Fibroblast growth factor-8 expression is regulated by intronic engrailed and Pbx1-binding sites

Differential modularity of the mammalian Engrailed 1 enhancer network directs sweat gland development

Enhancers are context-specific regulators of expression that drive biological complexity and variation through the redeployment of conserved genes. An example of this is the enhancer-mediated control of Engrailed 1 (EN1), a pleiotropic gene whose expression is required for the formation of mammalian eccrine sweat glands. We previously identified the En1 candidate enhancer (ECE) 18 cis-regulatory element that has been highly and repeatedly derived on the human lineage to potentiate ectodermal EN1 and induce our species' uniquely high eccrine gland density. Intriguingly, ECE18 quantitative activity is negligible outside of primates and ECE18 is not required for En1 regulation and eccrine gland formation in mice, raising the possibility that distinct enhancers have evolved to modulate the same trait. Here we report the identification of the ECE20 enhancer and show it has conserved functionality in mouse and human developing skin ectoderm. Unlike ECE18, knock-out of ECE20 in mice reduces ectodermal En1 and eccrine gland number. Notably, we find ECE20, but not ECE18, is also required for En1 expression in the embryonic mouse brain, demonstrating that ECE20 is a pleiotropic En1 enhancer. Finally, that ECE18 deletion does not potentiate the eccrine phenotype of ECE20 knock-out mice supports the secondary incorporation of ECE18 into the regulation of this trait in primates. Our findings reveal that the mammalian En1 regulatory machinery diversified to incorporate both shared and lineage-restricted enhancers to regulate the same phenotype, and also have implications for understanding the forces that shape the robustness and evolvability of developmental traits.

The TALE never ends: A comprehensive overview of the role of PBX1, a TALE transcription factor, in human developmental defects

PBX1 is a highly conserved atypical homeodomain transcription factor (TF) belonging to the TALE (three amino acid loop extension) family. Dimerized with other TALE proteins, it can interact with numerous partners and reach dozens of regulating sequences, suggesting its role as a pioneer factor. PBX1 is expressed throughout the embryonic stages (as early as the blastula stage) in vertebrates. In human, PBX1 germline variations are linked to syndromic renal anomalies (CAKUTHED). In this review, we summarized available data on PBX1 functions, PBX1-deficient animal models, and PBX1 germline variations in humans. Two types of genetic alterations were identified in PBX1 gene. PBX1 missense variations generate a severe phenotype including lung hypoplasia, cardiac malformations, and sexual development defects (DSDs). Conversely, truncating variants generate milder phenotypes (mainly cryptorchidism and deafness). We suggest that defects in PBX1 interactions with various partners, including proteins from the HOX (HOXA7, HOXA10, etc.), WNT (WNT9B, WNT3), and Polycomb (BMI1, EED) families are responsible for abnormal proliferation and differentiation of the embryonic mesenchyme. These alterations could explain most of the defects observed in humans. However, some phenotype variability (especially DSDs) remains poorly understood. Further studies are needed to explore the TALE family in greater depth.

The regulation of PBXs and their emerging role in cancer

Pre‐B‐cell leukaemia transcription factor (PBX) proteins are a subfamily of evolutionarily conserved, atypical homeodomain transcription factors that belong to the superfamily of three amino acid loop extension (TALE) homeodomain proteins. Members of the PBX family play crucial roles in regulating multiple pathophysiological processes, such as the development of organs, congenital cardiac defects and carcinogenesis. The dysregulation of PBXs has been shown to be closely associated with many diseases, particularly cancer. However, the detailed mechanisms of PBX dysregulation in cancer progression are still inconclusive. In this review, we summarize the recent advances in the structures, functions and regulatory mechanisms of PBXs, and discuss their underlying mechanisms in cancer progression. We also highlight the great potential of PBXs as biomarkers for the early diagnosis and prognostic evaluation of cancer as well as their therapeutic applications. The information reviewed here may expand researchers’ understanding of PBXs and could strengthen the clinical implication of PBXs in cancer treatment.

Delta-like ligand 4-mediated Notch signaling controls proliferation of second heart field progenitor cells by regulating Fgf8 expression

The role played by the Notch pathway in cardiac progenitor cell biology remains to be elucidated. Delta-like ligand 4 (Dll4), the arterial-specific Notch ligand, is expressed by second heart field (SHF) progenitors at time-points that are crucial in SHF biology. Dll4-mediated Notch signaling is required for maintaining an adequate pool of SHF progenitors, such that Dll4 knockout results in a reduction in proliferation and an increase in apoptosis. A reduced SHF progenitor pool leads to an underdeveloped right ventricle (RV) and outflow tract (OFT). In its most severe form, there is severe RV hypoplasia and poorly developed OFT resulting in early embryonic lethality. In its milder form, the OFT is foreshortened and misaligned, resulting in a double outlet right ventricle. Dll4-mediated Notch signaling maintains Fgf8 expression by transcriptional regulation at the promoter level. Combined heterozygous knockout of Dll4 and Fgf8 demonstrates genetic synergy in OFT alignment. Exogenous supplemental Fgf8 rescues proliferation in Dll4 mutants in ex-vivo culture. Our results establish a novel role for Dll4-mediated Notch signaling in SHF biology. More broadly, our model provides a platform for understanding oligogenic inheritance that results in clinically relevant OFT malformations.

'Building a perfect body': control of vertebrate organogenesis by PBX-dependent regulatory networks

Pbx genes encode transcription factors that belong to the TALE (three-amino-acid loop extension) superclass of homeodomain proteins. We have witnessed a surge in information about the roles of this gene family as leading actors in the transcriptional control of development. PBX proteins represent a clear example of how transcription factors can regulate developmental processes by combinatorial properties, acting within multimeric complexes to implement activation or repression of transcription depending on their interaction partners. Here, we revisit long-emphasized functions of PBX transcription factors as cofactors for HOX proteins, major architects of the body plan. We further discuss new knowledge on roles of PBX proteins in different developmental contexts as upstream regulators of Hox genes-as factors that interact with non-HOX proteins and can work independently of HOX-as well as potential pioneer factors. Committed to building a perfect body, PBX proteins govern regulatory networks that direct essential morphogenetic processes and organogenesis in vertebrate development. Perturbations of PBX-dependent networks can cause human congenital disease and cancer.

