA expression. Based on our data, we conclude that TFAP2A expression in human melanomas can be silenced by aberrant CpG methylation of the TFAP2A promoter. We have identified aberrant CpG DNA methylation as an epigenetic mark associated with TFAP2A silencing in human melanoma that could have significant implications for the therapy of human melanoma using epigenetic modifying drugs.

Tfap2a promotes specification and maturation of neurons in the inner ear through modulation of Bmp, Fgf and notch signaling

Neurons of the statoacoustic ganglion (SAG) transmit auditory and vestibular information from the inner ear to the hindbrain. SAG neuroblasts originate in the floor of the otic vesicle. New neuroblasts soon delaminate and migrate towards the hindbrain while continuing to proliferate, a phase known as transit amplification. SAG cells eventually come to rest between the ear and hindbrain before terminally differentiating. Regulation of these events is only partially understood. Fgf initiates neuroblast specification within the ear. Subsequently, Fgf secreted by mature SAG neurons exceeds a maximum threshold, serving to terminate specification and delay maturation of transit-amplifying cells. Notch signaling also limits SAG development, but how it is coordinated with Fgf is unknown. Here we show that transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis in the zebrafish inner ear. In both zebrafish and chick, Tfap2a is expressed in a ventrolateral domain of the otic vesicle that includes neurogenic precursors. Functional studies were conducted in zebrafish. Loss of Tfap2a elevated Fgf and Notch signaling, thereby inhibiting SAG specification and slowing maturation of transit-amplifying cells. Conversely, overexpression of Tfap2a inhibited Fgf and Notch signaling, leading to excess and accelerated SAG production. However, most SAG neurons produced by Tfap2a overexpression died soon after maturation. Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons. Analysis of genetic mosaics showed that Tfap2a acts non-autonomously to inhibit Fgf. This led to the discovery that Tfap2a activates expression of Bmp7a, which in turn inhibits both Fgf and Notch signaling. Blocking Bmp signaling reversed the effects of overexpressing Tfap2a. Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

Neurons of the statoacoustic ganglion (SAG) transmit impulses from the inner ear necessary for hearing and balance. SAG cells exhibit a complex pattern of development, regulation of which remains poorly understood. Here we show that transcription factor Tfap2a coordinates multiple cell signaling pathways needed to regulate the quantity and pace of SAG neuron production. SAG progenitors originate within the developing inner ear and then migrate out of the ear towards the hindbrain before forming mature neurons. We showed previously that Fgf initiates formation of SAG progenitors in the inner ear, but rising levels of Fgf signaling eventually terminate this process. Elevated Fgf also stimulates proliferation of SAG progenitors outside the ear and delays their maturation. Notch signaling is also known to limit SAG development. Tfap2a governs the strength of Fgf and Notch signaling by activating expression of Bmp7a, which inhibits Fgf and Notch. Together these signals stabilize the pool of SAG progenitors outside the ear by equalizing rates of maturation and proliferation. This balance is critical for sustained accumulation of SAG neurons during larval growth as well as regeneration following neural damage. These findings could inform development of stem cell therapies to correct auditory neuropathies in humans.

Genome-wide Chromatin Mapping Defines AP2α in the Etiology of Craniofacial Disorders

Objective

The aim of this study is to identify direct AP2α target genes implicated in craniofacial morphogenesis.

Design

AP2α, a product of the TFAP2A gene, is a master regulator of neural crest differentiation and development. AP2α is expressed in ectoderm and in migrating cranial neural crest (NC) cells that provide patterning information during orofacial development and generate most of the skull bones and the cranial ganglia. Mutations in TFAP2A cause branchio-oculofacial syndrome characterized by dysmorphic facial features including cleft or pseudocleft lip/palate. We hypothesize that AP2α primes a distinctive group of genes associated with NC development. Human promoter ChIP-chip arrays were used to define chromatin regions bound by AP2α in neural crest progenitors differentiated from human embryonic stem cells.

Results

High-confidence AP2α-binding peaks were detected in the regulatory regions of many target genes involved in the development of facial tissues including MSX1, IRF6, TBX22, and MAFB. In addition, we uncovered multiple single-nucleotide polymorphisms (SNPs) disrupting a conserved AP2α consensus sequence.

Conclusions

Knowledge of noncoding SNPs in the genomic loci occupied by AP2α provides an insight into the regulatory mechanisms underlying craniofacial development.

The development of zebrafish tendon and ligament progenitors

Despite the importance of tendons and ligaments for transmitting movement and providing stability to the musculoskeletal system, their development is considerably less well understood than that of the tissues they serve to connect. Zebrafish have been widely used to address questions in muscle and skeletal development, yet few studies describe their tendon and ligament tissues. We have analyzed in zebrafish the expression of several genes known to be enriched in mammalian tendons and ligaments, including scleraxis (scx), collagen 1a2 (col1a2) and tenomodulin (tnmd), or in the tendon-like myosepta of the zebrafish (xirp2a). Co-expression studies with muscle and cartilage markers demonstrate the presence of scxa, col1a2 and tnmd at sites between the developing muscle and cartilage, and xirp2a at the myotendinous junctions. We determined that the zebrafish craniofacial tendon and ligament progenitors are neural crest derived, as in mammals. Cranial and fin tendon progenitors can be induced in the absence of differentiated muscle or cartilage, although neighboring muscle and cartilage are required for tendon cell maintenance and organization, respectively. By contrast, myoseptal scxa expression requires muscle for its initiation. Together, these data suggest a conserved role for muscle in tendon development. Based on the similarities in gene expression, morphology, collagen ultrastructural arrangement and developmental regulation with that of mammalian tendons, we conclude that the zebrafish tendon populations are homologous to their force-transmitting counterparts in higher vertebrates. Within this context, the zebrafish model can be used to provide new avenues for studying tendon biology in a vertebrate genetic system.

