Involvement of AP-2 in regulation of the R-FABP gene in the developing chick retina

Conditional Deletion of AP-2α and AP-2β in the Developing Murine Retina Leads to Altered Amacrine Cell Mosaics and Disrupted Visual Function

Purpose

The combined action of the activating protein-2 (AP-2) transcription factors, AP-2α and AP-2β, is important in early retinal development, specifically in the formation of horizontal cells. However, in previous studies, it was not possible to analyze postnatal development and function of additional retinal subtypes.

Methods

We used a double conditional deletion of AP-2α and AP-2β from the retina to further examine the combinatory role of these genes in retinal cell patterning and function in postnatal adult mice as measured by Voronoi domain area and nearest-neighbor distance spatial analyses and ERGs, respectively.

Results

Conditional deletion of both AP-2α and AP-2β from the retina resulted in a variety of abnormalities, including the absence of horizontal cells, defects in the photoreceptor ribbons in which synapses failed to form, along with evidence of aberrant amacrine cell arrangement. Although no significant changes in amacrine cell population numbers were observed in the double mutants, significant irregularities in the mosaic patterning of amacrine cells was observed as demonstrated by both Voronoi domain areas and nearest-neighbor distances analyses. These changes were further accompanied by an alteration in the retinal response to light as recorded by ERGs. In particular, in the double-mutant mice lacking AP-2α and AP-2β, the b-wave amplitude, representative of interneuron signal processing, was significantly reduced compared with control littermates.

Conclusions

Together these findings demonstrate the requirement for both AP-2α and AP-2β in proper amacrine mosaic patterning and a normal functional light response in the retina.

Beyond MITF: Multiple transcription factors directly regulate the cellular phenotype in melanocytes and melanoma

MITF governs multiple steps in the development of melanocytes, including specification from neural crest, growth, survival, and terminal differentiation. In addition, the level of MITF activity determines the phenotype adopted by melanoma cells, whether invasive, proliferative, or differentiated. However, MITF does not act alone. Here, we review literature on the transcription factors that co-regulate MITF-dependent genes. ChIP-seq studies have indicated that the transcription factors SOX10, YY1, and TFAP2A co-occupy subsets of regulatory elements bound by MITF in melanocytes. Analyses at single loci also support roles for LEF1, RB1, IRF4, and PAX3 acting in combination with MITF, while sequence motif analyses suggest that additional transcription factors colocalize with MITF at many melanocyte-specific regulatory elements. However, the precise biochemical functions of each of these MITF collaborators and their contributions to gene expression remain to be elucidated. Analogous to the transcriptional networks in morphogen-patterned tissues during embryogenesis, we anticipate that the level of MITF activity is controlled not only by the concentration of activated MITF, but also by additional transcription factors that either quantitatively or qualitatively influence the expression of MITF-target genes.

AP-2ε Expression in Developing Retina: Contributing to the Molecular Diversity of Amacrine Cells

AP-2 transcription factors play important roles in the regulation of gene expression during development. Four of the five members of the AP-2 family (AP-2α, AP-2β, AP-2γ and AP-2δ) have previously been shown to be expressed in developing retina. Mouse knockouts have revealed roles for AP-2α, AP-2β and AP-2δ in retinal cell specification and function. Here, we show that the fifth member of the AP-2 family, AP-2ε, is also expressed in amacrine cells in developing mammalian and chicken retina. Our data indicate that there are considerably fewer AP-2ε-positive cells in the developing mouse retina compared to AP-2α, AP-2β and AP-2γ-positive cells, suggesting a specialized role for AP-2ε in a subset of amacrine cells. AP-2ε, which is restricted to the GABAergic amacrine lineage, is most commonly co-expressed with AP-2α and AP-2β, especially at early stages of retinal development. Co-expression of AP-2ε and AP-2γ increases with differentiation. Analysis of previously published Drop-seq data from single retinal cells supports co-expression of multiple AP-2s in the same cell. Since AP-2s bind to their target sequences as either homodimers or heterodimers, our work suggests spatially- and temporally-coordinated roles for combinations of AP-2 transcription factors in amacrine cells during retinal development.

