SELEX and missing phosphate contact analyses reveal flexibility within the AP-2[alpha] protein: DNA binding complex

Neural crest survival and differentiation in zebrafish depends on mont blanc/tfap2a gene function

Neural crest progenitor cells are the main contributors to craniofacial cartilage and connective tissue of the vertebrate head. These progenitor cells also give rise to the pigment, neuronal and glial cell lineages. To study the molecular basis of neural crest differentiation, we have cloned the gene disrupted in the mont blanc (mobm610) mutation,which affects all neural crest derivatives. Using a positional candidate cloning approach we identified an A to G transition within the 3′ splice site of the sixth intron of the tfap2a gene that abolishes the last exon encoding the crucial protein dimerization and DNA-binding domains. Neural crest induction and specification are not hindered in mobm610 mutant embryos, as revealed by normal expression of early neural crest specific genes such as snail2, foxd3and sox10. In addition, the initial stages of cranial neural crest migration appear undisturbed, while at a later phase the craniofacial primordia in pharyngeal arches two to seven fail to express their typical set of genes (sox9a, wnt5a, dlx2, hoxa2/b2). In mobm610 mutant embryos, the cell number of neuronal and glial derivatives of neural crest is greatly reduced, suggesting that tfap2a is required for their normal development. By tracing the fate of neural crest progenitors in live mont blanc(mobm610) embryos, we found that at 24 hpf neural crest cells migrate normally in the first pharyngeal arch while the preotic and postotic neural crest cells begin migration but fail to descend to the pharyngeal region of the head. TUNEL assay and Acridine Orange staining revealed that in the absence of tfap2a a subset of neural crest cells are unable to undergo terminal differentiation and die by apoptosis. Furthermore, surviving neural crest cells in tfap2a/mobm610 mutant embryos proliferate normally and later differentiate to individual derivatives. Our results indicate that tfap2a is essential to turn on the normal developmental program in arches 2-7 and in trunk neural crest. Thus, tfap2a does not appear to be involved in early specification and cell proliferation of neural crest, but it is a key regulator of an early differentiation phase and is required for cell survival in neural crest derived cell lineages.

AP-2 and HNK-1 define distinct populations of cranial neural crest cells

OBJECTIVE

To determine if distinct populations of cranial neural crest cells (CNCC) exist by characterization of their divergent gene expression patterns.

DESIGN

Identification of unique populations of CNCC was determined by a combination of lineage and immunohistochemical analyses.

SETTING

Department of Pathology, Children's Hospital of Philadelphia and the University of Pennsylvania School of Medicine, Philadelphia, PA 19104.

RESULTS

We found antibodies of two proteins previously described as identifying all CNCC, label three populations of CNCC at specific time-points. Furthermore, the activating protein 2 (AP-2) expressing CNCC become neural or mesenchymal NCC derivatives whereas the HNK-1 labeled cells do not participate in the mesenchymal lineage.

CONCLUSION

These data provide molecular markers for unique CNCC fates and thus will be invaluable in the characterizing of craniofacial anomalies related to defects in NCCS. In addition, our data suggest AP-2 may function in determining the unique mesenchymal fate of CNCCs.

Cell cycle arrest and apoptosis induction by activator protein 2alpha (AP-2alpha) and the role of p53 and p21WAF1/CIP1 in AP-2alpha-mediated growth inhibition

Activator protein 2alpha (AP-2alpha) is a sequence-specific DNA-binding transcription factor implicated in differentiation and transformation. In this study, we have made a replication-deficient recombinant adenovirus that expresses functional AP-2alpha (Ad-AP2). Cells infected with Ad-AP2 expressed induced levels of AP-2alpha protein, which bound to DNA in a sequence-specific manner and activated the AP-2-specific reporter 3X-AP2. Expression of AP-2alpha from Ad-AP2 inhibited cellular DNA synthesis and induced apoptosis. Ad-AP2 infection resulted in efficient inhibition of growth of cancer cells of six different types. In addition, prior expression of AP-2alpha increased the chemosensitivity of H460, a lung carcinoma cell line, to adriamycin (2.5-fold) and cisplatin (5-fold). Furthermore, the growth inhibition by AP-2alpha was found to be less efficient in the absence of p53 or p21, which correlated with reduced apoptosis in p53 null cells and lack of DNA synthesis inhibition in p21WAF1/CIP1 null cells by AP-2alpha, respectively. These results suggest that AP-2alpha inhibits the growth of cells by inducing cell cycle arrest and apoptosis and that the use of AP-2alpha should be explored as a therapeutic strategy either alone or in combination with chemotherapy.