Crosstalk of Intercellular Signaling Pathways in the Generation of Midbrain Dopaminergic Neurons In Vivo and from Stem Cells

Dopamine-synthesizing neurons located in the mammalian ventral midbrain are at the center stage of biomedical research due to their involvement in severe human neuropsychiatric and neurodegenerative disorders, most prominently Parkinson’s Disease (PD). The induction of midbrain dopaminergic (mDA) neurons depends on two important signaling centers of the mammalian embryo: the ventral midline or floor plate (FP) of the neural tube, and the isthmic organizer (IsO) at the mid-/hindbrain boundary (MHB). Cells located within and close to the FP secrete sonic hedgehog (SHH), and members of the wingless-type MMTV integration site family (WNT1/5A), as well as bone morphogenetic protein (BMP) family. The IsO cells secrete WNT1 and the fibroblast growth factor 8 (FGF8). Accordingly, the FGF8, SHH, WNT, and BMP signaling pathways play crucial roles during the development of the mDA neurons in the mammalian embryo. Moreover, these morphogens are essential for the generation of stem cell-derived mDA neurons, which are critical for the modeling, drug screening, and cell replacement therapy of PD. This review summarizes our current knowledge about the functions and crosstalk of these signaling pathways in mammalian mDA neuron development in vivo and their applications in stem cell-based paradigms for the efficient derivation of these neurons in vitro.

Fibroblast Growth Factor 8 Expression in GT1-7 GnRH-Secreting Neurons Is Androgen-Independent, but Can Be Upregulated by the Inhibition of DNA Methyltransferases

Fibroblast growth factor 8 (FGF8) is a potent morphogen that regulates the embryonic development of hypothalamic neuroendocrine cells. Indeed, using Fgf8 hypomorphic mice, we showed that reduced Fgf8 mRNA expression completely eliminated the presence of gonadotropin-releasing hormone (GnRH) neurons. These findings suggest that FGF8 signaling is required during the embryonic development of mouse GnRH neurons. Additionally, in situ hybridization studies showed that the embryonic primordial birth place of GnRH neurons, the olfactory placode, is highly enriched for Fgf8 mRNA expression. Taken together these data underscore the importance of FGF8 signaling for GnRH emergence. However, an important question remains unanswered: How is Fgf8 gene expression regulated in the developing embryonic mouse brain? One major candidate is the androgen receptor (AR), which has been shown to upregulate Fgf8 mRNA in 60-70% of newly diagnosed prostate cancers. Therefore, we hypothesized that ARs may be involved in the regulation of Fgf8 transcription in the developing mouse brain. To test this hypothesis, we used chromatin-immunoprecipitation (ChIP) assays to elucidate whether ARs interact with the 5'UTR region upstream of the translational start site of the Fgf8 gene in immortalized mouse GnRH neurons (GT1-7) and nasal explants. Our data showed that while AR interacts with the Fgf8 promoter region, this interaction was androgen-independent, and that androgen treatment did not affect Fgf8 mRNA levels, indicating that androgen signaling does not induce Fgf8 transcription. In contrast, inhibition of DNA methyltransferases (DNMT) significantly upregulated Fgf8 mRNA levels indicating that Fgf8 transcriptional activity may be dependent on DNA methylation status.

ChIP-Seq and RNA-Seq analyses identify components of the Wnt and Fgf signaling pathways as Prep1 target genes in mouse embryonic stem cells

The Prep1 (Pknox1) homeodomain transcription factor is essential at multiple stages of embryo development. In the E11.5 embryo trunk, we previously estimated that Prep1 binds about 3,300 genomic sites at a highly specific decameric consensus sequence, mainly governing basal cellular functions. We now show that in embryonic stem (ES) cells Prep1 binding pattern only partly overlaps that of the embryo trunk, with about 2,000 novel sites. Moreover, in ES cells Prep1 still binds mostly to promoters, as in total embryo trunk but, among the peaks bound exclusively in ES cells, the percentage of enhancers was three-fold higher. RNA-seq identifies about 1800 genes down-regulated in Prep1-/- ES cells which belong to gene ontology categories not enriched in the E11.5 Prep1i/i differentiated embryo, including in particular essential components of the Wnt and Fgf pathways. These data agree with aberrant Wnt and Fgf expression levels in the Prep1-/- ES cells with a deficient embryoid bodies (EBs) formation and differentiation. Re-establishment of the Prep1 level rescues the phenotype.

The lifelong maintenance of mesencephalic dopaminergic neurons by Nurr1 and engrailed

Specific vulnerability and degeneration of the dopaminergic neurons in the substantia nigra pars compacta of the midbrain is the pathological hallmark of Parkinson’s disease. A number of transcription factors regulate the birth and development of this set of neurons and some remain constitutively expressed throughout life. These maintenance transcription factors are closely associated with essential neurophysiological functions and are required ultimately for the long-term survival of the midbrain dopaminergic neurons. The current review describes the role of two such factors, Nurr1 and engrailed, in differentiation, maturation, and in normal physiological functions including acquisition of neurotransmitter identity. The review will also elucidate the relationship of these factors with life, vulnerability, degeneration and death of mesencephalic dopaminergic neurons in the context of Parkinson’s disease.

The atypical homeoprotein Pbx1a participates in the axonal pathfinding of mesencephalic dopaminergic neurons

BACKGROUND

The pre B-cell leukemia transcription factor 1 (Pbx1) genes belong to the three amino acid loop extension family of homeodomain proteins that form hetero-oligomeric complexes with other homeodomain transcription factors, thereby modulating target specificity, DNA binding affinity and transcriptional activity of their molecular associates.

RESULTS

Here, we provide evidence that Pbx1 is expressed in mesencephalic dopaminergic neurons from embryonic day 11 into adulthood and determines some of the cellular properties of this neuronal population. In Pbx1-deficient mice, the mesencephalic dopaminergic axons stall during mid-gestation at the border between di- and telencephalon before entering the ganglionic eminence, leading to a loose organization of the axonal bundle and partial misrouting. In Pbx1-deficient dopaminergic neurons, the high affinity netrin-1 receptor, deleted in colon cancer (DCC), is down-regulated. Interestingly, we found several conserved Pbx1 binding sites in the first intron of DCC, suggesting a direct regulation of DCC transcription by Pbx1.