Regulation of the HMOX1 gene by the transcription factor AP-2δ with unique DNA binding site

AP-2 transcription factors are important sequence-specific DNA-binding regulators that are expressed in the neural crest and other tissues during mammalian development. The human AP-2 family of transcription factors consists of five members, AP-2α, -β, -γ, -δ and -ε, which have an important role in the regulation of gene expression during development and in the differentiation of multiple organs and tissues. The present study aimed to investigate the mechanism by which AP-2δ mediates heme oxygenase-1 (HMOX1) gene expression. It was identified that the human AP-2δ protein exhibited weak binding to a suboptimal AP-2 sequence, 5'-GCCN3GGC-3', to which all other AP-2 proteins bind in vitro, providing the first example of DNA target specificity amongst the AP-2 family. AP-2δ protein bound to an optimized AP-2 consensus DNA sequence, 5'-GCCTGAGGC-3', in vitro and transactivated gene expression in eukaryotic cells. The transactivation domain of Ap-2δ differs notably from those in the other AP-2 proteins as it lacks the PY motif (XPPXY) and several other conserved residues that are important for the transcriptional activity of AP-2 proteins, yet it functions as an equally strong activator.

Eyes on DNA methylation: current evidence for DNA methylation in ocular development and disease

Epigenetic modulation of chromatin states constitutes a vital component of the cellular repertoire of transcriptional regulatory mechanisms. The development of new technologies capable of generating genome-wide maps of chromatin modifications has re-energized the field. We are now poised to determine how species- and tissue-specific patterns of DNA methylation, in concert with other chromatin modifications, function to establish and maintain cell- and tissue-specific patterns of gene expression during normal development, cellular differentiation, and disease. This review addresses our current understanding of the major mechanisms and function of DNA methylation in vertebrates with a historical perspective and an emphasis on what is known about DNA methylation in eye development and disease.

Expression of AP-2delta in the developing chick retina

AP-2 is a family of transcription factors that play important roles during embryonic development. Two AP-2 genes, AP-2alpha and AP-2beta, have previously been characterized in chick retina. Here, we demonstrate that a third member of the chicken AP-2 family, AP-2delta, is primarily expressed in the retina and brain, with highest levels at embryonic days 7 to 11. By in situ hybridization and immunohistochemical analysis, we show that AP-2delta RNA and protein are found in a subset of ganglion cells in embryonic chick retina. Co-immunostaining with anti-Brn3a and anti-AP-2delta antibodies indicates that approximately one-third of Brn3a-positive ganglion cells express AP-2delta. AP-2delta RNA but not AP-2delta protein is also found in cells located in the outer half of the inner nuclear layer. The spatial and temporal distribution of AP-2delta protein in the retina suggests a transient role in a subset of late-born ganglion cells likely involving axonal trafficking or pathfinding.

Proteome profiling of embryo chick retina

Background

Little is known regarding the molecular pathways that underlie the process of retinal development. The purpose of this study was to identify proteins which may be involved in development of retina. We used a proteomics-based approach to identify proteins that are up- or down-regulated during the development of the embryo chick retina.

Results

Two-dimensional gel electrophoresis was performed with the retina of embryo chicken, which was obtained from embryos of day 7 (ED7) and of day 11 (ED11). The protein spots showing significant differences were selected for identification by MALDI mass spectrometry. Thirteen proteins were differentially expressed; seven proteins were up-regulated in embryo retina of chicken at ED 11 and six proteins were down-regulated. Significant proteins were also evaluated in embryo day 15 (ED15). Some of identified proteins were known to regulate cell proliferation, cell death, transport, metabolism, organization and extracellular matrix, and others also included novel proteins.

Conclusion

We identified thirteen proteins which differentially expressed in embryonal retina of chicken at day 7, as compared to the retina of embryo of day 11. They were various regulatory proteins for cellular signaling.

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.

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.

