Identification of a retinoic acid response element upstream of the murine Hox-4.2 gene

Hox genes: Downstream "effectors" of retinoic acid signaling in vertebrate embryogenesis

One of the major regulatory challenges of animal development is to precisely coordinate in space and time the formation, specification, and patterning of cells that underlie elaboration of the basic body plan. How does the vertebrate plan for the nervous and hematopoietic systems, heart, limbs, digestive, and reproductive organs derive from seemingly similar population of cells? These systems are initially established and patterned along the anteroposterior axis (AP) by opposing signaling gradients that lead to the activation of gene regulatory networks involved in axial specification, including the Hox genes. The retinoid signaling pathway is one of the key signaling gradients coupled to the establishment of axial patterning. The nested domains of Hox gene expression, which provide a combinatorial code for axial patterning, arise in part through a differential response to retinoic acid (RA) diffusing from anabolic centers established within the embryo during development. Hence, Hox genes are important direct effectors of retinoid signaling in embryogenesis. This review focuses on describing current knowledge on the complex mechanisms and regulatory processes, which govern the response of Hox genes to RA in several tissue contexts including the nervous system during vertebrate development.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

DNA methylation-independent removable insulator controls chromatin remodeling at the HOXA locus via retinoic acid signaling

Chromatin insulators partition the genome into functional units to control gene expression, particularly in complex chromosomal regions. The CCCTC-binding factor (CTCF) is an insulator-binding protein that functions in transcriptional regulation and higher-order chromatin formation. Variable CTCF-binding sites have been identified to be cell type-specific partly due to differential DNA methylation. Here, we show that DNA methylation-independent removable CTCF insulator is responsible for retinoic acid (RA)-mediated higher-order chromatin remodeling in the human HOXA gene locus. Detailed chromatin analysis characterized multiple CTCF-enriched sites and RA-responsive enhancers at this locus. These regulatory elements and transcriptionally silent HOXA genes are closely positioned under basal conditions. Notably, upon RA signaling, the RAR/RXR transcription factor induced loss of adjacent CTCF binding and changed the higher-order chromatin conformation of the overall locus. Targeted disruption of a CTCF site by genome editing with zinc finger nucleases and CRISPR/Cas9 system showed that the site is required for chromatin conformations that maintain the initial associations among insulators, enhancers and promoters. The results indicate that the initial chromatin conformation affects subsequent RA-induced HOXA gene activation. Our study uncovers that a removable insulator spatiotemporally switches higher-order chromatin and multiple gene activities via cooperation of CTCF and key transcription factors.

HOX genes: Major actors in resistance to selective endocrine response modifiers

Long term treatment with therapies aimed at blocking the estrogen- (ER) or androgen receptor (AR) action often leads to the development of resistance to selective modulators of the estrogen receptor (SERMs) in ERα-positive breast cancer, or of the androgen receptor (SARMs) in AR-positive prostate cancer. Many underlying molecular events that confer resistance are known, but a unifying theme is yet to be revealed. Receptor tyrosine kinases (RTKs) such EGFR, ERBB2 and IGF1R are major mediators that can directly alter cellular response to the SERM, tamoxifen, but the mechanisms underlying increased expression of RTKs are not clear. A number of HOX genes and microRNAs and non-coding RNAs residing in the HOX cluster, have been identified as important independent predictors of endocrine resistant breast cancer. Recently, convincing evidence has accumulated that several members belonging to the four different HOX clusters contribute to endocrine therapy resistant breast cancer, but the mechanisms remain obscure. In this article, we have reviewed recent progress in understanding of the functioning of HOX genes and regulation of their expression by hormones. We also discuss, in particular, the contributions of several members of the HOX gene family to endocrine resistant breast cancer.

Thymosin-β4 is a determinant of drug sensitivity for Fenretinide and Vorinostat combination therapy in neuroblastoma