Suv39h1 mediates AP-2α-dependent inhibition of C/EBPα expression during adipogenesis

Previous studies have shown that CCAAT/enhancer-binding protein α (C/EBPα) plays a very important role during adipocyte terminal differentiation and that AP-2α (activator protein 2α) acts as a repressor to delay the expression of C/EBPα. However, the mechanisms by which AP-2α prevents the expression of C/EBPα are not fully understood. Here, we present evidence that Suv39h1, a histone H3 lysine 9 (H3K9)-specific trimethyltransferase, and G9a, a euchromatic methyltransferase, both interact with AP-2α and enhance AP-2α-mediated transcriptional repression of C/EBPα. Interestingly, we discovered that G9a mediates dimethylation of H3K9, thus providing the substrate, which is methylated by Suv39h1, to H3K9me3 on the C/EBPα promoter. The expression level of AP-2α was consistent with enrichment of H3K9me2 and H3K9me3 on the C/EBPα promoter in 3T3-L1 preadipocytes. Knockdown of Suv39h markedly increased C/EBPα expression and promoted adipogenesis. Conversely, ectopic expression of Suv39h1 delayed C/EBPα expression and impaired the accumulation of triglyceride, while simultaneous knockdown of AP-2α or G9a partially rescued this process. These findings indicate that Suv39h1 enhances AP-2α-mediated transcriptional repression of C/EBPα in an epigenetic manner and further inhibits adipocyte differentiation.

AP2α transcriptional activity is essential for retinoid-induced neuronal differentiation of mesenchymal stem cells

Pre-activation of the retinoid signaling pathway by all-trans retinoic acid facilitates neuronal differentiation of mesenchymal stem cells. Using protein/DNA based screening assays, we identified activator protein 2α as an important downstream target of all-trans retinoic acid. Although all-trans retinoic acid treatment significantly increased activator protein 2α transcriptional activity, it did not affect its expression. Inhibition of activator protein 2α with dominant-negative mutants reduced ATRA-induced differentiation of mesenchymal stem cells into neurons and reversed its associated functional recovery of memory impairment in the cell-based treatment of a hypoxic-ischemic brain damage rat model. Dominant-negative mutants of activator protein 2α inhibited the expression of neuronal markers which were induced by retinoic acid receptor β activation. All-trans retinoic acid treatment increased phosphorylation of activator protein 2α and resulted in its nuclear translocation. This was blocked by siRNA-mediated knockdown of retinoic acid receptor β. Furthermore, we found that retinoic acid receptor β directly interacted with activator protein 2α. In summary, the regulation of all-trans retinoic acid on activator protein 2α transcriptional activity was mediated by activation of retinoic acid receptor β and subsequent phosphorylation and nuclear translocation of activator protein 2α. Our results strongly suggest that activator protein 2α transcriptional activity is essential for all-trans retinoic acid-induced neuronal differentiation of mesenchymal stem cells.

Gene regulatory evolution and the origin of macroevolutionary novelties: insights from the neural crest

The appearance of novel anatomic structures during evolution is driven by changes to the networks of transcription factors, signaling pathways, and downstream effector genes controlling development. The nature of the changes to these developmental gene regulatory networks (GRNs) is poorly understood. A striking test case is the evolution of the GRN controlling development of the neural crest (NC). NC cells emerge from the neural plate border (NPB) and contribute to multiple adult structures. While all chordates have a NPB, only in vertebrates do NPB cells express all the genes constituting the neural crest GRN (NC-GRN). Interestingly, invertebrate chordates express orthologs of NC-GRN components in other tissues, revealing that during vertebrate evolution new regulatory connections emerged between transcription factors primitively expressed in the NPB and genes primitively expressed in other tissues. Such interactions could have evolved by two mechanisms. First, transcription factors primitively expressed in the NPB may have evolved new DNA and/or cofactor binding properties (protein neofunctionalization). Alternately, cis-regulatory elements driving NPB expression may have evolved near genes primitively expressed in other tissues (cis-regulatory neofunctionalization). Here we discuss how gene duplication can, in principle, promote either form of neofunctionalization. We review recent published examples of interspecies gene-swap, or regulatory-element-swap, experiments that test both models. Such experiments have yielded little evidence to support the importance of protein neofunctionalization in the emergence of the NC-GRN, but do support the importance of novel cis-regulatory elements in this process. The NC-GRN is an excellent model for the study of gene regulatory and macroevolutionary innovation.