Transcription factor AP-2 functions as a repressor that contributes to the liver-specific expression of serum amyloid A1 gene

We previously identified transcription factor AP-2 as the nuclear factor that interacts with the tissue-specific repressor element in the rat serum amyloid A1 (SAA1) promoter. In this report, we provide evidence for a second AP-2-binding site and show that both AP-2 sites participate in mediating the transcription repression of SAA1 promoter. This proximal AP-2 site overlaps with the NFkappaB-binding site known to be essential for SAA1 promoter activity. Protein binding competition experiments demonstrated that AP-2 and NFkappaB binding to these overlapping sites were mutually exclusive. Furthermore, the addition of AP-2 easily displaced prebound NFkappaB, whereas NFkappaB could not displace AP-2. These results thus suggest that one mechanism by which AP-2 negatively regulates SAA1 promoter activity may be by antagonizing the function of NFkappaB. Consistent with a repression function, transient expression of AP-2 in HepG2 cells inhibited conditioned medium-induced SAA1 promoter activation. This inhibition was dependent on functional AP-2-binding sites, since mutation of AP-2-binding sites abolished inhibitory effects of AP-2 in HepG2 cells as well as resulted in derepression of the SAA1 promoter in HeLa cells. In addition to SAA1, we found that several other liver gene promoters also contain putative AP-2-binding sites. Some of these sequences could specifically inhibit AP-2.DNA complex formation, and for the human complement C3 promoter, overexpression of AP-2 also could repress its cytokine-mediated activation. Finally, stable expression of AP-2 in hepatoma cells significantly reduced the expression of endogenous SAA, albumin, and alpha-fetoprotein genes. Taken together, our results suggest that AP-2 may function as a transcription repressor to inhibit the expression of not only SAA1 gene but also other liver genes in nonhepatic cells.

Expression of the human poliovirus receptor/CD155 gene during development of the central nervous system: implications for the pathogenesis of poliomyelitis

The gene for the human poliovirus receptor (hPVR/CD155) is the founding member of a new family of genes encoding proteins belonging to the immunoglobulin superfamily. To determine whether CD155 is expressed during mammalian development, we have made use of the previously characterized promoter of the CD155 gene and generated mice transgenic for a CD155 promoter-driven beta-galactosidase reporter gene. Expression of the reporter gene in transgenic embryos was observed during midgestation in anterior midline structures of the developing central nervous system and in the neuroretina. During that period, reporter gene expression appeared within the notochord and floor plate along the entire spinal cord reaching into the caudal diencephalon. In addition, transgene expression was observed in axonal projections emanating from retinal ganglion cells forming the optic nerve to reach the future region of the optic chiasm. Analysis of expression of CD155 during human embryonic development confirmed the distribution of reporter gene expression specified by CD155 promoter activity. The anatomical distribution of CD155 promoter activity during embryogenesis matches that of transacting factors previously identified to regulate transcription of the CD155 gene. Expression of CD155 within embryonic structures giving rise to spinal cord anterior horn motor neurons may explain the restrictive host cell tropism of poliovirus for this cellular compartment of the CNS.

The repressive function of AP2 transcription factor on the hepatocyte growth factor gene promoter

Hepatocyte growth factor is an important multifunctional growth factor whose gene expression is tightly regulated at the transcriptional level. Previous studies from our laboratory have shown that several cis-acting elements are present in the promoter and proximal promoter region of the HGF gene. In this study, we have uncovered that AP2 transcription factor specifically binds to a regulatory site located at -230 to -260 in the upstream region of the HGF gene promoter. Gelshift and supershift assays confirmed that AP2 has high binding affinity to this region. Functional studies which introduced a mutation in the AP2 core binding region as well as cotransfection experiments using an AP2 expression vector revealed that AP2 exerts a repressive role on the HGF gene promoter activity. The AP2 binding site overlaps with those of NF1 and USF/E-box binding sites which we have recently shown to constitute a composite multifunctional docking site for the members of the NF1 and USF transcription factor families. An inverse correlation was noted between AP2 binding activity to this composite site and HGF gene expression in different cell lines. Therefore, AP2-mediated repression of the HGF gene promoter may be part of the molecular mechanism responsible for regulating HGF expression.