CONCLUSIONS

The expression of Pbx1 in dopaminergic neurons and its regulation of DCC expression make it an important player in defining the axonal guidance of the midbrain dopaminergic neurons, with possible implications for the normal physiology of the nigro-striatal system as well as processes related to the degeneration of neurons during the course of Parkinson's disease.

Expression of Neurog1 instead of Atoh1 can partially rescue organ of Corti cell survival

In the mammalian inner ear neurosensory cell fate depends on three closely related transcription factors, Atoh1 for hair cells and Neurog1 and Neurod1 for neurons. We have previously shown that neuronal cell fate can be altered towards hair cell fate by eliminating Neurod1 mediated repression of Atoh1 expression in neurons. To test whether a similar plasticity is present in hair cell fate commitment, we have generated a knockin (KI) mouse line (Atoh1^KINeurog1) in which Atoh1 is replaced by Neurog1. Expression of Neurog1 under Atoh1 promoter control alters the cellular gene expression pattern, differentiation and survival of hair cell precursors in both heterozygous (Atoh1^+/KINeurog1) and homozygous (Atoh1^{KINeurog1/KINeurog1}) KI mice. Homozygous KI mice develop patches of organ of Corti precursor cells that express Neurog1, Neurod1, several prosensory genes and neurotrophins. In addition, these patches of cells receive afferent and efferent processes. Some cells among these patches form multiple microvilli but no stereocilia. Importantly, Neurog1 expressing mutants differ from Atoh1 null mutants, as they have intermittent formation of organ of Corti-like patches, opposed to a complete ‘flat epithelium’ in the absence of Atoh1. In heterozygous KI mice co-expression of Atoh1 and Neurog1 results in change in fate and patterning of some hair cells and supporting cells in addition to the abnormal hair cell polarity in the later stages of development. This differs from haploinsufficiency of Atoh1 (Pax2cre; Atoh1^f/+), indicating the effect of Neurog1 expression in developing hair cells. Our data suggest that Atoh1^KINeurog1 can provide some degree of functional support for survival of organ of Corti cells. In contrast to the previously demonstrated fate plasticity of neurons to differentiate as hair cells, hair cell precursors can be maintained for a limited time by Neurog1 but do not transdifferentiate as neurons.

Regulation of XFGF8 gene expression through SRY (sex-determining region Y)-box 2 in developing Xenopus embryos

Fibroblast growth factors (FGFs) function as mitogens and morphogens during vertebrate development. In the present study, to characterise the regulatory mechanism of FGF8 gene expression in developing Xenopus embryos the upstream region of the Xenopus FGF8 (XFGF8) gene was isolated. The upstream region of the XFGF8 gene contains two putative binding sites for the SRY (sex-determining region Y)-box 2 (SOX2) transcription factor. A reporter assay with serially deleted constructs revealed that the putative SOX2-binding motif may be a critical cis-element for XFGF8 gene activation in developing Xenopus embryos. Furthermore, Xenopus SOX2 (XSOX2) physically interacted with the SOX2-binding motif within the upstream region of the XFGF8 gene in vitro and in vivo. Depletion of endogenous XSOX2 resulted in loss of XFGF8 gene expression in midbrain–hindbrain junction, auditory placode, lens placode and forebrain in developing Xenopus embryos. Collectively, our results suggest that XSOX2 directly upregulates XFGF8 gene expression in the early embryonic development of Xenopus.

Polymorphisms of the steroid sulfatase (STS) gene are associated with attention deficit hyperactivity disorder and influence brain tissue mRNA expression

Previous studies in animals and humans have implicated the X-chromosome STS gene in the etiology of attentional difficulties and attention deficit hyperactivity disorder (ADHD). This family based association study has fine mapped a region of the STS gene across intron 1 and 2 previously associated with ADHD, in an extended sample of 450 ADHD probands and their parents. Significant association across this region is demonstrated individually with 7 of the 12 genotyped SNPs, as well as an allele specific haplotype of the 12 SNPs. The over transmitted risk allele of rs12861247 was also associated with reduced STS mRNA expression in normal human post-mortem frontal cortex brain tissue compared to the non-risk allele (P = 0.01). These results are consistent with the hypothesis arising from previous literature demonstrating that boys with deletions of the STS gene, and hence no STS protein are at a significantly increased risk of developing ADHD. Furthermore, this study has established the brain tissue transcript of STS, which except from adipose tissue, differs from that seen in all other tissues investigated. © 2010 Wiley-Liss, Inc.

Spatial analysis of expression patterns predicts genetic interactions at the mid-hindbrain boundary

The isthmic organizer mediating differentiation of mid- and hindbrain during vertebrate development is characterized by a well-defined pattern of locally restricted gene expression domains around the mid-hindbrain boundary (MHB). This pattern is established and maintained by a regulatory network between several transcription and secreted factors that is not yet understood in full detail. In this contribution we show that a Boolean analysis of the characteristic spatial gene expression patterns at the murine MHB reveals key regulatory interactions in this network. Our analysis employs techniques from computational logic for the minimization of Boolean functions. This approach allows us to predict also the interplay of the various regulatory interactions. In particular, we predict a maintaining, rather than inducing, effect of Fgf8 on Wnt1 expression, an issue that remained unclear from published data. Using mouse anterior neural plate/tube explant cultures, we provide experimental evidence that Fgf8 in fact only maintains but does not induce ectopic Wnt1 expression in these explants. In combination with previously validated interactions, this finding allows for the construction of a regulatory network between key transcription and secreted factors at the MHB. Analyses of Boolean, differential equation and reaction-diffusion models of this network confirm that it is indeed able to explain the stable maintenance of the MHB as well as time-courses of expression patterns both under wild-type and various knock-out conditions. In conclusion, we demonstrate that similar to temporal also spatial expression patterns can be used to gain information about the structure of regulatory networks. We show, in particular, that the spatial gene expression patterns around the MHB help us to understand the maintenance of this boundary on a systems level.