Distinct functional and temporal requirements for zebrafish Hdac1 during neural crest-derived craniofacial and peripheral neuron development

The regulation of gene expression is accomplished by both genetic and epigenetic means and is required for the precise control of the development of the neural crest. In hdac1(b382) mutants, craniofacial cartilage development is defective in two distinct ways. First, fewer hoxb3a, dlx2 and dlx3-expressing posterior branchial arch precursors are specified and many of those that are consequently undergo apoptosis. Second, in contrast, normal numbers of progenitors are present in the anterior mandibular and hyoid arches, but chondrocyte precursors fail to terminally differentiate. In the peripheral nervous system, there is a disruption of enteric, DRG and sympathetic neuron differentiation in hdac1(b382) mutants compared to wildtype embryos. Specifically, enteric and DRG-precursors differentiate into neurons in the anterior gut and trunk respectively, while enteric and DRG neurons are rarely present in the posterior gut and tail. Sympathetic neuron precursors are specified in hdac1(b382) mutants and they undergo generic neuronal differentiation but fail to undergo noradrenergic differentiation. Using the HDAC inhibitor TSA, we isolated enzyme activity and temporal requirements for HDAC function that reproduce hdac1(b382) defects in craniofacial and sympathetic neuron development. Our study reveals distinct functional and temporal requirements for zebrafish hdac1 during neural crest-derived craniofacial and peripheral neuron development.

Protein partners of KCTD proteins provide insights about their functional roles in cell differentiation and vertebrate development

The KCTD family includes tetramerization (T1) domain containing proteins with diverse biological effects. We identified a novel member of the KCTD family, BTBD10. A comprehensive analysis of protein-protein interactions (PPIs) allowed us to put forth a number of testable hypotheses concerning the biological functions for individual KCTD proteins. In particular, we predict that KCTD20 participates in the AKT-mTOR-p70 S6k signaling cascade, KCTD5 plays a role in cytokinesis in a NEK6 and ch-TOG-dependent manner, KCTD10 regulates the RhoA/RhoB pathway. Developmental regulator KCTD15 represses AP-2α and contributes to energy homeostasis by suppressing early adipogenesis. TNFAIP1-like KCTD proteins may participate in post-replication DNA repair through PCNA ubiquitination. KCTD12 may suppress the proliferation of gastrointestinal cells through interference with GABAb signaling. KCTD9 deserves experimental attention as the only eukaryotic protein with a DNA-like pentapeptide repeat domain. The value of manual curation of PPIs and analysis of existing high-throughput data should not be underestimated.

Copy number variation analysis implicates the cell polarity gene glypican 5 as a human spina bifida candidate gene

Neural tube defects (NTDs) are common birth defects of complex etiology. Family and population-based studies have confirmed a genetic component to NTDs. However, despite more than three decades of research, the genes involved in human NTDs remain largely unknown. We tested the hypothesis that rare copy number variants (CNVs), especially de novo germline CNVs, are a significant risk factor for NTDs. We used array-based comparative genomic hybridization (aCGH) to identify rare CNVs in 128 Caucasian and 61 Hispanic patients with non-syndromic lumbar-sacral myelomeningocele. We also performed aCGH analysis on the parents of affected individuals with rare CNVs where parental DNA was available (42 sets). Among the eight de novo CNVs that we identified, three generated copy number changes of entire genes. One large heterozygous deletion removed 27 genes, including PAX3, a known spina bifida-associated gene. A second CNV altered genes (PGPD8, ZC3H6) for which little is known regarding function or expression. A third heterozygous deletion removed GPC5 and part of GPC6, genes encoding glypicans. Glypicans are proteoglycans that modulate the activity of morphogens such as Sonic Hedgehog (SHH) and bone morphogenetic proteins (BMPs), both of which have been implicated in NTDs. Additionally, glypicans function in the planar cell polarity (PCP) pathway, and several PCP genes have been associated with NTDs. Here, we show that GPC5 orthologs are expressed in the neural tube, and that inhibiting their expression in frog and fish embryos results in NTDs. These results implicate GPC5 as a gene required for normal neural tube development.

A gene network that coordinates preplacodal competence and neural crest specification in zebrafish

Preplacodal ectoderm (PPE) and neural crest (NC) are specified at the interface of neural and nonneural ectoderm and together contribute to the peripheral nervous system in all vertebrates. Bmp activates early steps for both fates during late blastula stage. Low Bmp activates expression of transcription factors Tfap2a and Tfap2c in the lateral neural plate, thereby specifying neural crest fate. Elevated Bmp establishes preplacodal competence throughout the ventral ectoderm by coinducing Tfap2a, Tfap2c, Foxi1 and Gata3. PPE specification occurs later at the end of gastrulation and requires complete attenuation of Bmp, yet expression of PPE competence factors continues well past gastrulation. Here we show that competence factors positively regulate each other's expression during gastrulation, forming a self-sustaining network that operates independently of Bmp. Misexpression of Tfap2a in embryos blocked for Bmp from late blastula stage can restore development of both PPE and NC. However, Tfap2a alone is not sufficient to activate any other competence factors nor does it rescue individual placodes. On the other hand, misexpression of any two competence factors in Bmp-blocked embryos can activate the entire transcription factor network and support the development of NC, PPE and some individual placodes. We also show that while these factors are partially redundant with respect to PPE specification, they later provide non-redundant functions needed for development of specific placodes. Thus, we have identified a gene regulatory network that coordinates development of NC, PPE and individual placodes in zebrafish.