Ectopic expression of luteinizing hormone-releasing hormone and peripherin in the respiratory epithelium of mice lacking transcription factor AP-2alpha

The vertebrate transcription factor activator protein-2 (AP-2alpha) is involved in craniofacial morphogenesis. In the nasal placode AP-2alpha expression delineates presumptive respiratory epithelia from olfactory epithelia, with AP-2alpha expression restricted to the anterior region of the respiratory epithelium (absent from the olfactory epithelium) at later stages. To address the role AP-2alpha plays in differentiation of cell groups in the nasal placode, the spatiotemporal expression pattern of four markers normally associated with olfactory epithelial structures was analyzed in mice lacking AP-2alpha. These markers were the intermediate filament protein peripherin, the neuropeptide luteinizing hormone-releasing hormone (LHRH), the neural cell adhesion molecule (NCAM) and the olfactory transcription factor Olf-1. Development of cells expressing these markers was similar in both genotypes until embryonic day 12.5 (E12.5), indicating that the main olfactory epithelium and olfactory pit formation was normal. At E13.5 in mutant mice, ectopic LHRH neurons and peripherin axons were detected in respiratory epithelial areas, areas devoid of Olf-1 and NCAM staining. Over the next few days, an increase in total nasal LHRH neurons occurred. The increase in nasal LHRH neurons could be accounted for by LHRH neurons arising and migrating out of respiratory epithelial regions on peripherin-positive fibers. These results indicate that AP-2alpha is not essential for the separation of the olfactory and respiratory epithelium from the nasal placode and is consistent with AP-2alpha preventing recapitulation of developmental programs within the respiratory epithelium that lead to expression of LHRH and peripherin phenotypes.

Purification and identification of a tissue-specific repressor involved in serum amyloid A1 gene expression

We have previously demonstrated that the 5'-flanking regions from the rat serum amyloid A1 (SAA1) promoter are necessary and sufficient to confer specific cytokine-induced expression in cultured hepatoma cells. Deletion analysis identified a tissue-specific repressor (TSR) regulatory element, located between bp -289 and -256, that functioned as a silencer, contributing to the transcription repression on SAA1 promoter in nonliver cells. When this 34-base pair TSR-binding element was used as a probe in electrophoretic mobility shift assays, an intense DNA-protein complex was detected in nuclear extracts from HeLa and several other nonliver tissues. This TSR binding activity, however, was undetectable in extracts from liver or liver-derived cells. The distribution of TSR binding activity is therefore consistent with its regulatory role in repressing SAA1 expression in a tissue-specific manner. In this study, we purified TSR protein from HeLa nuclear extracts and showed that it has a molecular mass of approximately 50 kDa. Surprisingly, protein sequencing and antibody supershift experiments identified TSR as transcription factor AP-2. Subsequent functional analysis showed that forced expression of AP-2 in HepG2 cells could indeed inhibit conditioned medium-induced SAA1 promoter activation. Moreover, expression of a dominant-negative mutant of AP-2 in HeLa cells or mutation of the AP-2-binding site led to derepression of the SAA1 promoter, presumably by neutralizing the inhibitory effects of the endogenous wild-type AP-2. Our results therefore demonstrate a novel function for AP-2 in the transcriptional repression of SAA1 promoter. Together with its tissue distribution, AP-2 may contribute to SAA1's highly liver-specific expression pattern by restricting its expression in nonliver cells.

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

The AP-2 family of transcription factors are defined by the presence of a novel DNA binding domain, termed a 'basic helix-span-helix' motif. The AP-2 genes regulate important aspects of vertebrate embryogenesis and have also been linked to the control of cell proliferation and tumorigenesis, but the cellular targets that the AP-2 proteins control are largely undefined. In particular, since only a limited number of sequences have previously been utilized to define the nature of the AP-2 binding site, the range of DNA sequences recognized by the AP-2 proteins remains unknown. We have therefore utilized a SELEX analysis to identify multiple new AP-2[alpha] binding sites. Moreover, we have devised a novel missing phosphate and nucleotide competition analysis to characterize the residues in the binding site required for AP-2[alpha] protein:DNA contact. These studies suggest that the AP-2[alpha] protein:DNA complex is flexible and indicate that AP-2[alpha] can bind three related sequence motifs: GCC N3 GGC, GCC N4 GGC and GCC N3/4 GGG. The availability of these more refined consensus sequences should assist in the identification of target genes for this critical transcription factor.