Retinoids are an important component of neuroblastoma therapy at the stage of minimal residual disease, yet 40-50% of patients treated with 13-cis-retinoic acid (13-cis-RA) still relapse, indicating the need for more effective retinoid therapy. Vorinostat, or Suberoylanilide hydroxamic acid (SAHA), is a potent inhibitor of histone deacetylase (HDAC) classes I & II and has antitumor activity in vitro and in vivo. Fenretinide (4-HPR) is a synthetic retinoid which acts on cancer cells through both nuclear retinoid receptor and non-receptor mechanisms. In this study, we found that the combination of 4-HPR + SAHA exhibited potent cytotoxic effects on neuroblastoma cells, much more effective than 13-cis-RA + SAHA. The 4-HPR + SAHA combination induced caspase-dependent apoptosis through activation of caspase 3, reduced colony formation and cell migration in vitro, and tumorigenicity in vivo. The 4-HPR and SAHA combination significantly increased mRNA expression of thymosin-beta-4 (Tβ4) and decreased mRNA expression of retinoic acid receptor α (RARα). Importantly, the up-regulation of Tβ4 and down-regulation of RARα were both necessary for the 4-HPR + SAHA cytotoxic effect on neuroblastoma cells. Moreover, Tβ4 knockdown in neuroblastoma cells increased cell migration and blocked the effect of 4-HPR + SAHA on cell migration and focal adhesion formation. In primary human neuroblastoma tumor tissues, low expression of Tβ4 was associated with metastatic disease and predicted poor patient prognosis. Our findings demonstrate that Tβ4 is a novel therapeutic target in neuroblastoma, and that 4-HPR + SAHA is a potential therapy for the disease.

The development and growth of tissues derived from cranial neural crest and primitive mesoderm is dependent on the ligation status of retinoic acid receptor γ: evidence that retinoic acid receptor γ functions to maintain stem/progenitor cells in the absence of retinoic acid

Retinoic acid (RA) signaling is important to normal development. However, the function of the different RA receptors (RARs)--RARα, RARβ, and RARγ--is as yet unclear. We have used wild-type and transgenic zebrafish to examine the role of RARγ. Treatment of zebrafish embryos with an RARγ-specific agonist reduced somite formation and axial length, which was associated with a loss of hoxb13a expression and less-clear alterations in hoxc11a or myoD expression. Treatment with the RARγ agonist also disrupted formation of tissues arising from cranial neural crest, including cranial bones and anterior neural ganglia. There was a loss of Sox 9-immunopositive neural crest stem/progenitor cells in the same anterior regions. Pectoral fin outgrowth was blocked by RARγ agonist treatment. However, there was no loss of Tbx-5-immunopositive lateral plate mesodermal stem/progenitor cells and the block was reversed by agonist washout or by cotreatment with an RARγ antagonist. Regeneration of the caudal fin was also blocked by RARγ agonist treatment, which was associated with a loss of canonical Wnt signaling. This regenerative response was restored by agonist washout or cotreatment with the RARγ antagonist. These findings suggest that RARγ plays an essential role in maintaining stem/progenitor cells during embryonic development and tissue regeneration when the receptor is in its nonligated state.

YY1 acts as a transcriptional activator of Hoxa5 gene expression in mouse organogenesis

The Hox gene family encodes homeodomain-containing transcriptional regulators that confer positional information to axial and paraxial tissues in the developing embryo. The dynamic Hox gene expression pattern requires mechanisms that differentially control Hox transcription in a precise spatio-temporal fashion. This implies an integrated regulation of neighbouring Hox genes achieved through the sharing and the selective use of defined enhancer sequences. The Hoxa5 gene plays a crucial role in lung and gut organogenesis. To position Hoxa5 in the regulatory hierarchy that drives organ morphogenesis, we searched for cis-acting regulatory sequences and associated trans-acting factors required for Hoxa5 expression in the developing lung and gut. Using mouse transgenesis, we identified two DNA regions included in a 1.5-kb XbaI-XbaI fragment located in the Hoxa4-Hoxa5 intergenic domain and known to control Hoxa4 organ expression. The multifunctional YY1 transcription factor binds the two regulatory sequences in vitro and in vivo. Moreover, the mesenchymal deletion of the Yy1 gene function in mice results in a Hoxa5-like lung phenotype with decreased Hoxa5 and Hoxa4 gene expression. Thus, YY1 acts as a positive regulator of Hoxa5 expression in the developing lung and gut. Our data also support a role for YY1 in the coordinated expression of Hox genes for correct organogenesis.