Activated ALK collaborates with MYCN in neuroblastoma pathogenesis

Amplification of the MYCN oncogene in childhood neuroblastoma is often accompanied by mutational activation of ALK (anaplastic lymphoma kinase), suggesting their pathogenic cooperation. We generated a transgenic zebrafish model of neuroblastoma in which MYCN-induced tumors arise from a subpopulation of neuroblasts that migrate into the adrenal medulla analog following organogenesis. Coexpression of activated ALK with MYCN in this model triples the disease penetrance and markedly accelerates tumor onset. MYCN overexpression induces adrenal sympathetic neuroblast hyperplasia, blocks chromaffin cell differentiation, and ultimately triggers a developmentally-timed apoptotic response in the hyperplastic sympathoadrenal cells. Coexpression of activated ALK with MYCN provides prosurvival signals that block this apoptotic response and allow continued expansion and oncogenic transformation of hyperplastic neuroblasts, thus promoting progression to neuroblastoma.

Zebrafish arl6ip1 is required for neural crest development during embryogenesis

Background

Although the embryonic expression pattern of ADP ribosylation factor-like 6 interacting protein 1 (Arl6ip1) has been reported, its function in neural crest development is unclear.

Methods/Principal Findings

We found that knockdown of Arl6ip1 caused defective embryonic neural crest derivatives that were particularly severe in craniofacial cartilages. Expressions of the ectodermal patterning factors msxb, dlx3b, and pax3 were normal, but the expressions of the neural crest specifier genes foxd3, snai1b, and sox10 were greatly reduced. These findings suggest that arl6ip1 is essential for specification of neural crest derivatives, but not neural crest induction. Furthermore, we revealed that the streams of crestin- and sox10-expressing neural crest cells, which migrate ventrally from neural tube into trunk, were disrupted in arl6ip1 morphants. This migration defect was not only in the trunk neural crest, but also in the enteric tract where the vagal-derived neural crest cells failed to populate the enteric nervous system. We found that this migration defect was induced by dampened Shh signaling, which may have resulted from defective cilia. These data further suggested that arl6ip1 is required for neural crest migration. Finally, by double-staining of TUNEL and crestin, we confirmed that the loss of neural crest cells could not be attributed to apoptosis.

Conclusions/Significance

Therefore, we concluded that arl6ip1 is required for neural crest migration and sublineage specification.

Novel Tfap2-mediated control of soxE expression facilitated the evolutionary emergence of the neural crest

Gene duplication has been proposed to drive the evolution of novel morphologies. After gene duplication, it is unclear whether changes in the resulting paralogs' coding-regions, or in their cis-regulatory elements, contribute most significantly to the assembly of novel gene regulatory networks. The Transcription Factor Activator Protein 2 (Tfap2) was duplicated in the chordate lineage and is essential for development of the neural crest, a tissue that emerged with vertebrates. Using a tfap2-depleted zebrafish background, we test the ability of available gnathostome, agnathan, cephalochordate and insect tfap2 paralogs to drive neural crest development. With the exception of tfap2d (lamprey and zebrafish), all are able to do so. Together with expression analyses, these results indicate that sub-functionalization has occurred among Tfap2 paralogs, but that neo-functionalization of the Tfap2 protein did not drive the emergence of the neural crest. We investigate whether acquisition of novel target genes for Tfap2 might have done so. We show that in neural crest cells Tfap2 directly activates expression of sox10, which encodes a transcription factor essential for neural crest development. The appearance of this regulatory interaction is likely to have coincided with that of the neural crest, because AP2 and SoxE are not co-expressed in amphioxus, and because neural crest enhancers are not detected proximal to amphioxus soxE. We find that sox10 has limited ability to restore the neural crest in Tfap2-deficient embryos. Together, these results show that mutations resulting in novel Tfap2-mediated regulation of sox10 and other targets contributed to the evolution of the neural crest.

Tfap2a and Foxd3 regulate early steps in the development of the neural crest progenitor population

The neural crest is a stem cell-like population exclusive to vertebrates that gives rise to many different cell types including chondrocytes, neurons and melanocytes. Arising from the neural plate border at the intersection of Wnt and Bmp signaling pathways, the complexity of neural crest gene regulatory networks has made the earliest steps of induction difficult to elucidate. Here, we report that tfap2a and foxd3 participate in neural crest induction and are necessary and sufficient for this process to proceed. Double mutant tfap2a (mont blanc, mob) and foxd3 (mother superior, mos) mob;mos zebrafish embryos completely lack all neural crest-derived tissues. Moreover, tfap2a and foxd3 are expressed during gastrulation prior to neural crest induction in distinct, complementary, domains; tfap2a is expressed in the ventral non-neural ectoderm and foxd3 in the dorsal mesendoderm and ectoderm. We further show that Bmp signaling is expanded in mob;mos embryos while expression of dkk1, a Wnt signaling inhibitor, is increased and canonical Wnt targets are suppressed. These changes in Bmp and Wnt signaling result in specific perturbations of neural crest induction rather than general defects in neural plate border or dorso-ventral patterning. foxd3 overexpression, on the other hand, enhances the ability of tfap2a to ectopically induce neural crest around the neural plate, overriding the normal neural plate border limit of the early neural crest territory. Although loss of either Tfap2a or Foxd3 alters Bmp and Wnt signaling patterns, only their combined inactivation sufficiently alters these signaling gradients to abort neural crest induction. Collectively, our results indicate that tfap2a and foxd3, in addition to their respective roles in the differentiation of neural crest derivatives, also jointly maintain the balance of Bmp and Wnt signaling in order to delineate the neural crest induction domain.