Understanding brain formation during development is a tantalizing challenge. It is also essential for the fight against neurodegenerative diseases. In vertebrates, the central nervous system arises from a structure called the neural plate. This tissue is divided into four regions, which continue to develop into forebrain, midbrain, hindbrain and spinal cord. Interactions between locally expressed genes and signaling molecules are responsible for this patterning. Two key signaling molecules in this process are Fgf8 and Wnt1 proteins. They are secreted from a signaling center located at the boundary between prospective mid- and hindbrain (mid-hindbrain boundary, MHB) and mediate development of these two brain regions. Here, we logically analyze the spatial gene expression patterns at the MHB and predict interactions involved in the differentiation of mid- and hindbrain. In particular, our analysis indicates that Wnt1 depends on Fgf8 for stable maintenance. A time-course analysis of Wnt1 expression after implantation of Fgf8-coated beads in mouse neural plate/tube explants experimentally validates our prediction about the interactions between these two key patterning molecules. Subsequently, we demonstrate that available data allows construction of a mathematical model able to explain the maintenance of the signaling center at the MHB. We begin to understand this small aspect of brain formation on a systems level.

Transcriptional regulation of mesencephalic dopaminergic neurons: the full circle of life and death

Since mesencephalic dopaminergic neurons are associated to one of the most prominent human neurodegenerative ailments, Parkinson's disease, the molecular mechanism underlying their development and adult cellular properties has been the subject of intense investigations. Throughout life, transcription factors determine the fate of this neuronal population and control essential processes such as localization in the ventral midbrain, their neurotransmitter phenotype, their target innervations and synapse formation. Studies of transcription factors, such as Nurr1, Pitx3, Engrailed-1/2, and Lmx1a/b, have not only revealed importance of these genes during development, but also roles in the long-term survival and maintenance of these neurons. In this review, we will discuss the function of these transcription factors throughout the life of mesencephalic dopaminergic neurons and their value in the study of the disease mechanism.

Transcription of fgf8 is regulated by activating and repressive cis-elements at the midbrain-hindbrain boundary in zebrafish embryos

Fgf8 is expressed in the isthmic region of the developing brain, serving an organizing function in vertebrate embryos. We previously identified S4.2 downstream to the zebrafish fgf8 gene as a regulatory region that drives transcription in the anterior hindbrain. Here, we investigated the mechanism of fgf8 regulation by the S4.2 region during development. Reporter analyses in embryos revealed that S4.2 closely recapitulates fgf8 expression in the anteriormost hindbrain during somitogenesis. This region contains a sequence highly conserved in fgf8 of diverse vertebrates. Further analyses of S4.2 revealed a 342-bp core region composed of three subregions (#2, #3, and #4). Regions #3 and #4 drove expression broadly in the brain from the midbrain to r5 of the hindbrain, whereas a 28-bp sequence in #2 repressed ectopic expression in the midbrain and in r2 to r5. The enhancer function of S4.2 was absent in pax2a mutant embryos, while it was activated ectopically by pax2a misexpression in the hindbrain. We identified two sites in the core region that are bound by Pax2a in vitro and in vivo, the disruption of which abrogated the S4.2 activity. Thus, fgf8 expression in the anteriormost hindbrain involves activation and repression, with Pax2a as a pivotal regulator.

Characterisation of the genomic canine Fgf8 locus and screen for genetic variants in 4 dogs with different face types

We are investigating the genetic basis of morphological differences in skull shape between domestic dogs of different breeds using a candidate gene approach to identify genes involved in the genetic regulation. One such candidate is Fgf8. Fgf8 is a signalling molecule important in the embryonic development and patterning of the craniofacial region. Mice conditional null for the expression of Fgf8 after E9.5 have a short foreface and a wide skull (Trumpp et al. 1999). Using a combination of bioinformatics and PCR cloning, we have characterised the genomic loci of the canine Fgf8 gene. Like the mouse homologue, it is composed of six exons and we also predict that like the mouse, there are eight alternative isoforms that are generated by alternative splicing events. We have identified a short 200 bp sequence upstream of the Fgf8 gene that is highly conserved between species and have predicted putative transcription factor binding sites using the Transfac database. Genetic analysis of 4 dogs with different skull types identified genetic variation. None of the variants however, were predicted to have any functional significance.

Comparative genomics: a tool to functionally annotate human DNA

The availability of an increasing number of vertebrate genomes has enabled comparative methods to infer functional sequences based on evolutionary constraint. Although this has proved powerful for gene identification, significant progress has also been made in uncovering gene regulatory sequences such as distant acting transcriptional enhancers. These pursuits have led to the development of a variety of valuable databases and resources that should serve as a routine toolbox for biological discovery.

The engrailed transcription factors and the mesencephalic dopaminergic neurons

The engrailed genes belong to a large family of homeobox transcription factors. They are found throughout the animal kingdom, are highly conserved in the DNA binding domain and have been investigated for more than half a century. In the murine genome, two engrailed genes exist, called Engrailed-1 and Engrailed-2. Here, we summarize the properties of the engrailed genes and their functions, such as conserved structures, cellular localisation, secretion and internalisation, transcription factor activity, potential target genes and review their role in the development of mesencephalic dopaminergic neurons. During early development, they take part in the regionalization event, which specifies the neuroepithelium that provides the precursor cells of the mesencephalic dopaminergic neurons with the necessary signals for their induction. Later in the post-mitotic neurons, the two transcription factors participate in their specification and are cell-autonomously required for their survival.

Genomic organization, alternative splicing, and multiple regulatory regions of the zebrafish fgf8 gene

Fgf8 is among the members of the fibroblast growth factor (FGF) family that play pivotal roles in vertebrate development. In the present study, the genomic DNA of the zebrafish fgf8 gene was cloned to elucidate the regulatory mechanism behind the temporally and spatially restricted expression of the gene in vertebrate embryos. Structural analysis revealed that the exon-intron organization of fgf8 is highly conserved during vertebrate evolution, from teleosts to mammals. Close inspection of the genomic sequence and reverse transcription-polymerase chain reaction analysis revealed that zebrafish fgf8 encodes two splicing variants, corresponding to Fgf8a and Fgf8b, among the four to seven splicing variants known in mammals. Misexpression of the two variants in zebrafish embryos following mRNA injection showed that both variants have dorsalizing activities on zebrafish embryos, with Fgf8b being more potent. Reporter gene analysis of the transcriptional regulation of zebrafish fgf8 suggested that its complicated expression pattern, which is considered essential for its multiple roles in development, is mediated by combinations of different regulatory regions in the upstream and downstream regions of the gene. Furthermore, comparison of the genomic sequence of fgf8 among different vertebrate species suggests that this regulatory mechanism is conserved during vertebrate evolution.