Altered epigenetic regulation of homeobox genes in human oral squamous cell carcinoma cells

To gain insight into oral squamous cell carcinogenesis, we performed deep sequencing (RNAseq) of non-tumorigenic human OKF6-TERT1R and tumorigenic SCC-9 cells. Numerous homeobox genes are differentially expressed between OKF6-TERT1R and SCC-9 cells. Data from Oncomine, a cancer microarray database, also show that homeobox (HOX) genes are dysregulated in oral SCC patients. The activity of Polycomb repressive complexes (PRC), which causes epigenetic modifications, and retinoic acid (RA) signaling can control HOX gene transcription. HOXB7, HOXC10, HOXC13, and HOXD8 transcripts are higher in SCC-9 than in OKF6-TERT1R cells; using ChIP (chromatin immunoprecipitation) we detected PRC2 protein SUZ12 and the epigenetic H3K27me3 mark on histone H3 at these genes in OKF6-TERT1R, but not in SCC-9 cells. In contrast, IRX1, IRX4, SIX2 and TSHZ3 transcripts are lower in SCC-9 than in OKF6-TERT1R cells. We detected SUZ12 and the H3K27me3 mark at these genes in SCC-9, but not in OKF6-TERT1R cells. SUZ12 depletion increased HOXB7, HOXC10, HOXC13, and HOXD8 transcript levels and decreased the proliferation of OKF6-TERT1R cells. Transcriptional responses to RA are attenuated in SCC-9 versus OKF6-TERT1R cells. SUZ12 and H3K27me3 levels were not altered by RA at these HOX genes in SCC-9 and OKF6-TERT1R cells. We conclude that altered activity of PRC2 is associated with dysregulation of homeobox gene expression in human SCC cells, and that this dysregulation potentially plays a role in the neoplastic transformation of oral keratinocytes.

Directed neural differentiation of mouse embryonic stem cells is a sensitive system for the identification of novel Hox gene effectors

The evolutionarily conserved Hox family of homeodomain transcription factors plays fundamental roles in regulating cell specification along the anterior posterior axis during development of all bilaterian animals by controlling cell fate choices in a highly localized, extracellular signal and cell context dependent manner. Some studies have established downstream target genes in specific systems but their identification is insufficient to explain either the ability of Hox genes to direct homeotic transformations or the breadth of their patterning potential. To begin delineating Hox gene function in neural development we used a mouse ES cell based system that combines efficient neural differentiation with inducible Hoxb1 expression. Gene expression profiling suggested that Hoxb1 acted as both activator and repressor in the short term but predominantly as a repressor in the long run. Activated and repressed genes segregated in distinct processes suggesting that, in the context examined, Hoxb1 blocked differentiation while activating genes related to early developmental processes, wnt and cell surface receptor linked signal transduction and cell-to-cell communication. To further elucidate aspects of Hoxb1 function we used loss and gain of function approaches in the mouse and chick embryos. We show that Hoxb1 acts as an activator to establish the full expression domain of CRABPI and II in rhombomere 4 and as a repressor to restrict expression of Lhx5 and Lhx9. Thus the Hoxb1 patterning activity includes the regulation of the cellular response to retinoic acid and the delay of the expression of genes that commit cells to neural differentiation. The results of this study show that ES neural differentiation and inducible Hox gene expression can be used as a sensitive model system to systematically identify Hox novel target genes, delineate their interactions with signaling pathways in dictating cell fate and define the extent of functional overlap among different Hox genes.

Cdx1 refines positional identity of the vertebrate hindbrain by directly repressing Mafb expression

An interplay of transcription factors interprets signalling pathways to define anteroposterior positions along the vertebrate axis. In the hindbrain, these transcription factors prompt the position-appropriate appearance of seven to eight segmental structures, known as rhombomeres (r1-r8). The evolutionarily conserved Cdx caudal-type homeodomain transcription factors help specify the vertebrate trunk and tail but have not been shown to directly regulate hindbrain patterning genes. Mafb (Kreisler, Krml1, valentino), a basic domain leucine zipper transcription factor, is required for development of r5 and r6 and is the first gene to show restricted expression within these two segments. The homeodomain protein vHnf1 (Hnf1b) directly activates Mafb expression. vHnf1 and Mafb share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of Mafb and, therefore, cannot establish the posterior Mafb expression boundary. Upon identifying regulatory sequences responsible for posterior Mafb repression, we have used in situ hybridization, immunofluorescence and chromatin immunoprecipitation (ChIP) analyses to determine that Cdx1 directly inhibits early Mafb expression in the neural tube posterior of the r6/r7 boundary, which is the anteriormost boundary of Cdx1 expression in the hindbrain. Cdx1 dependent repression of Mafb is transient. After the 10-somite stage, another mechanism acts to restrict Mafb expression in its normal r5 and r6 domain, even in the absence of Cdx1. Our findings identify Mafb as one of the earliest direct targets of Cdx1 and show that Cdx1 plays a direct role in early hindbrain patterning. Thus, just as Cdx2 and Cdx4 govern the trunk-to-tail transition, Cdx1 may regulate the hindbrain-to-spinal cord transition.