Overexpression of transcription factor AP-2alpha suppresses mammary gland growth and morphogenesis

AP-2 transcription factors are key regulators of mouse embryonic development. Aberrant expression of these genes has also been linked to the progression of human breast cancer. Here, we have investigated the role of the AP-2 gene family in the postnatal maturation of the mouse mammary gland. Analysis of AP-2 RNA and protein levels demonstrates that these genes are expressed in the mammary glands of virgin and pregnant mice. Subsequently, AP-2 expression declines during lactation and then is reactivated during involution. The AP-2alpha and AP-2gamma proteins are localized in the ductal epithelium, as well as in the terminal end buds, suggesting that they may influence growth of the ductal network. We have tested this hypothesis by targeting AP-2alpha expression to the mouse mammary gland using the MMTV promoter. Our studies indicate that overexpression of AP-2alpha inhibits mammary gland growth and morphogenesis, and this coincides with a rise in PTHrP expression. Alveolar budding is severely curtailed in transgenic virgin mice, while lobuloalveolar development and functional differentiation are inhibited during pregnancy and lactation, respectively. Our studies strongly support a role for the AP-2 proteins in regulating the proliferation and differentiation of mammary gland epithelial cells in both mouse and human.

Identification of HTF (HER2 transcription factor) as an AP-2 (activator protein-2) transcription factor and contribution of the HTF binding site to ERBB2 gene overexpression

The ERBB2 gene is overexpressed in 30% of human breast cancers and this is correlated with poor prognosis. Overexpression of the ERBB2 gene is due to increased transcription and gene amplification. Our previous studies have identified a new cis element in the ERBB2 promoter which is involved in the gene's overexpression. This cis element, located 501 bp upstream from the main ERBB2 transcription initiation site, binds a transcription factor called HTF (HER2 transcription factor). We report here the identification of HTF as an AP-2 (activator protein-2) transcription factor. The new cis element is bound by AP-2 with high affinity, compared with a previously described AP-2 binding site located 284 bp downstream. Co-transfection of an AP-2alpha expression vector with a reporter vector containing the newly identified AP-2 binding site in front of a minimal ERBB2 promoter induced a dose-dependent increase in transcriptional activity. We examined the contribution of the new AP-2 binding site to ERBB2 overexpression. For this purpose we abolished the new and/or the previously described AP-2 binding sequence by site-directed mutagenesis. The results show that the two functional AP-2 sites in the first 700 bp of the ERBB2 promoter co-operate to achieve maximal transcriptional activity.

Auto/cross-regulation of Hoxb3 expression in posterior hindbrain and spinal cord

The complex and dynamic pattern of Hoxb3 expression in the developing hindbrain and the associated neural crest of mouse embryos is controlled by three separate cis-regulatory elements: element I (region A), element IIIa, and the r5 enhancer (element IVa). We have examined the cis-regulatory element IIIa by transgenic and mutational analysis to determine the upstream trans-acting factors and mechanisms that are involved in controlling the expression of the mouse Hoxb3 gene in the anterior spinal cord and hindbrain up to the r5/r6 boundary, as well as the associated neural crest which migrate to the third and posterior branchial arches and to the gut. By deletion analysis, we have identified the sequence requirements within a 482-bp element III482. Two Hox binding sites are identified in element III482 and we have shown that in vitro both Hoxb3 and Hoxb4 proteins can interact with these Hox binding sites, suggesting that auto/cross-regulation is required for establishing the expression of Hoxb3 in the neural tube domain. Interestingly, we have identified a novel GCCAGGC sequence motif within element III482, which is also required to direct gene expression to a subset of the expression domains except for rhombomere 6 and the associated neural crest migrating to the third and posterior branchial arches. Element III482 can direct a higher level of reporter gene expression in r6, which led us to investigate whether kreisler is involved in regulating Hoxb3 expression in r6 through this element. However, our transgenic and mutational analysis has demonstrated that, although kreisler binding sites are present, they are not required for the establishment or maintenance of reporter gene expression in r6. Our results have provided evidence that the expression of Hoxb3 in the neural tube up to the r5/r6 boundary is auto/cross-regulated by Hox genes and expression of Hoxb3 in r6 does not require kreisler.

