Wnt1-Cre-mediated deletion of AP-2alpha causes multiple neural crest-related defects

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.

Transcription factor MITF and remodeller BRG1 define chromatin organisation at regulatory elements in melanoma cells

Microphthalmia-associated transcription factor (MITF) is the master regulator of the melanocyte lineage. To understand how MITF regulates transcription, we used tandem affinity purification and mass spectrometry to define a comprehensive MITF interactome identifying novel cofactors involved in transcription, DNA replication and repair, and chromatin organisation. We show that MITF interacts with a PBAF chromatin remodelling complex comprising BRG1 and CHD7. BRG1 is essential for melanoma cell proliferation in vitro and for normal melanocyte development in vivo. MITF and SOX10 actively recruit BRG1 to a set of MITF-associated regulatory elements (MAREs) at active enhancers. Combinations of MITF, SOX10, TFAP2A, and YY1 bind between two BRG1-occupied nucleosomes thus defining both a signature of transcription factors essential for the melanocyte lineage and a specific chromatin organisation of the regulatory elements they occupy. BRG1 also regulates the dynamics of MITF genomic occupancy. MITF-BRG1 interplay thus plays an essential role in transcription regulation in melanoma.

Integration of comprehensive 3D microCT and signaling analysis reveals differential regulatory mechanisms of craniofacial bone development

Growth factor signaling regulates tissue-tissue interactions to control organogenesis and tissue homeostasis. Specifically, transforming growth factor beta (TGFβ) signaling plays a crucial role in the development of cranial neural crest (CNC) cell-derived bone, and loss of Tgfbr2 in CNC cells results in craniofacial skeletal malformations. Our recent studies indicate that non-canonical TGFβ signaling is activated whereas canonical TGFβ signaling is compromised in the absence of Tgfbr2 (in Tgfbr2(fl/fl);Wnt1-Cre mice). A haploinsufficiency of Tgfbr1 (aka Alk5) (Tgfbr2(fl/fl);Wnt1-Cre;Alk5(fl/+)) largely rescues craniofacial deformities in Tgfbr2 mutant mice by reducing ectopic non-canonical TGFβ signaling. However, the relative involvement of canonical and non-canonical TGFβ signaling in regulating specific craniofacial bone formation remains unclear. We compared the size and volume of CNC-derived craniofacial bones (frontal bone, premaxilla, maxilla, palatine bone, and mandible) from E18.5 control, Tgfbr2(fl/fl);Wnt1-Cre, and Tgfbr2(fl/fl);Wnt1-Cre;Alk5(fl/+)mice. By analyzing three dimensional (3D) micro-computed tomography (microCT) images, we found that different craniofacial bones were restored to different degrees in Tgfbr2(fl/fl);Wnt1-Cre;Alk5(fl/+) mice. Our study provides comprehensive information on anatomical landmarks and the size and volume of each craniofacial bone, as well as insights into the extent that canonical and non-canonical TGFβ signaling cascades contribute to the formation of each CNC-derived bone. Our data will serve as an important resource for developmental biologists who are interested in craniofacial morphogenesis.

Tfap2a-dependent changes in mouse facial morphology result in clefting that can be ameliorated by a reduction in Fgf8 gene dosage

Failure of facial prominence fusion causes cleft lip and palate (CL/P), a common human birth defect. Several potential mechanisms can be envisioned that would result in CL/P, including failure of prominence growth and/or alignment as well as a failure of fusion of the juxtaposed epithelial seams. Here, using geometric morphometrics, we analyzed facial outgrowth and shape change over time in a novel mouse model exhibiting fully penetrant bilateral CL/P. This robust model is based upon mutations in Tfap2a, the gene encoding transcription factor AP-2α, which has been implicated in both syndromic and non-syndromic human CL/P. Our findings indicate that aberrant morphology and subsequent misalignment of the facial prominences underlies the inability of the mutant prominences to fuse. Exencephaly also occured in some of the Tfap2a mutants and we observed additional morphometric differences that indicate an influence of neural tube closure defects on facial shape. Molecular analysis of the CL/P model indicates that Fgf signaling is misregulated in the face, and that reducing Fgf8 gene dosage can attenuate the clefting pathology by generating compensatory changes. Furthermore, mutations in either Tfap2a or Fgf8 increase variance in facial shape, but the combination of these mutations restores variance to normal levels. The alterations in variance provide a potential mechanistic link between clefting and the evolution and diversity of facial morphology. Overall, our findings suggest that CL/P can result from small gene-expression changes that alter the shape of the facial prominences and uncouple their coordinated morphogenesis, which is necessary for normal fusion.

Multiple tissue-specific requirements for the BMP antagonist Noggin in development of the mammalian craniofacial skeleton

Proper morphogenesis is essential for both form and function of the mammalian craniofacial skeleton, which consists of more than twenty small cartilages and bones. Skeletal elements that support the oral cavity are derived from cranial neural crest cells (NCCs) that develop in the maxillary and mandibular buds of pharyngeal arch 1 (PA1). Bone Morphogenetic Protein (BMP) signaling has been implicated in most aspects of craniofacial skeletogenesis, including PA1 development. However, the roles of the BMP antagonist Noggin in formation of the craniofacial skeleton remain unclear, in part because of its multiple domains of expression during formative stages. Here we used a tissue-specific gene ablation approach to assess roles of Noggin (Nog) in two different tissue domains potentially relevant to mandibular and maxillary development. We found that the axial midline domain of Nog expression is critical to promote PA1 development in early stages, necessary for adequate outgrowth of the mandibular bud. Subsequently, Nog expression in NCCs regulates craniofacial cartilage and bone formation. Mice lacking Nog in NCCs have an enlarged mandible that results from increased cell proliferation in and around Meckel׳s cartilage. These mutants also show complete secondary cleft palate, most likely due to inhibition of posterior palatal shelf elevation by disrupted morphology of the developing skull base. Our findings demonstrate multiple roles of Noggin in different domains for craniofacial skeletogenesis, and suggest an indirect mechanism for secondary cleft palate in Nog mutants that may be relevant to human cleft palate as well.