Activator protein-2α functions as a master regulator of multiple transcription factors in the mouse liver

AIM

  Activator protein 2α (AP-2α) belongs to the AP-2 family of transcription factors that are involved in the regulation of cell proliferation, differentiation, apoptosis and carcinogenesis and has been suggested to function as a tumor suppressor in many cancers. However, the physiological role of AP-2α in hepatocytes is unknown. The present study is to characterize the expression and function of AP-2α in the liver of conscience mouse.

METHODS

  Exogenous AP-2α was overexpressed in the mouse liver by in vivo gene delivery and changes in transcription factor expression were identified by using protein-DNA arrays and immunoblotting.

RESULTS

  Western blotting and protein/DNA arrays showed that AP-2α is expressed in the nuclei of mouse hepatocytes. Overexpression of AP-2αin vivo significantly suppressed transcription factors AP-1, CREB and c-Myc, and markedly increased CBF, c-Myb, NF-1, Pax-5, RXR, Smad3/4, TR(DR-4), USF-1 and GATA. Among all GATA proteins, only GATA-4 level was dramatically elevated and there was a concomitant loss of phospho-GATA-4. Corresponding changes were detected in upstream kinases Akt, GSK-3β and PKA, which regulates the phosphorylation status and stability of GATA-4 protein.

CONCLUSIONS

  AP-2α is expressed in mouse hepatocytes and it acts as a master regulator of numerous transcription factors in the liver.

FOXD1 Duplication Causes Branchial Defects and Interacts with the TFAP2A Gene Implicated in the Branchio-Oculo-Facial Syndrome in Causing Eye Effects in Zebrafish

Branchio-oculo-facial syndrome (BOFS) is a rare disorder characterized by maldevelopment of the first and second branchial arches, skin defects, facial dysmorphism, auricular, ophthalmological and oral abnormalities. A high clinical variability has been reported. Recently, mutations in TFAP2A were found to underlie this condition. A small duplication on 5q13 was detected in 2 family members with mild BOFS features. Molecular cytogenetic delineation of the duplication demonstrated that only 7 genes are affected: LOC100289045, RGNEF, UTP15, ANKRA2, FUNDC2P1, BTF3 and FOXD1. The latter is expressed in the developing branchial arches and involved in cranio-facial development. Zebrafish embryos with combined inhibition of the expression of foxd1l and tfap2a show optic axis defects. We identified a novel locus associated with a mild BOFS-like phenotype. The functional in vivo experiments suggest an interaction between FOXD1 and TFAP2A.

Reduced TFAP2A function causes variable optic fissure closure and retinal defects and sensitizes eye development to mutations in other morphogenetic regulators

Mutations in the transcription factor encoding TFAP2A gene underlie branchio-oculo-facial syndrome (BOFS), a rare dominant disorder characterized by distinctive craniofacial, ocular, ectodermal and renal anomalies. To elucidate the range of ocular phenotypes caused by mutations in TFAP2A, we took three approaches. First, we screened a cohort of 37 highly selected individuals with severe ocular anomalies plus variable defects associated with BOFS for mutations or deletions in TFAP2A. We identified one individual with a de novo TFAP2A four amino acid deletion, a second individual with two non-synonymous variations in an alternative splice isoform TFAP2A2, and a sibling-pair with a paternally inherited whole gene deletion with variable phenotypic expression. Second, we determined that TFAP2A is expressed in the lens, neural retina, nasal process, and epithelial lining of the oral cavity and palatal shelves of human and mouse embryos--sites consistent with the phenotype observed in patients with BOFS. Third, we used zebrafish to examine how partial abrogation of the fish ortholog of TFAP2A affects the penetrance and expressivity of ocular phenotypes due to mutations in genes encoding bmp4 or tcf7l1a. In both cases, we observed synthetic, enhanced ocular phenotypes including coloboma and anophthalmia when tfap2a is knocked down in embryos with bmp4 or tcf7l1a mutations. These results reveal that mutations in TFAP2A are associated with a wide range of eye phenotypes and that hypomorphic tfap2a mutations can increase the risk of developmental defects arising from mutations at other loci.

prdm1a Regulates sox10 and islet1 in the development of neural crest and Rohon-Beard sensory neurons

The PR domain containing 1a, with ZNF domain factor, gene (prdm1a) plays an integral role in the development of a number of different cell types during vertebrate embryogenesis, including neural crest cells, Rohon-Beard (RB) sensory neurons and the cranial neural crest-derived craniofacial skeletal elements. To better understand how Prdm1a regulates the development of various cell types in zebrafish, we performed a microarray analysis comparing wild type and prdm1a mutant embryos and identified a number of genes with altered expression in the absence of prdm1a. Rescue analysis determined that two of these, sox10 and islet1, lie downstream of Prdm1a in the development of neural crest cells and RB neurons, respectively. In addition, we identified a number of other novel downstream targets of Prdm1a that may be important for the development of diverse tissues during zebrafish embryogenesis.

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been is critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular, and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play, and equally importantly, where disparities in experimental results suggest areas of profitable study.