Effects of retinoic acid on Dominant hemimelia expression in mice

BACKGROUND

Dominant hemimelia (Dh) is an autosomal dominant mutation that arose spontaneously in mice. Dh animals are asplenic and they exhibit asymmetric hindlimb defects in association with reduced numbers of lumbar vertebrae. These defects suggest that Dh acts early in embryonic development to affect patterning of the anterior-posterior (A-P) and left-right axes. This study was undertaken to determine whether retinoic acid (RA), which is involved in A-P patterning and coordination of bilaterally synchronized somitogenesis, affects phenotypic expression of the Dh gene.

METHODS

Thirty-four pregnant females were given, by oral intubation, a single dose of 50 or 75 mg all-trans RA per kilogram body weight at GD 9, 10, or 11. The pregnant females were then euthanized at GD 18 and fetuses removed by cesarean section. A total of 326 fetuses were identified by phenotype and linked DNA and their skeletons were analyzed.

RESULTS

There was a differential effect of RA on the axial skeleton and hindlimb of Dh/+ mice as compared to their wild-type littermates. Dose- and stage-specific effects on sternebrae and vertebrae were observed.

CONCLUSIONS

The effects of RA dosing on numbers of sternebrae and vertebrae suggest that Dh embryos have a primary defect in retinoid-mediated A-P patterning. Dosing with RA may produce the observed effects on phenotypic expression of Dh/+ by indirectly or directly modifying an already existing altered Hox expression pattern. As the relationship between axial patterning and the asymmetric limb is unknown, Dh is an important model for studying this relationship.

Hox genes and segmentation of the vertebrate hindbrain

In the vertebrate central nervous system, the hindbrain is an important center for coordinating motor activity, posture, equilibrium, sleep patterns, and essential unconscious functions, such as breathing rhythms and blood circulation. During development, the vertebrate hindbrain depends upon the process of segmentation or compartmentalization to create and organize regional properties essential for orchestrating its highly conserved functional roles. The process of segmentation in the hindbrain differs from that which functions in the paraxial mesoderm to generate somites and the axial skeleton. In the prospective hindbrain, cells in the neural epithelia transiently alter their ability to interact with their neighbors, resulting in the formation of seven lineage-restricted cellular compartments. These different segments or rhombomeres each go on to adopt unique characters in response to environmental signals. The Hox family of transcription factors is coupled to this process. Overlapping or nested patterns of Hox gene expression correlate with segmental domains and provide a combinatorial code and molecular framework for specifying the unique identities of hindbrain segments. The segmental organization and patterns of Hox expression and function are highly conserved among vertebrates and, as a consequence, comparative studies between different species have greatly enhanced our ability to build a picture of the regulatory cascades that control early hindbrain development. The purpose of this chapter is to review what is known about the regulatory mechanisms which establish and maintain Hox gene expression and function in hindbrain development.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

HOXA5 acts directly downstream of retinoic acid receptor beta and contributes to retinoic acid-induced apoptosis and growth inhibition

The promise of retinoids as chemopreventive agents in breast cancer is based on the differentiation and apoptosis induced upon their binding to the retinoic acid (RA) receptor beta (RARbeta). We have previously shown that HOXA5 induces apoptosis in breast cancer cells. In this study, we investigated whether RA/RARbeta and HOXA5 actions intersect to induce apoptosis and differentiation in breast cancer cells. We found that HOXA5 expression can be induced by RA only in RARbeta-positive breast cancer cells. We have, for the first time, identified the RA response element in HOXA5, which was found to be located in the 3' end of the gene. Chromatin immunoprecipitation assays showed that RARbeta binds directly to this region in vivo. Overexpression of RARbeta strongly enhances RA responsiveness, and knocking down RARbeta expression abolishes RA-mediated induction of HOXA5 expression in breast cancer cells. In addition, there is coordinated loss of both HOXA5 and RARbeta expression during neoplastic transformation and progression in the breast epithelial cell model, MCF10A. Knockdown of HOXA5 expression partially abrogates retinoid-induced apoptosis and promotes cell survival upon RA treatment. These results strongly suggest that HOXA5 acts directly downstream of RARbeta and may contribute to retinoid-induced anticancer and chemopreventive effects.