AP-2α is required after lens vesicle formation to maintain lens integrity

BACKGROUND

Transcription factors are critical in regulating lens development. The AP-2 family of transcription factors functions in differentiation, cell growth and apoptosis, and in lens and eye development. AP-2α, in particular, is important in early lens development, and when conditionally deleted at the placode stage defective separation of the lens vesicle from the surface ectoderm results. AP-2α's role during later stages of lens development is unknown. To address this, the MLR10-Cre transgene was used to delete AP-2α from the lens epithelium beginning at embryonic day (E) 10.5.

RESULTS

The loss of AP-2α after lens vesicle separation resulted in morphological defects beginning at E18.5. By P4, a small highly vacuolated lens with a multilayered epithelium was evident in the MLR10-AP-2α mutants. Epithelial cells appeared elongated and expressed fiber cell specific βB1 and γ-crystallins. Epithelial cell polarity and lens cell adhesion was disrupted and accompanied by the misexpression of ZO-1, N-Cadherin, and β-catenin. Cell death was observed in the mutant lens epithelium between postnatal day (P) 14 and P30, and correlated with altered arrangements of cells within the epithelium.

CONCLUSIONS

Our findings demonstrate that AP-2α continues to be required after lens vesicle separation to maintain a normal lens epithelial cell phenotype and overall lens integrity and to ensure correct fiber cell 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.

Insights into neural crest development and evolution from genomic analysis

The neural crest is an excellent model system for the study of cell type diversification during embryonic development due to its multipotency, motility, and ability to form a broad array of derivatives ranging from neurons and glia, to cartilage, bone, and melanocytes. As a uniquely vertebrate cell population, it also offers important clues regarding vertebrate origins. In the past 30 yr, introduction of recombinant DNA technology has facilitated the dissection of the genetic program controlling neural crest development and has provided important insights into gene regulatory mechanisms underlying cell migration and differentiation. More recently, new genomic approaches have provided a platform and tools that are changing the depth and breadth of our understanding of neural crest development at a “systems” level. Such advances provide an insightful view of the regulatory landscape of neural crest cells and offer a new perspective on developmental as well as stem cell and cancer biology.

Prdm1a directly activates foxd3 and tfap2a during zebrafish neural crest specification

The neural crest comprises multipotent precursor cells that are induced at the neural plate border by a series of complex signaling and genetic interactions. Several transcription factors, termed neural crest specifiers, are necessary for early neural crest development; however, the nature of their interactions and regulation is not well understood. Here, we have established that the PR/SET domain-containing transcription factor Prdm1a is co-expressed with two essential neural crest specifiers, foxd3 and tfap2a, at the neural plate border. Through rescue experiments, chromatin immunoprecipitation and reporter assays, we have determined that Prdm1a directly binds to and transcriptionally activates enhancers for foxd3 and tfap2a and that they are functional, direct targets of Prdm1a at the neural plate border. Additionally, analysis of dominant activator and dominant repressor Prdm1a constructs suggests that Prdm1a is required both as a transcriptional activator and transcriptional repressor for neural crest development in zebrafish embryos.

The Ptch1(DL) mouse: a new model to study lambdoid craniosynostosis and basal cell nevus syndrome-associated skeletal defects

Mouse models provide valuable opportunities for probing the underlying pathology of human birth defects. By using an N-ethyl-N-nitrosourea-based screen for recessive mutations affecting craniofacial anatomy, we isolated a mouse strain, Dogface-like (DL), with abnormal skull and snout morphology. Examination of the skull indicated that these mice developed craniosynostosis of the lambdoid suture. Further analysis revealed skeletal defects related to the pathology of basal cell nevus syndrome (BCNS) including defects in development of the limbs, scapula, ribcage, secondary palate, cranial base, and cranial vault. In humans, BCNS is often associated with mutations in the Hedgehog receptor PTCH1 and genetic mapping in DL identified a point mutation at a splice donor site in Ptch1. By using genetic complementation analysis we determined that DL is a hypomorphic allele of Ptch1, leading to increased Hedgehog signaling. Two aberrant transcripts are generated by the mutated Ptch1(DL) gene, which would be predicted to reduce significantly the levels of functional Patched1 protein. This new Ptch1 allele broadens the mouse genetic reagents available to study the Hedgehog pathway and provides a valuable means to study the underlying skeletal abnormalities in BCNS. In addition, these results strengthen the connection between elevated Hedgehog signaling and craniosynostosis.

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.

The widely used Wnt1-Cre transgene causes developmental phenotypes by ectopic activation of Wnt signaling

The Wnt1-Cre transgenic mouse line is extensively used in the study of the development of the neural crest and its derivatives and the midbrain. The Wnt1 gene has important developmental roles in formation of the midbrain-hindbrain boundary, regulation of midbrain size, and neurogenesis of ventral midbrain dopaminergic (mDA) neurons. Here, we report that Wnt1-Cre transgenic mice exhibit phenotypes in multiple aspects of midbrain development. Significant expansion of the midbrain and increased proliferation in the developing inferior colliculus is associated with ectopic expression of Wnt1. Marked elevation of Wnt1 expression in the ventral midbrain is correlated with disruption of the differentiation program of ventral mDA neurons. We find that these phenotypes can be attributed to ectopic expression of Wnt1 from the Wnt1-Cre transgene leading to the ectopic activation of canonical Wnt/β-catenin signaling. Since these caveats could complicate the utility of Wnt1-Cre in some developmental circumstances, we report a new Wnt1-Cre2 transgenic mouse line that can serve the same purposes as the original without the associated phenotypic complications. These studies reveal an important caveat to a widely-used reagent, provide an improved version of this reagent, and indicate that the original Wnt1-Cre transgenic mouse line may be useful as a gain of function model for interrogating Wnt signaling mechanisms in multiple aspects of midbrain development.

Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina

BACKGROUND

We have previously shown that the transcription factor AP-2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina-specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP-2β and AP-2γ, showed expression patterns similar to AP-2α.

RESULTS

Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP-2α and AP-2β are also co-expressed in developing horizontal cells. AP-2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP-2α/β-immunopositive population. To address possible redundant roles for AP-2α and AP-2β during retinogenesis, Tcfap2a/b-deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone.

CONCLUSIONS

These studies have uncovered critical roles for AP-2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development.

Analysis of TFAP2A mutations in Branchio-Oculo-Facial Syndrome indicates functional complexity within the AP-2α DNA-binding domain

Multiple lines of evidence indicate that the AP-2 transcription factor family has an important regulatory function in human craniofacial development. Notably, mutations in TFAP2A, the gene encoding AP-2α, have been identified in patients with Branchio-Oculo-Facial Syndrome (BOFS). BOFS is an autosomal-dominant trait that commonly presents with facial clefting, eye defects and branchial skin anomalies. Examination of multiple cases has suggested either simple haploinsufficiency or more complex genetic causes for BOFS, especially as the clinical manifestations are variable, with no clear genotype-phenotype correlation. Mutations occur throughout TFAP2A, but mostly within conserved sequences within the DNA contact domain of AP-2α. However, the consequences of the various mutations for AP-2α protein function have not been evaluated. Therefore, it remains unclear if all BOFS mutations result in similar changes to the AP-2α protein or if they each produce specific alterations that underlie the spectrum of phenotypes. Here, we have investigated the molecular consequences of the mutations that localize to the DNA-binding region. We show that although individual mutations have different effects on DNA binding, they all demonstrate significantly reduced transcriptional activities. Moreover, all mutant derivatives have an altered nuclear:cytoplasmic distribution compared with the predominantly nuclear localization of wild-type AP-2α and several can exert a dominant-negative activity on the wild-type AP-2α protein. Overall, our data suggest that the individual TFAP2A BOFS mutations can generate null, hypomorphic or antimorphic alleles and that these differences in activity, combined with a role for AP-2α in epigenetic events, may influence the resultant pathology and the phenotypic variability.

Inhibition of neural crest formation by Kctd15 involves regulation of transcription factor AP-2

The neural crest develops in vertebrate embryos within a discrete domain at the neural plate boundary and eventually gives rise to a migrating population of cells that differentiate into a multitude of derivatives. We have shown that the broad-complex, tramtrack and bric a brac (BTB) domain-containing factor potassium channel tetramerization domain containing 15 (Kctd15) inhibits neural crest formation, and we proposed that its function is to delimit the neural crest domain. Here we report that Kctd15 is a highly effective inhibitor of transcription factor activating enhancer binding protein 2 (AP-2) in zebrafish embryos and in human cells; AP-2 is known to be critical for several steps of neural crest development. Kctd15 interacts with AP-2α but does not interfere with its nuclear localization or binding to cognate sites in the genome. Kctd15 binds specifically to the activation domain of AP-2α and efficiently inhibits transcriptional activation by a hybrid protein composed of the regulatory protein Gal4 DNA binding and AP-2α activation domains. Mutation of one proline residue in the activation domain to an alanine (P59A) yields a protein that is highly active but largely insensitive to Kctd15. These results indicate that Kctd15 acts in the embryo at least in part by specifically binding to the activation domain of AP-2α, thereby blocking the function of this critical factor in the neural crest induction hierarchy.

Targeted deletion of Hand2 in enteric neural precursor cells affects its functions in neurogenesis, neurotransmitter specification and gangliogenesis, causing functional aganglionosis

Targeted deletion of the bHLH DNA-binding protein Hand2 in the neural crest, impacts development of the enteric nervous system (ENS), possibly by regulating the transition from neural precursor cell to neuron. We tested this hypothesis by targeting Hand2 deletion in nestin-expressing neural precursor (NEP) cells. The mutant mice showed abnormal ENS development, resulting in lethal neurogenic pseudo-obstruction. Neurogenesis of neurons derived from NEP cells identified a second nestin non-expressing neural precursor (NNEP) cell in the ENS. There was substantial compensation for the loss of neurons derived from the NEP pool by the NNEP pool but this was insufficient to abrogate the negative impact of Hand2 deletion. Hand2-mediated regulation of proliferation affected both neural precursor and neuron numbers. Differentiation of glial cells derived from the NEP cells was significantly decreased with no compensation from the NNEP pool of cells. Our data indicate differential developmental potential of NEPs and NNEPs; NNEPs preferentially differentiate as neurons, whereas NEPs give rise to both neurons and glial cells. Deletion of Hand2 also resulted in complete loss of NOS and VIP and a significant decrease in expression of choline acetyltransferase and calretinin, demonstrating a role for Hand2 in neurotransmitter specification and/or expression. Loss of Hand2 resulted in a marked disruption of the developing neural network, exemplified by lack of a myenteric plexus and extensive overgrowth of fibers. Thus, Hand2 is essential for neurogenesis, neurotransmitter specification and neural network patterning in the developing ENS.

Chemokine-mediated migration of mesencephalic neural crest cells

Clefts of the lip and/or palate are among the most prevalent birth defects affecting approximately 7000 newborns in the United States annually. Disruption of the developmentally programmed migration of neural crest cells (NCCs) into the orofacial region is thought to be one of the major causes of orofacial clefting. Signaling of the chemokine SDF-1 (Stromal Derived Factor-1) through its specific receptor, CXCR4, is required for the migration of many stem cell and progenitor cell populations from their respective sites of emergence to the regions where they differentiate into complex cell types, tissues and organs. In the present study, "transwell" assays of chick embryo mesencephalic (cranial) NCC migration and ex ovo whole embryo "bead implantation" assays were utilized to determine whether SDF-1/CXCR4 signaling mediates mesencephalic NCC migration. Results from this study demonstrate that attenuation of SDF-1 signaling, through the use of specific CXCR4 antagonists (AMD3100 and TN14003), disrupts the migration of mesencephalic NCCs into the orofacial region, suggesting a novel role for SDF-1/CXCR4 signaling in the directed migration of mesencephalic NCCs in the early stage embryo.