Transcription factor AP-2γ is essential in the extra-embryonic lineages for early postimplantation development

The members of the AP-2 family of transcription factors play important roles during mammalian development and morphogenesis. AP-2γ (Tcfap2c – Mouse Genome Informatics) is a retinoic acid-responsive gene implicated in placental development and the progression of human breast cancer. We show that AP-2γ is present in all cells of preimplantation embryos and becomes restricted to the extra-embryonic lineages at the time of implantation. To study further the biological function of AP-2γ, we have generated Tcfap2c-deficient mice by gene disruption. The majority of Tcfap2c–/– mice failed to survive beyond 8.5 days post coitum (d.p.c.). At 7.5 d.p.c., Tcfap2c–/– mutants were typically arrested or retarded in their embryonic development in comparison to controls. Morphological and molecular analyses of mutants revealed that gastrulation could be initiated and that anterior-posterior patterning of the epiblast remained intact. However, the Tcfap2c mutants failed to establish a normal maternal-embryonic interface, and the extra-embryonic tissues were malformed. Moreover, the trophoblast-specific expression of eomesodermin and Cdx2, two genes implicated in FGF-responsive trophoblast stem cell maintenance, was significantly reduced. Chimera studies demonstrated that AP-2γ plays no major autonomous role in the development of the embryo proper. By contrast, the presence of AP-2γ in the extra-embryonic membranes is required for normal development of this compartment and also for survival of the mouse embryo.

AP-2 gamma and the homeodomain protein distal-less 3 are required for placental-specific expression of the murine 3 beta-hydroxysteroid dehydrogenase VI gene, Hsd3b6

The enzyme 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) is essential for the biosynthesis of all active steroid hormones. It exists as multiple isoforms in humans and rodents, each the product of a distinct gene. Human 3 beta-HSD I in placenta is essential for placental progesterone biosynthesis and thus is essential for the maintenance of pregnancy. The murine ortholog, 3 beta-HSD VI, is the only isoform expressed in giant trophoblast cells during the first half of mouse pregnancy. This study was designed to identify the cis-acting element(s) and the associated transcription factors required for trophoblast-specific expression of 3 beta-HSD VI. Transfection studies in placental and nonplacental cells identified a novel 66-bp trophoblast-specific enhancer element located between -2896 and -2831 of the 3 beta-HSD VI promoter. DNase protection analysis of the enhancer element identified three trophoblast-specific binding sites, FPI, FPII, and FPIII. Electrophoretic mobility shift assays with oligonucleotides representing the protected sequences, FPI and FPIII, and nuclear extracts isolated from human JEG-3 cells and from mouse trophoblast cells, demonstrated the same binding pattern that was distinct from the binding pattern with mouse Leydig cell nuclear proteins. Further electrophoretic mobility shift assays identified AP-2 gamma and the homeodomain protein, Dlx 3, as the transcription factors that specifically bind to FPI and FPIII, respectively. Site-specific mutations in each of the binding sites eliminated enhancer activity indicating that AP-2 gamma and Dlx 3, together with an additional transcription factor(s) that are conserved between humans and mice, are required for trophoblast-specific expression of 3 beta-HSD VI.