NF-kappaB activation upregulates fibroblast growth factor 8 expression in prostate cancer cells

BACKGROUND

Fibroblast growth factor 8 (FGF8) is over-expressed in prostate cancer (CaP) correlating with high-grade disease and reduced survival. The role of acetylation in transcriptional regulation of FGF8 was investigated using the histone deacetylase (HDAC) inhibitor Trichostatin A (TSA).

METHODS

FGF8 transcriptional response to TSA was investigated by gene reporter assays, RT-PCR, and Western blotting. Chromatin immunoprecipitation (ChIP) assays were also performed.

RESULTS

FGF8 is upregulated in response to TSA treatment along with NF-kappaB transcriptional activity. Over-expression of p65 activated FGF8 transcription. ChIP assays revealed p65 recruitment to the fgf8 promoter, containing putative NF-kappaB binding sites, post TSA stimulation. PI-3K activity is required for TSA mediated FGF8 upregulation.

CONCLUSION

Using TSA treatment in prostate cancer cells, a requirement of PI-3K activity in mediating TSA function is demonstrated and a novel role for NF-kappaB in the regulation of FGF8 expression is uncovered.

Characterisation of the genomic structure of chick Fgf8

The Fgf8 gene encodes a series of secreted signalling molecules important in the normal development of the face, brain and limbs. The genomic structure of the chick Fgf8 gene has been analysed and compared to the human and mouse sequences. Divergence between the chick, human and mouse genomic structure was observed. Data indicates that the long alternatively spliced form of exon 1b observed in mouse and exon 1c observed in human and mouse do not exist in the chick Fgf8 gene. RT-PCR analysis indicates that chick Fgf8, like its mouse and human counterpart is alternatively spliced. This data along with the genomic structure data indicates that in the chick there are only two isoforms of Fgf8. This is in contrast to the human and mouse, where evidence suggests that there are 4 and 8 isoforms, respectively. Approximately 400 bp of intron 1d is highly conserved between chick, human and mouse genomic sequences. Using TRANSFAC possible conserved regulatory element binding sites within this domain were identified.

Identification of evolutionarily conserved regulatory elements in the mouse Fgf8 locus

The secreted signaling molecule fibroblast growth factor 8 (Fgf8) is an essential component of certain embryonic signaling centers including the mid-hindbrain (isthmic) organizer, the first branchial arch (BA1), and the apical ectodermal ridge (AER). In these signaling centers Fgf8 transcripts are expressed in a dynamic and transient fashion, but the mechanism by which this highly specific expression pattern is established remains largely unknown. We used DNA sequence comparisons coupled to transgenic approaches to obtain insight into the structure and function of regulatory elements in the Fgf8 locus. First, a bacterial artificial chromosome (BAC) containing the mouse Fgf8 gene partially rescues the embryonic lethality of Fgf8-deficient mice and controls Fgf8-specific gene expression of a coinjected lacZ reporter transgene. Second, sequence comparison of vertebrate Fgf8 loci revealed evolutionarily highly conserved noncoding sequences that were unexpectedly located mainly 3' of the Fgf8 coding region. Third, in transgenic mice some of these elements were sufficient to target expression to the AER, tail bud, and brain, including the isthmic organizer, indicating that they may represent Fgf8 cis-acting elements. Collectively, these data identify novel regulatory elements of the Fgf8 gene sufficient to drive expression to regions of known Fgf8 activity.

Engrailed: complexity and economy of a multi-functional transcription factor

Engrailed is a homeodomain-containing transcription factor with numerous, overlapping roles in neural development. Its multifunctional nature depends upon an extremely diverse group of molecular functions including the ability to regulate both transcription and translation, and to be released from and internalized by cells. Recent findings have shown how some of these functions relate to specific roles in development and disease.

Upper beak truncation in chicken embryos with the cleft primary palate mutation is due to an epithelial defect in the frontonasal mass

In this study, we used the chicken mutant strain known as cleft primary palate (cpp) to study the mechanisms of beak outgrowth. cpp mutants have complete truncation of the upper beak with normal development of the lower beak. Based on structural analysis and grafts of facial prominences, we localized the defect to the frontonasal mass and its derivatives. Several explanations that would account for the outgrowth defect were investigated, including abnormal expression of genes in the frontonasal epithelium, intrinsic defects in epithelium and/or mesenchyme defects in epithelial-mesenchymal signalling, a localized decrease in cell proliferation or a localized increase in programmed cell death. One of the genes expressed in the frontonasal epithelial growth zone, Fgf8, failed to down-regulate and was maintained for at least 48 hr beyond the time when down-regulation normally occurs. Recombination experiments further illustrated that the frontonasal mass epithelium was abnormal in the cpp mutants, whereas mutant mesenchyme was capable of normal outgrowth when combined with wild-type epithelium. Cell proliferation was not decreased in mutant embryos nor was cell death initially increased. Later, at stages 31-32, when the prenasal cartilage begins directed outgrowth, there was an increase in cell death within the mutant upper but not lower beak cartilage. The cpp beak truncation, therefore, is due to an epithelial defect in the frontonasal mass that is coincident with a failure to down-regulate expression of Fgf8.

Tbx1 regulates fibroblast growth factors in the anterior heart field through a reinforcing autoregulatory loop involving forkhead transcription factors

Birth defects, which occur in one out of 20 live births, often affect multiple organs that have common developmental origins. Human and mouse studies indicate that haploinsufficiency of the transcription factor TBX1 disrupts pharyngeal arch development, resulting in the cardiac and craniofacial features associated with microdeletion of 22q11 (del22q11), the most frequent human deletion syndrome. Here, we have generated an allelic series of Tbx1 deficiency that reveals a lower critical threshold for Tbx1 activity in the cardiac outflow tract compared with other pharyngeal arch derivatives, including the palatal bones. Mice hypomorphic for Tbx1 failed to activate expression of the forkhead transcription factor Foxa2 in the pharyngeal mesoderm, which contains cardiac outflow precursors derived from the anterior heart field. We identified a Fox-binding site upstream of Tbx1 that interacted with Foxa2 and was necessary for pharyngeal mesoderm expression of Tbx1, revealing an autoregulatory loop that may explain the increased cardiac sensitivity to Tbx1 dose. Downstream of Tbx1, we found a fibroblast growth factor 8 (Fgf8) enhancer that was dependent on Tbx1 in vivo for regulating expression in the cardiac outflow tract, but not in pharyngeal arches. Consistent with its role in regulating cardiac outflow tract cells Tbx1 gain of function resulted in expansion of the cardiac outflow tract segment derived from the anterior heart field as marked by Fgf10. These findings reveal a Tbx1-dependent transcriptional and signaling network in the cardiac outflow tract that renders mouse cardiovascular development more susceptible than craniofacial development to a reduction in Tbx1 dose, similar to humans with del22q11.