Neural crest and ectodermal cells intermix in the nasal placode to give rise to GnRH-1 neurons, sensory neurons, and olfactory ensheathing cells

The origin of GnRH-1 cells and olfactory ensheathing cells has been controversial. Genetic Cre-lox lineage tracing of the neural crest (NC) versus ectodermal contribution to the developing nasal placode was performed using two complementary mouse models, the NC-specific Wnt1Cre mouse line and an ectodermal-specific Crect mouse line. Using these lines we prove that the NC give rise to the olfactory ensheathing cells and subpopulations of GnRH-1 neurons, olfactory and vomeronasal cells. These data demonstrate that Schwann cells and olfactory ensheathing cells share a common developmental origin. Furthermore, the results indicate that certain conditions that impact olfaction and sexual development, such as Kallmann syndrome, may be in part neurocristopathies.

Hydrocephalus and abnormal subcommissural organ in mice lacking presenilin-1 in Wnt1 cell lineages

Presenilin-1 (PS1) is a transmembrane protein that is in many cases responsible for the development of familial Alzheimer's disease. PS1 is widely expressed in embryogenesis and is essential for neurogenesis, somitogenesis, angiogenesis, and cardiac morphogenesis. To further investigate the role of PS1 in the brain, we inactivated the PS1 gene in Wnt1 cell lineages using the Cre-loxP recombination system. Here we show that conditional inactivation of PS1 in Wnt1 cell lineages results in congenital hydrocephalus and subcommissural organ abnormalities, suggesting a possible role of PS1 in the regulation of cerebrospinal fluid homeostasis.

Transcription factor AP-2β regulates the neurotransmitter phenotype and maturation of chromaffin cells

During development, sympathetic neurons and chromaffin cells originate from bipotential sympathoadrenal (SA) progenitors arising from neural crests (NC) in the trunk regions. Recently, we showed that AP-2β, a member of the AP2 family, plays a critical role in the development of sympathetic neurons and locus coeruleus and their norepinephrine (NE) neurotransmitter phenotype. In the present study, we investigated the potential role of AP-2β in the development of NC-derived neuroendocrine chromaffin cells of the adrenal medulla and the epinephrine (EPI) phenotype determination. In support of its role in chromaffin cell development, AP-2β is prominently expressed in both embryonic and adult adrenal medulla. In adrenal chromaffin cells of the AP-2β(-/-) mouse, the expression levels of catecholamine biosynthesizing enzymes, dopamine β-hydroxylase (DBH) and phenylethanolamine-N-methyl-transferase (PNMT), as well as the SA-specific transcription factor, Phox2b, are significantly reduced compared to wild type. In addition, ultrastructural analysis demonstrated that the formation of large secretory vesicles, a hallmark of differentiated chromaffin cells, is defective in AP-2β(-/-) mice. Furthermore, the level of EPI content is largely diminished (>80%) in the adrenal gland of AP-2β(-/-) mice. Chromatin immunoprecipitation (ChIP) assays of rat adrenal gland showed that AP-2β binds to the upstream promoter of the PNMT gene in vivo; strongly suggesting that it is a direct target gene. Overall, our data suggest that AP-2β plays critical roles in the epinephrine phenotype and maturation of adrenal chromaffin cells.

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.

Generation and characterization of a novel neural crest marker allele, Inka1-LacZ, reveals a role for Inka1 in mouse neural tube closure

Previous studies identified Inka1 as a gene regulated by AP-2alpha in the neural crest required for craniofacial morphogenesis in fish and frog. Here, we extend the analysis of Inka1 function and regulation to the mouse by generating a LacZ knock-in allele. Inka1-LacZ allele expression occurs in the cephalic mesenchyme, heart, and paraxial mesoderm prior to E8.5. Subsequently, expression is observed in the migratory neural crest cells and their derivatives. Consistent with expression of Inka1 in tissues of the developing head during neurulation, a low percentage of Inka1(-/-) mice show exencephaly while the remainder are viable and fertile. Further studies indicate that AP-2alpha is not required for Inka1 expression in the mouse, and suggest that there is no significant genetic interaction between these two factors during embryogenesis. Together, these data demonstrate that while the expression domain of Inka1 is conserved among vertebrates, its function and regulation are not.

AP-2alpha knockout mice exhibit optic cup patterning defects and failure of optic stalk morphogenesis

Appropriate development of the retina and optic nerve requires that the forebrain-derived optic neuroepithelium undergoes a precisely coordinated sequence of patterning and morphogenetic events, processes which are highly influenced by signals from adjacent tissues. Our previous work has suggested that transcription factor activating protein-2 alpha (AP-2alpha; Tcfap2a) has a non-cell autonomous role in optic cup (OC) development; however, it remained unclear how OC abnormalities in AP-2alpha knockout (KO) mice arise at the morphological and molecular level. In this study, we show that patterning and morphogenetic defects in the AP-2alpha KO optic neuroepithelium begin at the optic vesicle stage. During subsequent OC formation, ectopic neural retina and optic stalk-like tissue replaced regions of retinal pigment epithelium. AP-2alpha KO eyes also displayed coloboma in the ventral retina, and a rare phenotype in which the optic stalk completely failed to extend, causing the OCs to be drawn inward to the midline. We detected evidence of increased sonic hedgehog signaling in the AP-2alpha KO forebrain neuroepithelium, which likely contributed to multiple aspects of the ocular phenotype, including expansion of PAX2-positive optic stalk-like tissue into the OC. Our data suggest that loss of AP-2alpha in multiple tissues in the craniofacial region leads to severe OC and optic stalk abnormalities by disturbing the tissue-tissue interactions required for ocular development. In view of recent data showing that mutations in human TFAP2A result in similar eye defects, the current findings demonstrate that AP-2alpha KO mice provide a valuable model for human ocular disease.