Inhibitory module of Ets-1 allosterically regulates DNA binding through a dipole-facilitated phosphate contact

DNA binding of the transcription factor Ets-1 is negatively regulated by three inhibitory helices that lie near the ETS domain. The current model suggests that this negative regulation, termed autoinhibition, is caused by the energetic expense of a DNA-induced structural transition that includes the unfolding of one inhibitory helix. This report investigates the role of helix H1 of the ETS domain in the autoinhibition mechanism. Previous structural studies modeled the inhibitory helices packing together and connecting with helix H1, suggesting a role of this helix in the configuration of an inhibitory module. Recently, high-resolution structures of the ETS domain-DNA interface indicate that the N terminus of helix H1 directly contacts DNA. The contact, which is augmented by the macrodipole of helix H1, consists of a hydrogen bond between the amide NH of leucine 337 in helix H1 and the oxygen of a corresponding phosphate. We propose that this hydrogen bond positions helix H1 to be a link between autoinhibition and DNA binding. Four independent approaches tested this hypothesis. First, the hydrogen bond was disrupted by removal of the phosphate in a missing phosphate analysis. Second, base pairs that surround the helix H1-contacting phosphate and appear to dictate DNA backbone conformation were mutated. Next, a hydrophobic residue in helix H1 that is expected to position the N terminus of the helix was altered. Finally, a residue on the surface of helix H1 that may contact the inhibitory elements was changed. In each case DNA binding and autoinhibition was affected. Taken together, the results demonstrate the role of the dipole-facilitated phosphate contact in DNA binding. Furthermore, the findings support a model in which helix H1 links the inhibitory elements to the ETS domain. We speculate that this helix, which is conserved in all Ets proteins, provides a common route to regulation.

Cloning and characterization of a novel human transcription factor AP-2 beta like gene (TFAP2BL1)

The AP-2 transcription factor has been shown to play an important role in development, morphogenesis, apoptosis, cell-cycle control and has also been implicated in mammary oncogenesis. Here we report the cloning and characterization of a novel human transcription factor AP-2 like gene (TFAP2BL1), which is located on human chromosome 6p12.1-21.1. The TFAP2BL1 cDNA is 2076 base pairs in length, encoding a 452-amino acid polypeptide related to human Ap-2protein. TFAP2BL1 gene has significantly high homology to transcription factor AP-2 gene of human, mouse, chicken, sheep, fruit fly, and C. elegans at amino acid level. RT-PCR analysis shows its relatively high expression level in adult thymus, prostate, small intestine, skeletal muscle, placenta, brain, and testis tissues.

Cloning and characterization of a novel mouse AP-2 transcription factor, AP-2delta, with unique DNA binding and transactivation properties

AP-2 transcription factors are sequence-specific DNA-binding proteins expressed in neural crest and other tissues during mammalian development. Three mammalian genes, AP-2alpha, AP-2beta, and AP-2gamma, have been reported previously. A partial predicted AP-2 gene was identified in tandem with AP-2beta on human chromosome 6p12-p21.1. The orthologous mouse gene, which we named Ap-2delta, was identified from a fetal mouse head cDNA library. Northern analysis revealed two transcripts in embryonic and newborn mouse brain, with markedly higher steady-state levels in the former. The predicted Ap-2delta protein comprised 452 amino acids and was highly similar to other AP-2 proteins across the DNA-binding and dimerization domains. Ap-2delta formed homodimers and heterodimers in vitro, bound an optimized AP-2 consensus DNA sequence, and transactivated gene expression in eukaryotic cells. Ap-2delta dimers bound poorly to an AP-2 binding sequence from the human metallothionein IIa promoter in vitro, revealing a sequence specificity not previously observed among other AP-2 proteins. The PY motif and critical residues in the transactivation domain, which are highly conserved in the AP-2 family and believed necessary for transactivation, were divergent in Ap-2delta. The unique protein sequence and functional features of Ap-2delta suggest mechanisms, besides tissue-specific AP-2 gene expression, for specific control of target gene activation.