Differentiation of zebrafish melanophores depends on transcription factors AP2 alpha and AP2 epsilon

A model of the gene-regulatory-network (GRN), governing growth, survival, and differentiation of melanocytes, has emerged from studies of mouse coat color mutants and melanoma cell lines. In this model, Transcription Factor Activator Protein 2 alpha (TFAP2A) contributes to melanocyte development by activating expression of the gene encoding the receptor tyrosine kinase Kit. Next, ligand-bound Kit stimulates a pathway activating transcription factor Microphthalmia (Mitf), which promotes differentiation and survival of melanocytes by activating expression of Tyrosinase family members, Bcl2, and other genes. The model predicts that in both Tfap2a and Kit null mutants there will be a phenotype of reduced melanocytes and that, because Tfap2a acts upstream of Kit, this phenotype will be more severe, or at least as severe as, in Tfap2a null mutants in comparison to Kit null mutants. Unexpectedly, this is not the case in zebrafish or mouse. Because many Tfap2 family members have identical DNA–binding specificity, we reasoned that another Tfap2 family member may work redundantly with Tfap2a in promoting Kit expression. We report that tfap2e is expressed in melanoblasts and melanophores in zebrafish embryos and that its orthologue, TFAP2E, is expressed in human melanocytes. We provide evidence that Tfap2e functions redundantly with Tfap2a to maintain kita expression in zebrafish embryonic melanophores. Further, we show that, in contrast to in kita mutants where embryonic melanophores appear to differentiate normally, in tfap2a/e doubly-deficient embryonic melanophores are small and under-melanized, although they retain expression of mitfa. Interestingly, forcing expression of mitfa in tfap2a/e doubly-deficient embryos partially restores melanophore differentiation. These findings reveal that Tfap2 activity, mediated redundantly by Tfap2a and Tfap2e, promotes melanophore differentiation in parallel with Mitf by an effector other than Kit. This work illustrates how analysis of single-gene mutants may fail to identify steps in a GRN that are affected by the redundant activity of related proteins.

Neural crest-derived pigment cells, known as melanocytes, are important to an organism's survival because they protect skin cells from ultraviolet radiation, camouflage the organism from predators, and contribute to sexual selection. Networks of regulatory proteins control the steps of melanocyte development, including lineage specification, migration, survival, and differentiation. Gaps in our understanding of these networks hamper progress in effective prevention and treatment of diseases of melanocytes, including metastatic melanoma and vitiligo. Studies conducted in tissue-culture cells and mouse embryos implicate regulatory proteins including the transcription factor TFAP2A, the growth factor receptor KIT, and the transcription factor MITF as being important for multiple steps in melanocyte development. Abnormalities in TFAP2A, KIT, and MITF expression in melanoma highlight the importance of this pathway in human disease. Here we show that a gene closely related to TFAP2A, tfap2e, is expressed in zebrafish embryonic melanocytes and human melanocytes. We provide evidence that Tfap2e cooperates with Tfap2a to promote expression of zebrafish kita in embryonic melanocytes. Further we show that an effector of Tfap2a/e activity other than Kita is required for melanocyte differentiation and that this effector acts upstream or in parallel with Mitfa activity. These findings reveal unexpected complexity to the gene-regulatory network governing melanocyte differentiation.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Identification of a novel nonsense mutation on the Pax3 gene in ENU-derived white belly spotting mice and its genetic interaction with c-Kit

In the course of a large-scale screening program of N-ethyl-N-nitrosourea mutagenesis, we isolated two semidominant mutation lines with white belly spotting, named as wps and wbs. Direct sequencing detected a nucleotide G-to-A transversion in exon 2 of the c-Kit gene in wps, which resulted in a missense D60N mutation. Another mutant, wbs, was mapped to chromosome 1 by genome-wide linkage analysis. In 93 meioses, the wbs locus was confined to a 5.2-Mb region between D1Mit380 and D1Mit215, including the Pax3 gene. A nonsense mutation K107X on the Pax3 coding region in wbs mice was identified, causing the loss of Pax3 protein in the homozygous mutant. We further demonstrated that Pax3 exhibited genetic interaction with c-Kit by intercrossing the wps and wbs mice. Further, Pax3 transactivated the c-Kit promoter in different cell lines. However, electrophoretic mobility shift assays showed that Pax3 did not bind to the c-Kit promoter, indicating that Pax3 may interact with c-Kit in an indirect way. This expands our understanding of the intricate regulatory network governing the melanocyte development.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

Roles of AP-2 transcription factors in the regulation of cartilage and skeletal development

During embryogenesis, most of the mammalian skeletal system is preformed as cartilaginous structures that ossify later. The different stages of cartilage and skeletal development are well described, and several molecular factors are known to influence the events of this enchondral ossification, especially transcription factors. Members of the AP-2 family of transcription factors play important roles in several cellular processes, such as apoptosis, migration and differentiation. Studies with knockout mice demonstrate that a main function of AP-2s is the suppression of terminal differentiation during embryonic development. Additionally, the specific role of these molecules as regulators during chondrogenesis has been characterized. This review gives an overview of AP-2s, and discusses the recent findings on the AP-2 family, in particular AP-2alpha, AP-2beta, and AP-2epsilon, as regulators of cartilage and skeletal development.