Targeted deletion of Hand2 in cardiac neural crest-derived cells influences cardiac gene expression and outflow tract development

The basic helix-loop-helix DNA binding protein Hand2 has critical functions in cardiac development both in neural crest-derived and mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest has allowed us to genetically dissect Hand2-dependent defects specifically in outflow tract and cardiac cushion independent of Hand2 functions in mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest results in misalignment of the aortic arch arteries and outflow tract, contributing to development of double outlet right ventricle (DORV) and ventricular septal defects (VSD). These neural crest-derived developmental anomalies are associated with altered expression of Hand2-target genes we have identified by gene profiling. A number of Hand2 direct target genes have been identified using ChIP and ChIP-on-chip analyses. We have identified and validated a number of genes related to cell migration, proliferation/cell cycle and intracellular signaling whose expression is affected by Hand2 deletion in the neural crest and which are associated with development of VSD and DORV. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting expression of target genes associated with a number of functional interactions in neural crest-derived cells required for proper patterning of the outflow tract, generation of the appropriate number of neural crest-derived cells for elongation of the conotruncus and cardiac cushion organization. Our genetic model has made it possible to investigate the molecular genetics of neural crest contributions to outflow tract morphogenesis and cell differentiation.

Restricted growth and insulin-like growth factor-1 deficiency in mice lacking presenilin-1 in the neural crest cell lineage

Presenilin-1 (PS1) is a transmembrane protein that is in many cases responsible for the development of early-onset familial Alzheimer's disease. PS1 is essential for neurogenesis, somitogenesis, angiogenesis, and cardiac morphogenesis. We report here that PS1 is also required for maturation and/or maintenance of the pituitary gland. We generated PS1-conditional knockout (PS1-cKO) mice by crossing floxed PS1 and Wnt1-cre mice, in which PS1 was lacking in the neural crest-derived cell lineage. Although the PS1-cKO mice exhibited no obvious phenotypic abnormalities for several days after birth, reduced body weight in the mutant was evident by the age of 3-5 weeks. Pituitary weight and serum insulin-like growth factor (IGF)-1 level were also reduced in the mutant. Histologic analysis revealed severe atrophy of the cytosol in the anterior and intermediate pituitary lobes of the mutant. Immunohistochemistry did not reveal clear differences in the expression levels of thyroid-stimulating hormone, adrenocorticotropic hormone, or prolactin in the mutant pituitary. In contrast, growth hormone expression levels were reduced in the anterior lobe of the mutant. PS1 was defective in the posterior lobe, but not the anterior or intermediate lobes, in the mutant pituitary. These findings suggest that PS1 indirectly mediates the development and/or maintenance of the anterior and intermediate lobes in the pituitary gland via actions in other regions, such as the posterior lobe.

BCOR regulates mesenchymal stem cell function by epigenetic mechanisms

BCOR (BCL6 co-repressor) represses gene transcription by interacting with BCL-6 1, 2. BCOR mutation is responsible for oculo-facio-cardio-dental (OFCD) syndrome, characterized by canine teeth with extremely long roots, congenital cataracts, craniofacial defects and congenital heart disease3–5. Here we show that BCOR mutation increased osteo/dentinogenic potentials of mesenchymal stem cells (MSCs) isolated from an OFCD patient, providing a molecular explanation for abnormal root growth. AP-2α was identified as a repressive target of BCOR, and BCOR mutation resulted in abnormal activation of AP-2α. Gain- and loss-of-function assays suggested that AP-2α was a key factor that mediated increased osteo/dentinogenic capacity of MSCs. Moreover, we found that BCOR maintained tissue homeostasis and gene silencing by epigenetic mechanisms. BCOR mutation increased histone H3K4/36 methylation in MSCs, thereby reactivating transcription of silenced target genes. In summary, by studying a rare human genetic disease, we unravel an epigenetic mechanism for control of human adult stem cell function.

Disorganized olfactory bulb lamination in mice deficient for transcription factor AP-2epsilon

Within the olfactory bulb, neurons and their axonal connections are organized into distinct layers corresponding to different functionalities. Here we demonstrate that transcription factor AP-2epsilon is required for olfactory bulb development, specifically the establishment of appropriate neuronal lamination. During normal mouse embryogenesis, AP-2epsilon transcripts are one of the earliest markers of olfactory bulb formation, and expression eventually becomes refined to the projection neurons, the mitral and tufted cells. To assess the function of AP-2epsilon in olfaction, we generated a null allele (the "AK" allele) by inserting a Cre recombinase transgene into the endogenous AP-2epsilon genomic locus. AP-2epsilon-null mice exhibited defective olfactory bulb architecture. The mitral cell layer was disorganized, typified by misoriented and aberrantly positioned projection neurons, and the adjacent internal plexiform layer was absent. Despite the significant disruption of olfactory bulb organization, AP-2epsilon null mice were viable and could discriminate a variety of odors. AP-2epsilon-null mice thus provide compelling evidence for the robust nature of the mouse olfactory system, and serve as a model system to probe both the regulation of neuronal lamination and the functional circuitry of the olfactory bulb. We also show that Cre recombinase expression directed by the AP-2epsilon locus can specifically target floxed genes within the olfactory bulb, extending the utility of this AK allele.

AP-2alpha regulates migration of GN-11 neurons via a specific genetic programme involving the Axl receptor tyrosine kinase

Background

Neuronal migration is a crucial process that allows neurons to reach their correct target location to allow the nervous system to function properly. AP-2α is a transcription factor essential for neural crest cell migration and its mutation results in apoptosis within this cell population, as demonstrated by genetic models.