Transcription factor AP-2 mRNA and DNA binding activity are constitutively expressed in SV40-immortalized but not normal human lung fibroblasts

Large T antigen (LT) expressed by the oncogenic DNA virus SV40 transforms cells by interacting with and perturbing the normal function of several important cellular proteins including P53, RB, c-MYC, and AP-2. AP-2 binds to regulatory elements within the SV40 enhancer and is therefore of particular interest for mechanisms relating to viral transcription, replication, and packaging. LT antigen has been previously shown to inhibit transcription factor AP-2 from binding to its cognate cis-element in DNA in vitro, and this is believed to occur through a direct physical interaction between the LT and AP-2 proteins. Recently LT and AP-2 were shown to interact at the protein level in vivo and this interaction appeared to mediated by the RB protein. Although LT inhibited AP-2 DNA binding in vitro, the effects of LT on AP-2 expression and DNA binding activity in vivo have not been previously reported. We report here that transcription factor AP-2alpha is constitutively expressed in SV40-transformed cells compared to their normal cell counterparts. The overexpression of AP-2alpha in SV40 transformed cells occurred at the levels of mRNA, protein, and DNA binding activity. The increase in AP-2 DNA binding in vivo was particularly interesting since previous studies in vitro would have predicted that AP-2 DNA binding should be decreased in the presence of LT. AP-2 is a plieotropic regulator of gene expression, activating some and repressing others. Thus, increased cellular AP-2 activity may be an important downstream effector for the transforming ability of SV40.

Differential expression of AP-2alpha and AP-2beta in the developing chick retina: repression of R-FABP promoter activity by AP-2

Retinal fatty acid binding protein (R-FABP) is the avian counterpart of murine brain FABP implicated in glial cell differentiation and neuronal cell migration. R-FABP is highly expressed in the undifferentiated retina and brain of chick embryos. We have previously shown by in vitro studies that the AP-2 transcription factor binds to a consensus AP-2 binding site in the R-FABP promoter region. Based on the expression pattern of AP-2 in the developing retina and on mutational analysis of the AP-2 binding site in DNA transfection experiments, we proposed that AP-2 could be involved in the down-regulation of R-FABP transcription. Here, we describe the cDNA isolation of two members of the AP-2 family expressed in the chick retina, AP-2alpha and AP-2beta. We show that R-FABP mRNA and the AP-2 factors are expressed in mutually exclusive patterns in the differentiating retina: whereas AP-2alpha and AP-2beta are selectively expressed either in amacrine, or in amacrine and horizontal cells, respectively, R-FABP mRNAis found in Muller glial cells and/or bipolar cells. Furthermore, a decrease in R-FABP-dependent expression is obtained upon cotransfection of primary retinal cultures with AP-2 expression vectors and a CAT reporter construct. The early and cell-specific expression of AP-2alpha and AP-2beta in the developing retina suggest a role for this transcription factor family in the early steps of amacrine and horizontal cell differentiation. Repression of the R-FABP gene in these cells may be an important component of their developmental program.

Derepression of the C/EBPalpha gene during adipogenesis: identification of AP-2alpha as a repressor

During adipogenesis, CCAAT/enhancer binding protein alpha (C/EBPalpha) serves as a pleiotropic transcriptional activator of adipocyte genes. Previously, we identified dual repressive elements in the C/EBPalpha gene and a putative transacting factor (C/EBPalpha undifferentiated protein, or CUP) expressed by preadipocytes, but not adipocytes, that bind to these elements. In the present investigation, CUP was purified 17,000-fold from nuclear extracts of 3T3-L1 preadipocytes. Amino acid sequence and mass spectral analysis of tryptic peptides derived from purifed CUP (molecular mass approximately 50 kDa) revealed that the repressor is (or contains) an isoform of the transcription factor, AP-2alpha. Electrophoretic mobility shift and Western blot analysis on purified CUP and preadipocyte nuclear extracts confirmed the identity of CUP as AP-2alpha. Both AP-2alpha protein and CUP binding activity are expressed by preadipocytes and then decrease concomitantly during differentiation of 3T3-L1 preadipocytes into adipocytes. Consistent with a repressive role of AP-2alpha/CUP, an AP-2alpha1 expression vector, cotransfected with a C/EBPalpha promoter-reporter construct into 3T3-L1 adipocytes, inhibited reporter gene transcription. Taken together with previous results, these findings suggest that in preadipocytes the C/EBPalpha gene is repressed by AP-2alpha/CUP, which, upon induction of differentiation, is down-regulated, allowing expression of the gene.