Effect of single nucleotide polymorphisms on expression of the gene encoding thrombin-activatable fibrinolysis inhibitor: a functional analysis

Thrombin-activable fibrinolysis inhibitor (TAFI) is a plasma zymogen that acts as a molecular link between coagulation and fibrinolysis. Numerous single nucleotide polymorphisms (SNPs) have been identified in CPB2, the gene encoding TAFI, and are located in the 5'-flanking region, in the coding sequences, and in the 3'-untranslated region (UTR) of the CPB2 mRNA transcript. Associations between CPB2 SNPs and variation in plasma TAFI antigen concentrations have been described, but the identity of SNPs that are causally linked to this variation is not known. In the current study, we investigated the effect of the SNPs in the 5'-flanking region on CPB2 promoter activity and SNPs in the 3'-UTR on CPB2 mRNA stability. Whereas the 5'-flanking region SNPs (with 2 exceptions) did not have a significant effect on promoter activity, either alone or in haplotypic combinations seen in the human population, all of the 3'-UTR SNPs substantially affected mRNA stability. We speculate that these SNPs, in part, contribute to variation in plasma TAFI concentrations via modulation of CPB2 gene expression through an effect on mRNA stability.

Interplay between chromatin and trans-acting factors regulating the Hoxd4 promoter during neural differentiation

Correct patterning of the antero-posterior axis of the embryonic trunk is dependent on spatiotemporally restricted Hox gene expression. In this study, we identified components of the Hoxd4 P1 promoter directing expression in neurally differentiating retinoic acid-treated P19 cells. We mapped three nucleosomes that are subsequently remodeled into an open chromatin state upon retinoic acid-induced Hoxd4 transcription. These nucleosomes spanned the Hoxd4 transcriptional start site in addition to a GC-rich positive regulatory element located 3' to the initiation site. We further identified two major cis-acting regulatory elements. An autoregulatory element was shown to recruit HOXD4 and its cofactor PBX1 and to positively regulate Hoxd4 expression in differentiating P19 cells. Conversely, the Polycomb group (PcG) protein Ying-Yang 1 (YY1) binds to an internucleosomal linker and represses Hoxd4 transcription before and during transcriptional activation. Sequential chromatin immunoprecipitation studies revealed that the PcG protein MEL18 was co-recruited with YY1 only in undifferentiated P19 cells, suggesting a role for MEL18 in silencing Hoxd4 transcription in undifferentiated P19 cells. This study links for the first time local chromatin remodeling events that take place during transcriptional activation with the dynamics of transcription factor association and DNA accessibility at a Hox regulatory region.

Endocrine regulation of HOX genes

Hox genes have a well-characterized role in embryonic development, where they determine identity along the anteroposterior body axis. Hox genes are expressed not only during embryogenesis but also in the adult, where they are necessary for functional differentiation. Despite the known function of these genes as transcription factors, few regulatory mechanisms that drive Hox expression are known. Recently, several hormones and their cognate receptors have been shown to regulate Hox gene expression and thereby mediate development in the embryo as well as functional differentiation in the adult organism. Estradiol, progesterone, testosterone, retinoic acid, and vitamin D have been shown to regulate Hox gene expression. In the embryo, the endocrine system directs axial Hox gene expression; aberrant Hox gene expression due to exposure to endocrine disruptors contributes to the teratogenicity of these compounds. In the adult, endocrine regulation of Hox genes is necessary to enable such diverse functions as hematopoiesis and reproduction; endocrinopathies can result in dysregulated HOX gene expression affecting physiology. By regulating HOX genes, hormonal signals utilize a conserved mechanism that allows generation of structural and functional diversity in both developing and adult tissues. This review discusses endocrine Hox regulation and its impact on physiology and human pathology.

Molecular approaches to developmental malformations using analogous forms of valproic acid

The teratogenic potential of valproic acid has been well established both in experimental models and in human clinical studies. Evidence from many previous studies has shown that VPA is an appropriate drug model for studying chemical structure-teratogenicity relationships. Using molecular techniques of DNA microarray (GeneChip system) or quantitative real-time polymerase chain reaction with low teratogenic VPA analogs as comparative control drugs, we attempted to identify the genes involved with the molecular mechanisms of VPA teratogenicity in the neural tube and the axial skeleton of the mouse embryo. The recent development of DNA microarray enables a genome-wide approach to the identification of genes correlated with the teratogenicity of chemicals (teratogenomics). The VPA-induced changes in gene expression seen during mouse embryogenesis provides information for understanding how VPA disrupts normal embryonic development, and also provides leads for the development of safer medicines.