Results

We down-modulated AP-2α expression in GN-11 neurons by RNA interference and observe reduced neuron migration following the activation of a specific genetic programme including the Adhesion Related Kinase (Axl) gene. We prove that Axl is able to coordinate migration per se and by ChIP and promoter analysis we observe that its transcription is directly driven by AP-2α via the binding to one or more functional AP-2α binding sites present in its regulatory region. Analysis of migration in AP-2α null mouse embryo fibroblasts also reveals an essential role for AP-2α in cell movement via the activation of a distinct genetic programme.

Conclusion

We show that AP-2α plays an essential role in cell movement via the activation of cell-specific genetic programmes. Moreover, we demonstrate that the AP-2α regulated gene Axl is an essential player in GN-11 neuron migration.

Biomedical discovery acceleration, with applications to craniofacial development

The profusion of high-throughput instruments and the explosion of new results in the scientific literature, particularly in molecular biomedicine, is both a blessing and a curse to the bench researcher. Even knowledgeable and experienced scientists can benefit from computational tools that help navigate this vast and rapidly evolving terrain. In this paper, we describe a novel computational approach to this challenge, a knowledge-based system that combines reading, reasoning, and reporting methods to facilitate analysis of experimental data. Reading methods extract information from external resources, either by parsing structured data or using biomedical language processing to extract information from unstructured data, and track knowledge provenance. Reasoning methods enrich the knowledge that results from reading by, for example, noting two genes that are annotated to the same ontology term or database entry. Reasoning is also used to combine all sources into a knowledge network that represents the integration of all sorts of relationships between a pair of genes, and to calculate a combined reliability score. Reporting methods combine the knowledge network with a congruent network constructed from experimental data and visualize the combined network in a tool that facilitates the knowledge-based analysis of that data. An implementation of this approach, called the Hanalyzer, is demonstrated on a large-scale gene expression array dataset relevant to craniofacial development. The use of the tool was critical in the creation of hypotheses regarding the roles of four genes never previously characterized as involved in craniofacial development; each of these hypotheses was validated by further experimental work.

Growth of cranial synchondroses and sutures requires polycystin-1

In vertebrates, coordinated embryonic and postnatal growth of the craniofacial bones and the skull base is essential during the expansion of the rostrum and the brain. Identification of molecules that regulate skull growth is important for understanding the nature of craniofacial defects and for development of non-invasive biologically based diagnostics and therapies. Here we report on spatially restricted growth defects at the skull base and in craniofacial sutures of mice deficient for polycystin-1 (Pkd1). Mutant animals reveal a premature closure of both presphenoid and sphenooccipital synchondroses at the cranial base. Furthermore, knockout mice lacking Pkd1 in neural crest cells are characterized by impaired postnatal growth at the osteogenic fronts in craniofacial sutures that are subjected to tensile forces. Our data suggest that polycystin-1 is required for proliferation of subpopulations of cranial osteochondroprogenitor cells of both mesodermal and neural crest origin during skull growth. However, the Erk1/2 signalling pathway is up-regulated in the Pkd1-deficient skeletal tissue, similarly to that previously reported for polycystic kidney.

Cell autonomous roles for AP-2alpha in lens vesicle separation and maintenance of the lens epithelial cell phenotype

In this study, we have created a conditional deletion of AP-2alpha in the developing mouse lens (Le-AP-2alpha mutants) to determine the cell-autonomous requirement(s) for AP-2alpha in lens development. Embryonic and adult Le-AP-2alpha mutants exhibited defects confined to lens placode derivatives, including a persistent adhesion of the lens to the overlying corneal epithelium (or lens stalk). Expression of known regulators of lens vesicle separation, including Pax6, Pitx3, and Foxe3 was observed in the Le-AP-2alpha mutant lens demonstrating that these genes do not lie directly downstream of AP-2alpha. Unlike germ-line mutants, Le-AP-2alpha mutants did not exhibit defects in the optic cup, further defining the tissue specific role(s) for AP-2alpha in eye development. Finally, comparative microarray analysis of lenses from the Le-AP-2alpha mutants vs. wild-type littermates revealed differential expression of 415 mRNAs, including reduced expression of genes important for maintaining the lens epithelial cell phenotype, such as E-cadherin.

Regulation of the noradrenaline neurotransmitter phenotype by the transcription factor AP-2beta

AP-2 family transcription factors are essential for development and morphogenesis of diverse tissues and organs, but their precise roles in specification of neural crest stem cell (NCSC)-derived cell types have not been determined. Among three members known to be expressed in the NCSC (i.e. AP-2alpha, AP-2beta, and AP-2gamma), we found that only AP-2beta is predominantly expressed in the sympathetic ganglia of developing mouse embryos, supporting its role in sympathetic development. Indeed, AP-2beta null mice expressed significantly reduced levels of both noradrenaline (NA) and NA-synthesizing dopamine beta-hydroxylase in the peripheral nervous system. Strikingly, we also found that NA neuron development was significantly compromised in the locus coeruleus as well. Pharmacological treatment with an NA intermediate during pregnancy significantly rescues the neonatal lethality of AP-2beta(-/-) mice, indicating that NA deficiency is one of the main causes for lethality found in AP-2beta(-/-) mice. We also showed that forced expression of AP-2beta, but not other AP-2 factors, in NCSC favors their differentiation into NA neurons. In summary, we propose that AP-2beta plays critical and distinctive roles in the NA phenotype specification in both the peripheral and central nervous system during development.