Neoplasms with schwannian differentiation express transcription factors known to regulate normal schwann cell development

A number of transcription factors have been identified as important in guiding normal Schwann cell development. This study used immunohistochemistry on tissue arrays to assess the expression of some of these transcription factors (Sox5, Sox9, Sox10, AP-2α, Pax7, and FoxD3) on 76 schwannomas, 105 neurofibromas, and 34 malignant peripheral nerve sheath tumors (MPNSTs). Sox9 and Sox10 were found to be widely expressed in all tumor types. FoxD3 reactivity was stronger and more frequently found in schwannomas and MPNSTs than neurofibromas. AP-2α was positive in 31% to 49% of all tumors, but strong reactivity was limited to MPNSTs and schwannomas. Pax7 and Sox5 expression was restricted to subsets of MPNSTs. Statistical analysis showed significant differences between the 3 tumor types in the expression of these markers. No differences were found in the analyzed tumor subgroups, including schwannomas of different sites, schwannomas with or without NF2 association, neurofibromas of different types, or sporadic versus NF1-associated MPNSTs. These results suggest that the transcription factors that guide normal Schwann cell development also play a role in the biology of neoplastic cells with Schwannian differentiation. FoxD3, AP-2α, Pax7, and Sox5 are upregulated in MPNSTs compared with neurofibromas and may be markers of malignant transformation. Screening the expression of FoxD3, Sox9, and Sox10 on 23 cases of other spindle-cell proliferations that may be considered in the differential diagnosis of MPNST, including synovial sarcoma and spindle cell melanoma, suggests that these 3 are helpful markers of Schwannian differentiation in the context of diagnosing MPNSTs.

Genetic ablation of neural crest cell diversification

The neural crest generates multiple cell types during embryogenesis but the mechanisms regulating neural crest cell diversification are incompletely understood. Previous studies using mutant zebrafish indicated that foxd3 and tfap2a function early and differentially in the development of neural crest sublineages. Here, we show that the simultaneous loss of foxd3 and tfap2a function in zebrafish foxd3(zdf10);tfap2a(low) double mutant embryos globally prevents the specification of developmentally distinct neural crest sublineages. By contrast, neural crest induction occurs independently of foxd3 and tfap2a function. We show that the failure of neural crest cell diversification in double mutants is accompanied by the absence of neural crest sox10 and sox9a/b gene expression, and that forced expression of sox10 and sox9a/b differentially rescues neural crest sublineage specification and derivative differentiation. These results demonstrate the functional necessity for foxd3 and tfap2a for neural crest sublineage specification and that this requirement is mediated by the synergistic regulation of the expression of SoxE family genes. Our results identify a genetic regulatory pathway functionally discrete from the process of neural crest induction that is required for the initiation of neural crest cell diversification during embryonic development.

Evolution of the neural crest viewed from a gene regulatory perspective

Neural crest cells are a vertebrate innovation and form a wide variety of embryonic cell types as diverse as peripheral neurons and facial skeleton. They undergo complex migration and differentiation processes from their site of origin in the developing central nervous system to their final destinations in the periphery. In this review, we summarize recent data on the current formulation of a gene regulatory network underlying neural crest formation and its roots at the base of the vertebrate lineage. Analyzing neural crest formation from a gene regulatory viewpoint provides insights into both the developmental mechanisms and evolutionary origins of this vertebrate-specific cell type.

Dissecting early regulatory relationships in the lamprey neural crest gene network

The neural crest, a multipotent embryonic cell type, originates at the border between neural and nonneural ectoderm. After neural tube closure, these cells undergo an epithelial-mesenchymal transition, migrate to precise, often distant locations, and differentiate into diverse derivatives. Analyses of expression and function of signaling and transcription factors in higher vertebrates has led to the proposal that a neural crest gene regulatory network (NC-GRN) orchestrates neural crest formation. Here, we interrogate the NC-GRN in the lamprey, taking advantage of its slow development and basal phylogenetic position to resolve early inductive events, 1 regulatory step at the time. To establish regulatory relationships at the neural plate border, we assess relative expression of 6 neural crest network genes and effects of individually perturbing each on the remaining 5. The results refine an upstream portion of the NC-GRN and reveal unexpected order and linkages therein; e.g., lamprey AP-2 appears to function early as a neural plate border rather than a neural crest specifier and in a pathway linked to MsxA but independent of ZicA. These findings provide an ancestral framework for performing comparative tests in higher vertebrates in which network linkages may be more difficult to resolve because of their rapid development.

Maternal Interferon Regulatory Factor 6 is required for the differentiation of primary superficial epithelia in Danio and Xenopus embryos

Early in the development of animal embryos, superficial cells of the blastula form a distinct lineage and adopt an epithelial morphology. In different animals, the fate of these primary superficial epithelial (PSE) cells varies, and it is unclear whether pathways governing segregation of blastomeres into the PSE lineage are conserved. Mutations in the gene encoding Interferon Regulatory Factor 6 (IRF6) are associated with syndromic and non-syndromic forms of cleft lip and palate, consistent with a role for Irf6 in development of oral epithelia, and mouse Irf6 targeted null mutant embryos display abnormal differentiation of oral epithelia and skin. In Danio rerio (zebrafish) and Xenopus laevis (African clawed frog) embryos, zygotic irf6 transcripts are present in many epithelial tissues including the presumptive PSE cells and maternal irf6 transcripts are present throughout all cells at the blastula stage. Injection of antisense oligonucleotides with ability to disrupt translation of irf6 transcripts caused little or no effect on development. By contrast, injection of RNA encoding a putative dominant negative Irf6 caused epiboly arrest, loss of gene expression characteristic of the EVL, and rupture of the embryo at late gastrula stage. The dominant negative Irf6 disrupted EVL gene expression in a cell autonomous fashion. These results suggest that Irf6 translated in the oocyte or unfertilized egg suffices for early development. Supporting the importance of maternal Irf6, we show that depletion of maternal irf6 transcripts in X. laevis embryos leads to gastrulation defects and rupture of the superficial epithelium. These experiments reveal a conserved role for maternally-encoded Irf6 in differentiation of a simple epithelium in X. laevis and D. rerio. This epithelium constitutes a novel model tissue in which to explore the Irf6 regulatory pathway.