Midbrain dopaminergic neurons: control of their cell fate by the engrailed transcription factors

As for any other cell population, the development, cell fate, and properties of mesencephalic dopaminergic (mesDA) neurons are ultimately controlled at the transcriptional level. The genes for two transcription factors Engrailed-1 ( En1) and Engrailed-2 ( En2) play an essential role in the development and maintenance of these cells. They belong to a family of genes that have been investigated in Drosophila for more than half a century. The products of these genes are all characterized by homeotic tissue transformation and a highly conserved protein sequence, the homeobox. En1 and En2 act upon at least two steps of the differentiation of mesDA neurons. They take part in the regionalization event, which gives rise to the neuroepithelium that provides the precursor cells in the ventral midbrain with the fibroblast growth factor 8 signal necessary for their induction. Additionally, these genes are required in postmitotic mesDA neurons in which they are expressed from embryonic day 12 continuously into adulthood. In mutant mice homozygous null for En1 and En2, the neurons are generated in the ventral midbrain, become postmitotic, and begin to express their neurotransmitter phenotype. However, thereafter, they rapidly die by apoptosis. Cell mixing experiments in vitro and in vivo have demonstrated that the engrailed requirement for the survival of mesDA neurons is cell-autonomous. The inactivation of engrailed by RNA interference induces apoptosis in less than 24 h. These data suggest that the engrailed genes control an essential mechanism for the survival of mesDA neurons.

Large-scale characterization of genes specific to the larval nervous system in the ascidian Ciona intestinalis

The central and peripheral nervous systems (CNS and PNS) of the ascidian tadpole larva are comparatively simple, consisting of only about 350 cells. However, studies of the expression of neural patterning genes have demonstrated overall similarity between the ascidian CNS and the vertebrate CNS, suggesting that the ascidian CNS is sufficiently complex to be relevant to those of vertebrates. Recent progress in the Ciona intestinalis genome project and cDNA project together with considerable EST information has made Ciona an ideal model for investigating molecular mechanisms underlying the formation and function of the chordate nervous system. Here, we characterized 56 genes specific to the nervous system by determining their full-length cDNA sequences and confirming their spatial expression patterns. These genes included those that function in the nervous systems of other animals, especially those involved in photoreceptor-mediated signaling and neurotransmitter release. Thus, the nervous system-specific genes in Ciona larvae will provide not only probes for determining their function but also clues for exploring the complex network of nervous system-specific genes.

The intracrine hypothesis and intracellular peptide hormone action

There is evidence that many peptide growth factors and hormones act in the intracellular space after either internalization or retention in their cells of synthesis. These factors, commonly called intracrines, are structurally diverse while sharing some common functional features. Reports of intracellular peptide hormone binding and action are reviewed here. Also, this laboratory has made proposals regarding the origin and actions of intracrines and these areas are further explored. Intracrine interactions and the relationship of intracrines to transcription factors are discussed. The intracellular/intracrine renin-angiotensin system (iRAS) is reviewed to illustrate the intracrine analogue of a well-established physiological system. The role of intracrine action in metazoan development is also considered.

Orientation-dependent influence of an intergenic enhancer on the promoter activity of the divergently transcribed mouse Shsp/alpha B-crystallin and Mkbp/HspB2 genes

The mouse Shsp/alphaB-crystallin and Mkbp/HspB2 genes are closely linked and divergently transcribed. In this study, we have analyzed the contribution of the intergenic enhancer to Shsp/alphaB-crystallin and Mkbp/HspB2 promoter activity using dual-reporter vectors in transient transfection and transgenic mouse experiments. Deletion of the enhancer reduced Shsp/alphaB-crystallin promoter activity by 30- and 93-fold and Mkbp/HspB2 promoter activity by 6- and 10-fold in transiently transfected mouse lens alpha-TN4 and myoblast C2C12 cells, respectively. Surprisingly, inversion of the enhancer reduced Shsp/alphaB-crystallin promoter activity by 17-fold, but did not affect Mkbp/HspB2 promoter activity in the transfected cells. In contrast, enhancer activity was orientation-independent in combination with a heterologous promoter in transfected cells. Transgenic mouse experiments established the orientation dependence and Shsp/alphaB-crystallin promoter preference of the intergenic enhancer in its native context. The orientation dependence and preferential effect of the Shsp/alphaB-crystallin enhancer on the Shsp/alphaB-crystallin promoter provide an example of adaptive changes in gene regulation accompanying the functional diversification of duplicated genes during evolution.

Neuron-restricted expression of the rat gonadotropin-releasing hormone gene is conferred by a cell-specific protein complex that binds repeated CAATT elements

GnRH gene expression is restricted to a tiny population of neurons scattered throughout the mediobasal hypothalamus. The combination of a 300-bp enhancer and the 173-bp promoter from the rat GnRH gene can confer this narrow specificity in transgenic mice and in transfections of hypothalamic GT1-7 cells. In the present study, we identify repeated CAATT elements in the 3' region of the rat GnRH enhancer that bind a tissue-restricted protein complex and play a significant role in cell-restricted expression of the GnRH gene. Deletions of multiple repeats demonstrate their importance in transcriptional activity. In fact, even mutation of a single repeat reduces expression. This reduction can be compensated by the conserved GnRH promoter, which also contains such elements and binds this protein complex. In Southwestern analysis, three proteins from GT1-7 nuclear extract bind to the CAATT element, and these proteins are not found in NIH3T3 cells. This cell-specific protein complex has properties of the Q50 homeodomain family of transcription factors and binds to as many as seven binding sites in the enhancer and promoter to play a key role in GnRH gene expression in the hypothalamus.