Variation in WNT genes is associated with non-syndromic cleft lip with or without cleft palate

Non-syndromic cleft lip with or without cleft palate (NSCLP) is a common birth defect. Genetic and environmental factors have been causally implicated and studies have begun to delineate genetic contributions. The Wnt genes are involved in regulating mid-face development and upper lip fusion and are therefore strong candidates for an etiological role in NSCLP. Furthermore, the clf1 region in A/WyN clefting susceptible mice contains the Wnt3 and Wnt9B genes. To assess the role of the Wnt family of genes in NSCLP, we interrogated seven Wnt genes (Wnt3, Wnt3A, Wnt5A, Wnt7A, Wnt8A, Wnt9B and Wnt11) in our well-defined NSCLP dataset. Thirty-eight single nucleotide polymorphisms were genotyped in 132 multiplex NSCLP families and 354 simplex parent-child trios. In the entire dataset, single-nucleotide polymorphisms (SNPs) in three genes, Wnt3A (P = 0.006), Wnt 5A (P = 0.002) and Wnt11 (P = 0.0001) were significantly associated with NSCLP after correction for multiple testing. When stratified by ethnicity, the strongest associations were found for SNPs in Wnt3A (P = 0.0007), Wnt11 (P = 0.0012) and Wnt8A (P = 0.0013). Multiple haplotypes in Wnt genes were associated with NSCLP, and gene-gene interactions were observed between Wnt3A and both Wnt3 and Wnt5A (P = 0.004 and P = 0.039, respectively). This data suggests that alteration in Wnt gene function may perturb formation and/or fusion of the facial processes and predispose to NSCLP.

Conditional deletion of Hand2 reveals critical functions in neurogenesis and cell type-specific gene expression for development of neural crest-derived noradrenergic sympathetic ganglion neurons

Neural crest-derived structures that depend critically upon expression of the basic helix-loop-helix DNA binding protein Hand2 for normal development include craniofacial cartilage and bone, the outflow tract of the heart, cardiac cushion, and noradrenergic sympathetic ganglion neurons. Loss of Hand2 is embryonic lethal by E9.5, obviating a genetic analysis of its in-vivo function. We have overcome this difficulty by specific deletion of Hand2 in neural crest-derived cells by crossing our line of floxed Hand2 mice with Wnt1-Cre transgenic mice. Our analysis of Hand2 knock-out in neural crest-derived cells reveals effects on development in all neural crest-derived structures where Hand2 is expressed. In the autonomic nervous system, conditional disruption of Hand2 results in a significant and progressive loss of neurons as well as a significant loss of TH expression. Hand2 affects generation of the neural precursor pool of cells by affecting both the proliferative capacity of the progenitors as well as affecting expression of Phox2a and Gata3, DNA binding proteins important for the cell autonomous development of noradrenergic neurons. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting differentiation and cell type-specific gene expression in neural crest-derived noradrenergic sympathetic ganglion neurons. Hand2 has a pivotal function in a non-linear cross-regulatory network of DNA binding proteins that affect cell autonomous control of differentiation and cell type-specific gene expression.

The etiopathogenesis of cleft lip and cleft palate: usefulness and caveats of mouse models

Cleft lip and cleft palate are frequent human congenital malformations with a complex multifactorial etiology. These orofacial clefts can occur as part of a syndrome involving multiple organs or as isolated clefts without other detectable defects. Both forms of clefting constitute a heavy burden to the affected individuals and their next of kin. Human and mouse facial traits are utterly dissimilar. However, embryonic development of the lip and palate are strikingly similar in both species, making the mouse a model of choice to study their normal and abnormal development. Human epidemiological and genetic studies are clearly important for understanding the etiology of lip and palate clefting. However, our current knowledge about the etiopathogenesis of these malformations has mainly been gathered throughout the years from mouse models, including those with mutagen-, teratogen- and targeted mutation-induced clefts as well as from mice with spontaneous clefts. This review provides a comprehensive description of the numerous mouse models for cleft lip and/or cleft palate. Despite a few weak points, these models have revealed a high order of molecular complexity as well as the stringent spatiotemporal regulations and interactions between key factors which govern the development of these orofacial structures.

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.

Identification and analysis of a conserved Tcfap2a intronic enhancer element required for expression in facial and limb bud mesenchyme

Tcfap2a, the gene encoding the mouse AP-2alpha transcription factor, is required for normal development of multiple structures during embryogenesis, including the face and limbs. Using comparative sequence analysis and transgenic-mouse experiments we have identified an intronic enhancer within this gene that directs expression to the face and limb mesenchyme. There are two conserved sequence blocks within this intron, and the larger of these directs tissue-specific activity and is found in all vertebrate Tcfap2a genes analyzed. To assess the role of the enhancer in regulating endogenous mouse Tcfap2a expression, we have deleted this cis-regulatory sequence from the genome. Loss of this element severely impairs Tcfap2a expression in the limb bud mesenchyme but generates only a modest reduction in the facial mesenchyme. The reduction in Tcfap2a transcription is accompanied by altered patterning of the forelimb, resulting in postaxial polydactyly. These results indicate that the major role for this enhancer resides within the limb bud, and it serves to maintain a level of Tcfap2a expression that limits the size of the hand plate and the associated number of digit primordia. The potential role of this cis-acting sequence in modeling the size and shape of the face and limbs during evolution is discussed.

Neural crest cell deficiency of c-myc causes skull and hearing defects

The proto-oncogene c-myc has a central role in multiple processes important for embryonic development, including cell proliferation, growth, apoptosis, and differentiation. We have investigated the role of c-myc in neural crest by using Wnt1-Cre-mediated deletion of a conditional mutation of the c-myc gene. c-myc deficiency in neural crest resulted in viable adult mice that have defects in coat color, skull frontal bone, and middle ear ossicle development. Physiological hearing studies demonstrated a significant hearing deficit in the mutant mice. In this report, we focus on the craniofacial and hearing defects. To further examine neural crest cells affected by c-myc deficiency, we fate mapped Wnt1-Cre expressing neural crest cells using the ROSA26 Cre reporter transgene. The phenotype obtained demonstrates the critical role that c-myc has in neural crest during craniofacial development as well as in providing a model for examining human congenital skull defects and deafness.