Developmental regulation and overexpression of the transcription factor AP-2, a potential regulator of the timing of Schwann cell generation

There is now evidence from in vivo and in vitro studies that the rate of Schwann cell generation is regulated by the balance of two opposing signals, beta neuregulins and endothelins. The beta neuregulins promote the development of precursors to Schwann cells whereas endothelins retard it through an action on endothelin-B receptors. The present work has shown additional controls of this transition, and implicates AP-2 transcription factors, in particular AP-2 alpha, as negative regulators of Schwann cell generation. We found that both AP-2 alpha and AP-2 gamma are present in early embryonic nerves, whereas AP-2 beta was not. Isoform-specific analysis of AP-2 alpha showed that isoform 3 was most abundant with isoforms 1 and 2 present in lesser amounts; isoform 4 was absent. Maximal AP-2 alpha and AP-2 gamma mRNA expression occurred at embryonic day (E) 12/13 in the mouse and at E14/15 in the rat, which correlates with the presence of Schwann cell precursors in the nerve. In both rats and in mice, in vivo and in vitro, downregulation of AP-2 alpha mRNA and protein coincided with one of the main steps in Schwann cell development, the precursor-Schwann cell transition. Moreover, Schwann cell generation was delayed if this downregulation was prevented by enforced expression of AP-2 alpha in precursors. These studies suggest that AP-2 is involved in the control of the timing of Schwann cell development.

Regulatory roles of AP-2 transcription factors in vertebrate development, apoptosis and cell-cycle control

AP-2 transcription factors represent a family of three closely related and evolutionarily conserved sequence-specific DNA-binding proteins, AP-2alpha, -beta and -gamma. Subsequent studies have identified spatially and temporally regulated embryonic expression patterns in a number of different tissues including neural crest derivatives, neural, epidermal and urogenital tissues. Here, we review the current understanding of developmental defects in AP-2-deficient mice and consider regulatory functions of AP-2 in control of apoptosis, cell cycle, and gene expression. Recently, the first inherited human disorder, Char syndrome, was identified to be caused by AP-2beta missense mutations. In light of the manifold and essential functions of AP-2 proteins in cell growth, differentiation and programmed death, mutations or changes in precisely programmed expression patterns are likely to contribute to other congenital malformations or neoplastic diseases.

Identification of amino acid residues of transcription factor AP-2 involved in DNA binding

AP-2 is a cell-type specific, developmentally regulated transcription factor which has been described as a critical regulator of gene expression during vertebrate development and embryogenesis. Although the overall domains of this factor necessary for their activity have been identified, the exact identity of AP-2 amino acid residues responsible for its interaction with the DNA structure has not yet been described. Here, we describe the identification of a region of AP-2 which was protected by an oligonucleotide probe containing its binding site from trypsin digestion, monitored by peptide mapping by MALDI-TOF mass spectrometry. Furthermore, we analyzed the relative in vitro DNA-binding activity, the stimulatory potency on the AP-2-dependent APOE promoter, as well as the ability to inhibit the effect of the wild-type protein of each one of a set of single-site substitution AP-2 mutants spanning the identified region. Taken together, our data clearly demonstrate that the region between amino acid residues 252-260 of AP-2 is essential for its DNA-binding activity. Particularly, the individual substitution in any of the residues 253, 254, 255, 257 or 260 is sufficient for completely abolishing the interaction with DNA and the stimulation of APOE promoter activity. These results indicate a crucial role of this region in the formation of an active DNA-binding domain and strongly suggest that these residues provide direct contacts with the DNA structure at the AP-2 binding site.

Experimental analysis and computer prediction of CTF/NFI transcription factor DNA binding sites

Accurate prediction of transcription factor binding sites is needed to unravel the function and regulation of genes discovered in genome sequencing projects. To evaluate current computer prediction tools, we have begun a systematic study of the sequence-specific DNA-binding of a transcription factor belonging to the CTF/NFI family. Using a systematic collection of rationally designed oligonucleotides combined with an in vitro DNA binding assay, we found that the sequence specificity of this protein cannot be represented by a simple consensus sequence or weight matrix. For instance, CTF/NFI uses a flexible DNA binding mode that allows for variations of the binding site length. From the experimental data, we derived a novel prediction method using a generalised profile as a binding site predictor. Experimental evaluation of the generalised profile indicated that it accurately predicts the binding affinity of the transcription factor to natural or synthetic DNA sequences. Furthermore, the in vitro measured binding affinities of a subset of oligonucleotides were found to correlate with their transcriptional activities in transfected cells. The combined computational-experimental approach exemplified in this work thus resulted in an accurate prediction method for CTF/NFI binding sites potentially functioning as regulatory regions in vivo.