Lysophosphatidylcholine stimulates monocyte chemoattractant protein-1 gene expression in rat aortic smooth muscle cells

OBJECTIVE

Monocyte chemoattractant protein (MCP)-1 is a proatherogenic factor that is responsible for approximately 60% of plaque macrophages in mouse models of atherosclerosis. We investigated whether lysophosphatidylcholine (LPC), enriched in oxidized low density lipoprotein, can modulate the expression of MCP-1 in arterial wall cells.

METHODS AND RESULTS

LPC induced a 3-fold increase in MCP-1 mRNA in rat vascular smooth muscle cells (VSMCs) in a time- and dose-dependent manner. Nuclear runon analysis showed that this increase was attributable to increased MCP-1 gene transcription. There was a 2-fold increase in MCP-1 protein in the conditioned media of cells treated with LPC. LPC-associated increases of MCP-1 mRNA and protein were similar to those produced by platelet-derived growth factor-BB, a known inducer of MCP-1. Analyses of the MCP-1 promoter in transiently transfected VSMCs indicated an LPC-responsive element(s) between base pairs -146 and -261 (relative to transcription initiation). Further studies suggested that LPC-induced MCP-1 expression partially involves mitogen-activated protein kinase/extracellular signal-regulated kinase, a tyrosine kinase(s), and (to a lesser extent) protein kinase C but not the activation of the platelet-derived growth factor receptor.

CONCLUSIONS

LPC stimulates MCP-1 expression at the transcriptional level in VSMCs, suggesting a molecular mechanism by which LPC contributes to the atherogenicity of oxidized low density lipoprotein.

Regulation of FGF8 expression by the androgen receptor in human prostate cancer

Fibroblast growth factor 8 (FGF8) has been shown to play a key role in prostate carcinogenesis. It was initially cloned as an androgen induced protein in mouse mammary cancer SC3 cells. In this study, we examined if FGF8 was also regulated by the androgen receptor in human prostate cancer. FGF8b protein expression in resected clinical prostate cancer correlated closely with expression of the androgen receptor (AR). In the androgen sensitive CWR22 prostate xenograft, we observed up-regulation of FGF8b immunoreactivity in testosterone supplemented mice while castration markedly reduced its signal. Furthermore, FGF8b protein expression in AR positive LNCaP cells was similarly enhanced by androgens. The proximal promoter of the human FGF8 gene was cloned into a luciferase reporter construct (FGF8.luc). FGF8.luc activity in AR positive LNCaP and SC3 cells was increased 2.5-fold by androgens. In AR negative DU145 cells, maximal induction of FGF8.luc required both co-transfection of the AR and the presence of androgens. The anti-androgen bicalutamide completely abolished AR mediated FGF8.luc induction. Deletion constructs from FGF8.luc have further defined an active promoter region and an androgen responsive region. Nucleotide analysis of this androgen responsive region has revealed putative androgen response elements. Finally, using ChIP assays we confirmed in vivo interaction between the AR and the androgen responsive region of the FGF8 promoter. Taken together these data provide first evidence that expression of the mitogen FGF8 in prostate cancer is, at least in part, regulated by the androgen receptor at the transcriptional level.

A novel positive transcriptional feedback loop in midbrain-hindbrain boundary development is revealed through analysis of the zebrafish pax2.1 promoter in transgenic lines

The pax2.1 gene encodes a paired-box transcription factor that is one of the earliest genes to be specifically activated in development of the midbrain and midbrain-hindbrain boundary (MHB), and is required for the development and organizer activity of this territory. To understand how this spatially restricted transcriptional activity of pax2.1 is achieved, we have isolated and characterized the pax2.1-promoter using a lacZ and a GFP reporter gene in transient injection assays and transgenic lines. Stable transgenic expression of this reporter gene shows that a 5.3-kb fragment of the 5′ region contains most, but not all, elements required for driving pax2.1 expression. The expressing tissues include the MHB, hindbrain, spinal cord, ear and pronephros. Transgene activation in the pronephros and developing ear suggests that these pax2.1-expressing tissues are composed of independently regulated subdomains. In addition, ectopic but spatially restricted activation of the reporter genes in rhombomeres 3 and 5 and in the forebrain, which do not normally express endogenous pax2.1, demonstrates the importance of negative regulation of pax2.1.

Comparison of transgene expression in wild-type and homozygous pax2.1 mutant no isthmus (noi) embryos reveals that the transgene contains control element(s) for a novel, positive transcriptional feedback loop in MHB development. Transcription of endogenous pax2.1 at the MHB is known to be initially Pax2.1 independent, during activation in late gastrulation. In contrast, transgene expression requires the endogenous Pax2.1 function. Transplantations, mRNA injections and morpholino knock-down experiments show that this feedback regulation of pax2.1 transcription occurs cell-autonomously, and that it requires eng2 and eng3 as known targets for Pax2.1 regulation. We suggest that this novel feedback loop may allow continuation of pax2.1 expression, and hence development of the MHB organizer, to become independent of the patterning machinery of the gastrula embryo.

Distinct regulators control the expression of the mid-hindbrain organizer signal FGF8

Local expression of FGF8 at the mid/hindbrain boundary (MHB) governs the development of multiple neurons and support cells. Here we show that the paired-domain protein Pax2 is necessary and sufficient for the induction of FGF8 in part by regulating the expression of Pax5&8. A network of transcription and secreted factors, including En1, Otx2, Gbx2, Grg4 and Wnt1&4, that is established independently of Pax2, further refines the expression domain and level of FGF8 at the MHB through opposing effects on Pax2 activity. Our results indicate that the expression of local organizing factors is controlled by combinatorial interaction between inductive and modulatory factors.