Conditional deletion of activating protein 2alpha (AP-2alpha) in the developing retina demonstrates non-cell-autonomous roles for AP-2alpha in optic cup development

Activating protein 2alpha (AP-2alpha) is known to be expressed in the retina, and AP-2alpha-null mice exhibit defects in the developing optic cup, including patterning of the neural retina (NR) and a replacement of the dorsal retinal pigmented epithelium (RPE) with NR. In this study, we analyzed the temporal and spatial retinal expression patterns of AP-2alpha and created a conditional deletion of AP-2alpha in the developing retina. AP-2alpha exhibited a distinct expression pattern in the developing inner nuclear layer of the retina, and colocalization studies indicated that AP-2alpha was exclusively expressed in postmitotic amacrine cell populations. Targeted deletion of AP-2alpha in the developing retina did not result in observable retinal defects. Further examination of AP-2alpha-null mutants revealed that the severity of the RPE defect was variable and, although defects in retinal lamination occur at later embryonic stages, earlier stages showed normal lamination and expression of markers for amacrine and ganglion cells. Together, these data demonstrate that, whereas AP-2alpha alone does not play an intrinsic role in retinogenesis, it has non-cell-autonomous effects on optic cup development. Additional expression analyses showed that multiple AP-2 proteins are present in the developing retina, which will be important to future studies.

The genetic regulation of pigment cell development

Pigment cells in developing vertebrates are derived from a transient and pluripotent population of cells called neural crest. The neural crest delaminates from the developing neural tube and overlying ectoderm early in development. The pigment cells are the only derivative to migrate along the dorso-lateral pathway. As they migrate, the precursor pigment cell population differentiates and expands through proliferation and pro-survival processes, ultimately contributing to the coloration of organisms. The types of pigment cells that develop, timing of these processes, and final destination can vary between organisms. Studies from mice, chick, Xenopus, zebrafish, and medaka have led to the identification of many genes that regulate pigment cell development. These include several classes of proteins: transcription factors, transmembrane receptors, and extracellular ligands. This chapter discusses an overview of pigment cell development and the genes that regulate this important process.

Hand2 determines the noradrenergic phenotype in the mouse sympathetic nervous system

The basic helix-loop-helix (bHLH) transcription factor Hand2 has been shown to play a role in the development of the mammalian sympathetic nervous system (SNS); however, its precise role could not be uncovered because Hand2 is required for early embryonic survival. We therefore generated a conditional Hand2 knockout mouse line by excising Hand2 in Wnt1-Cre-expressing neural crest-derived cells. These mice die at 12.5 dpc with embryos showing severe cardiovascular and facial defects. Crest-derived cells, however, populate sites of SNS development and proliferate normally. Sympathetic precursors differentiate into neurons and express the pan-neuronal markers, beta3-tubulin (Tuj1) and Hu showing that Hand2 is not essential for SNS neuronal differentiation. To determine whether Hand2 regulates noradrenergic differentiation, the levels of the norepinephrine biosynthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) was examined. Both enzymes were dramatically reduced in mutant embryos suggesting that the primary role of Hand2 in the SNS is determination of neuronal phenotype. Loss of Hand2 did not affect the expression of other members of the transcriptional circuit regulating SNS development, including Phox2a/b, Mash1 and Gata2/3; however, Hand2 was required for Hand1 expression. Our data suggest that the major role of Hand2 during SNS development is to permit sympathetic neurons to acquire a catecholaminergic phenotype.

Multiple functions of Snail family genes during palate development in mice

Palate development requires precise regulation of gene expression changes, morphogenetic movements and alterations in cell physiology. Defects in any of these processes can result in cleft palate, a common human birth defect. The Snail gene family encodes transcriptional repressors that play essential roles in the growth and patterning of vertebrate embryos. Here we report the functions of Snail (Snai1) and Slug (Snai2) genes during palate development in mice. Snai2(-/-) mice exhibit cleft palate, which is completely penetrant on a Snai1 heterozygous genetic background. Cleft palate in Snai1(+/-) Snai2(-/-) embryos is due to a failure of the elevated palatal shelves to fuse. Furthermore, while tissue-specific deletion of the Snai1 gene in neural crest cells does not cause any obvious defects, neural-crest-specific Snai1 deletion on a Snai2(-/-) genetic background results in multiple craniofacial defects, including a cleft palate phenotype distinct from that observed in Snai1(+/-) Snai2(-/-) embryos. In embryos with neural-crest-specific Snai1 deletion on a Snai2(-/-) background, palatal clefting results from a failure of Meckel's cartilage to extend the mandible and thereby allow the palatal shelves to elevate, defects similar to those seen in the Pierre Robin Sequence in humans.

The Hectd1 ubiquitin ligase is required for development of the head mesenchyme and neural tube closure

Closure of the cranial neural tube depends on normal development of the head mesenchyme. Homozygous-mutant embryos for the ENU-induced open mind (opm) mutation exhibit exencephaly associated with defects in head mesenchyme development and dorsal-lateral hinge point formation. The head mesenchyme in opm mutant embryos is denser than in wildtype embryos and displays an abnormal cellular organization. Since cells that originate from both the cephalic paraxial mesoderm and the neural crest populate the head mesenchyme, we explored the origin of the abnormal head mesenchyme. opm mutant embryos show apparently normal development of neural crest-derived structures. Furthermore, the abnormal head mesenchyme in opm mutant embryos is not derived from the neural crest, but instead expresses molecular markers of cephalic mesoderm. We also report the identification of the opm mutation in the ubiquitously expressed Hectd1 E3 ubiquitin ligase. Two different Hectd1 alleles cause incompletely penetrant neural tube defects in heterozygous animals, indicating that Hectd1 function is required at a critical threshold for neural tube closure. This low penetrance of neural tube defects in embryos heterozygous for Hectd1 mutations suggests that Hectd1 should be considered as candidate susceptibility gene in human neural tube defects.