Endoderm-derived Sonic hedgehog and mesoderm Hand2 expression are required for enteric nervous system development in zebrafish

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest cells (NCC). The developmental controls that govern the migration, proliferation and patterning of the ENS precursors are not well understood. We have investigated the roles of endoderm and Sonic hedgehog (SHH) in the development of the ENS. We show that endoderm is required for the migration of ENS NCC from the vagal region to the anterior end of the intestine. We show that the expression of shh and its receptor ptc-1 correlate with the development of the ENS and demonstrate that hedgehog (HH) signaling is required in two phases, a pre-enteric and an enteric phase, for normal ENS development. We show that HH signaling regulates the proliferation of vagal NCC and ENS precursors in vivo. We also show the zebrafish hand2 is required for the normal development of the intestinal smooth muscle and the ENS. Furthermore we show that endoderm and HH signaling, but not hand2, regulate gdnf expression in the intestine, highlighting a central role of endoderm and SHH in patterning the intestine and the ENS.

Insights from a sea lamprey into the evolution of neural crest gene regulatory network

The neural crest is a vertebrate innovation that forms at the embryonic neural plate border, transforms from epithelial to mesenchymal, migrates extensively throughout the embryo along well-defined pathways, and differentiates into a plethora of derivatives that include elements of peripheral nervous system, craniofacial skeleton, melanocytes, etc. The complex process of neural crest formation is guided by multiple regulatory modules that define neural crest gene regulatory network (NC GRN), which allows the neural crest to progressively acquire all of its defining characteristics. The molecular study of neural crest formation in lamprey, a basal extant vertebrate, consisting in identification and functional tests of molecular elements at each regulatory level of this network, has helped address the question of the timing of emergence of NC GRN and define its basal state. The results have revealed striking conservation in deployment of upstream factors and regulatory modules, suggesting that proximal portions of the network arose early in vertebrate evolution and have been tightly conserved for more than 500 million years. In contrast, certain differences were observed in deployment of some neural crest specifier and downstream effector genes expected to confer species-specific migratory and differentiation properties.

The mych gene is required for neural crest survival during zebrafish development

Background

Among Myc family genes, c-Myc is known to have a role in neural crest specification in Xenopus and in craniofacial development in the mouse. There is no information on the function of other Myc genes in neural crest development, or about any developmental role of zebrafish Myc genes.

Principal Findings

We isolated the zebrafish mych (myc homologue) gene. Knockdown of mych leads to severe defects in craniofacial development and in certain other tissues including the eye. These phenotypes appear to be caused by cell death in the neural crest and in the eye field in the anterior brain.

Significance

Mych is a novel factor required for neural crest cell survival in zebrafish.

Tfap2 transcription factors in zebrafish neural crest development and ectodermal evolution

Transcription factor AP2 (Tfap2) genes play essential roles in development of the epidermis and migratory cells of the neural crest (NC) in vertebrate embryos. These transcriptional activators are among the earliest genes expressed in the ectoderm and specify fates within the epidermis/crest through both direct and indirect mechanisms. The Tfap2 family arose from a single ancestral gene in a chordate ancestor that underwent gene duplication to give up to five family members in living vertebrates. This coincided with the acquisition of important roles in NC development by Tfap2 genes suggesting that this gene family was important in ectodermal evolution and possibly in the origin of NC. Here, we show that a zebrafish tfap2c is expressed in the nonneural ectoderm during early development and functions redundantly with tfap2a in NC specification. In zebrafish embryos depleted of both tfap2a and tfap2c, NC cells are virtually eliminated. Cell transplantation experiments indicate that tfap2c functions cell-autonomously in NC specification. Cells of the enveloping layer, which forms a temporary skin layer surrounding the ectoderm, also fail to differentiate or to express appropriate keratins in tfap2c deficient embryos. The role of Tfap2 genes in epidermal and NC development is considered here in the broader context of ectodermal evolution. Distinct, tissue-specific functions for Tfap2 genes in different vertebrates may reflect subfunctionalisation of an ancestral gene that consequently led to the gain of novel roles for different subfamily members in patterning the epidermis and NC.

Control of cell migration in the zebrafish lateral line: implication of the gene "tumour-associated calcium signal transducer," tacstd

The sensory organs of the zebrafish lateral-line system (neuromasts) originate from migrating primordia that move along precise pathways. The posterior primordium, which deposits the neuromasts on the body and tail of the embryo, migrates along the horizontal myoseptum from the otic region to the tip of the tail. This migration is controlled by the chemokine SDF1, which is expressed along the prospective pathway, and by its receptor CXCR4, which is expressed by the migrating cells. In this report, we describe another zebrafish gene that is heterogeneously expressed in the migrating cells, tacstd. This gene codes for a membrane protein that is homologous to the TACSTD1/2 mammalian proteins. Inactivation of the zebrafish tacstd gene results in a decrease in proneuromast deposition, suggesting that tacstd is required for the deposition process.