Regionalisation and acquisition of polarity in the optic tectum

The optic tectum differentiates from the alar plate of the mesencephalon and receives retinal fibres in a precise retinotopic manner. Here, mechanisms of tectum polarisation and regionalisation are reviewed. Misexpression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to that of the tectum. Ephrin A2 and A5 are expressed in a gradient in the tectum, caudal high and rostral low, and may play important roles in the formation of a precise retinotectal projection map. Retinal fibres that express receptors for these ligands, and which come from the temporal retina, are repulsed by the ligands and do not invade the caudal tectum. Both En1 and En2 can regulate posterior characteristics in the tectum by inducing ephrin A2 and A5. Transplantation experiments in chick have indicated that the mes/metencephalic boundary works as an organiser for the tectum and the cerebellum. Fgf8 is a candidate signalling molecule in the organiser. Pax2/5, En, and Fgf8 are in a positive feedback loop for their expression such that misexpression of one of these genes in the diencephalon turns on the feedback loop and can result in induction of an optic tectum. Otx2 and Gbx2 appear to repress each other's expression and contribute to defining the posterior border of the tectum. Misexpression of Otx2 in the metencephalon can change the fate of its alar plate to a tectum, and misexpression of Gbx2 in the mesencephalon can cause anterior shifting of the caudal limit of the tectum. The anterior border of the tectum may be determined as a result of repressive interactions between Pax6 and En1/Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to ventralize the tissue; that is, Shh can change the fate of the presumptive tectum to that of the tegmentum that is the ventral structure. It is proposed that the brain vesicle that expresses Otx2, Pax2, and En1 may differentiate into the tectum.

An E-box motif residing in the exon/intron 1 junction regulates both transcriptional activation and splicing of the human norepinephrine transporter gene

The norepinephrine transporter (NET) is responsible for the rapid NaCl-dependent uptake of norepinephrine into presynaptic noradrenergic nerve endings. Recently, we have characterized the structural organization of the 5' upstream promoter region of the human NET (hNET) gene. A new intron of 476 base pairs was found in the middle of the 5'-untranslated leader sequence and was shown to robustly enhance the promoter activity. Here, we show that the first hNET intron enhances both the homologous hNET and the heterologous thymidine kinase promoter activities in an orientation- and position-dependent manner. The first hNET intron exhibited a similar promoter-enhancing effect in both SK-N-BE(2)C (NET-positive) and HeLa (NET-negative) cell lines, showing that its function is not cell-specific. Transient transfection assays of a series of deletional constructs show that the first hNET intron contains subdomains with either positive or negative regulatory functions. Furthermore, DNase I footprinting analysis demonstrated that the 5' side of the intron, encompassing the splice donor site, is prominently protected by nuclear proteins isolated from both SK-N-BE(2)C and HeLa cells. The protected nucleotide sequence contains a consensus E-box motif, known to regulate diverse eukaryotic genes, which overlaps with the splice donor site of the first intron. We demonstrate that two basic helix-loop-helix proteins, upstream stimulatory factors 1 and 2, are major proteins interacting at this site and that the E-box is at least in part responsible for the promoter-enhancing activity of the first intron. Furthermore, site-directed mutagenesis of the splice donor site of the first intron affects both correct splicing and transcriptional activity. Taken together, our results indicate that a cis-element residing at the first exon/intron junction, encompassing an E-box motif, has a unique dual role in basal transcriptional activation and splicing of hNET mRNA.

The neuronal microtubule-associated protein 1B is under homeoprotein transcriptional control

To identify genes regulated by homeoprotein transcription factors in postnatal neurons, the DNA-binding domain (homeodomain) of Engrailed homeoprotein was internalized into rat cerebellum neurons. The internalized homeodomain (EnHD) acts as a competitive inhibitor of Engrailed and of several homeoproteins (Mainguy et al., 2000). Analysis by differential display revealed that microtubule-associated protein 1B (MAP1B) mRNA is upregulated by EnHD. This upregulation does not require protein synthesis, suggesting a direct effect of the homeodomain on MAP1B transcription. The promoter region of MAP1B was cut into several subdomains, and each subdomain was tested for its ability to bind Engrailed and EnHD and to associate with Engrailed-containing cerebellum nuclear extracts. In addition, the activity, and regulation by Engrailed, of each subdomain and of the entire promoter were evaluated in vivo by electroporation in the chick embryo neural tube. These experiments demonstrate that MAP1B promoter is regulated by Engrailed in vivo. Moreover, they show that one promoter domain that contains all ATTA homeoprotein cognate binding sites common to the rat and human genes is an essential element of this regulation. It is thus proposed that MAP1B, a cytoskeleton protein involved in neuronal growth and regeneration, is under homeoprotein transcriptional regulation.

Enhanced proliferation, survival, and dopaminergic differentiation of CNS precursors in lowered oxygen

Standard cell culture systems impose environmental oxygen (O(2)) levels of 20%, whereas actual tissue O(2) levels in both developing and adult brain are an order of magnitude lower. To address whether proliferation and differentiation of CNS precursors in vitro are influenced by the O(2) environment, we analyzed embryonic day 12 rat mesencephalic precursor cells in traditional cultures with 20% O(2) and in lowered O(2) (3 +/- 2%). Proliferation was promoted and apoptosis was reduced when cells were grown in lowered O(2), yielding greater numbers of precursors. The differentiation of precursor cells into neurons with specific neurotransmitter phenotypes was also significantly altered. The percentage of neurons of dopaminergic phenotype increased to 56% in lowered O(2) compared with 18% in 20% O(2). Together, the increases in total cell number and percentage of dopaminergic neurons resulted in a ninefold net increase in dopamine neuron yield. Differential gene expression analysis revealed more abundant messages for FGF8, engrailed-1, and erythropoietin in lowered O(2). Erythropoietin supplementation of 20% O(2) cultures partially mimicked increased dopaminergic differentiation characteristic of CNS precursors cultured in lowered O(2). These data demonstrate increased proliferation, reduced cell death, and enhanced dopamine neuron generation in lowered O(2), making this method an important advance in the ex vivo generation of specific neurons for brain repair.

Regionalization of the optic tectum: combinations of gene expression that define the tectum

The optic tectum differentiates from the alar plate of the mesencephalon. Here, the molecular mechanisms for differentiation of the tectum are reviewed. Mis-expression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to become the tectum. En, Fgf8, Pax2 and Pax5, exist in a positive feedback loop for their expression so that mis-expression of any of these genes acts on the feedback loop resulting in induction of the optic tectum in the diencephalon. Otx2 and Gbx2 can repress the expression of each other and contribute to the formation of the posterior border of the tectum. Mis-expression of Otx2 in the metencephalon changed the fate of its alar plate to the tectum. The anterior border of the tectum might be determined as a result of repressive interaction of Pax6 with En1 and Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to the ventralization of the tissue, that is, the area affected by Shh differentiates into the tegmentum. It is proposed that the brain vesicle that expresses Otx2, Pax2 and En1 might differentiate into the tectum.