RARgamma is critical for maintaining a balance between hematopoietic stem cell self-renewal and differentiation

Hematopoietic stem cells (HSCs) sustain lifelong production of all blood cell types through finely balanced divisions leading to self-renewal and differentiation. Although several genes influencing HSC self-renewal have been identified, to date no gene has been described that, when activated, enhances HSC self-renewal and, when inactivated [corrected] promotes HSC differentiation. We observe that the retinoic acid receptor (RAR)gamma is selectively expressed in primitive hematopoietic precursors and that the bone marrow of RARgamma knockout mice exhibit markedly reduced numbers of HSCs associated with increased numbers of more mature progenitor cells compared with wild-type mice. In contrast, RARalpha is widely expressed in hematopoietic cells, but RARalpha knockout mice do not exhibit any HSC or progenitor abnormalities. Primitive hematopoietic precursors overexpressing RARalpha differentiate predominantly to granulocytes in short-term culture, whereas those overexpressing RARgamma exhibit a much more undifferentiated phenotype. Furthermore, loss of RARgamma abrogated the potentiating effects of all-trans retinoic acid on the maintenance of HSCs in ex vivo culture. Finally, pharmacological activation of RARgamma ex vivo promotes HSC self-renewal, as demonstrated by serial transplant studies. We conclude that the RARs have distinct roles in hematopoiesis and that RARgamma is a critical physiological and pharmacological regulator of the balance between HSC self-renewal and differentiation.

Polycomb homologs are involved in teratogenicity of valproic acid in mice

BACKGROUND

Valproic acid (VPA) is widely used to treat epilepsy and bipolar disorder and is also a potent teratogen, but its teratogenic mechanisms are unknown. We have attempted to describe a fundamental role of the Polycomb group (Pc-G) in VPA-induced transformations of the axial skeleton.

METHODS

Pregnant NMRI mice were given a single subcutaneous injection of vehicle or VPA (800 mg/kg) on gestation day (GD) 8. The expression of genes encoding Polycomb and trithorax groups was measured by quantitative real-time RT-PCR using total RNA isolated from the embryos exposed to vehicle or VPA for 1, 3, and 6 hr. In addition, the use of two less teratogenic antiepileptic chemicals valpromide (VPD) and valnoctamide (VCD) provide reliable evidence to support the relationship between VPA teratogenicity and the Polycomb group.

RESULTS

At a teratogenic level, VPA inhibits the expression of the Polycomb group genes, including Eed, Ezh2, Zfp144, Bmi1, Cbx2, Rnf2, and YY1 in the mouse embryos. In contrast, neither VPD nor VCD have significant effects on the expression of those genes affected by VPA. The trithorax group (trx-G) gene MLL, which is known to be required to maintain homeobox gene expression such as the Polycomb gene, is not affected by a teratogenic dose of VPA.

CONCLUSIONS

We propose that, during embryonic development, VPA may affect the gene silencing pathway mediated by the Polycomb group complex. The epigenetic mechanism of VPA teratogenicity on anteroposterior patterning is suspected.

Molecular Regulation of Müllerian Development by Hox Genes

HOX genes are a family of regulatory molecules that encode conserved transcription factors controlling aspects of morphogenesis and cell differentiation during normal embryonic development. All metazoans possess a common genetic system for embryonic patterning, and this system is also used in the reproductive tract. Hox genes are also expressed in the adult uterus. Hox genes are essential both for the development of mullerian tract in the embryonic period and adult function. Sex steroids regulate Hox gene expression during embryonic and endometrial development in the menstrual cycle. EMX2 and beta(3)-integrin acting downstream of Hoxa10 gene are likely involved in both these developmental processes. This article reviews the role and molecular regulation of Hox genes in reproductive tract development.

Pre-B cell leukemia transcription factor (PBX) proteins are important mediators for retinoic acid-dependent endodermal and neuronal differentiation of mouse embryonal carcinoma P19 cells

Pre-B cell leukemia transcription factors (PBXs) act as cofactors in the transcriptional regulation mediated by Homeobox proteins during embryonic development and cellular differentition. PBX1 protein is expressed throughout murine embryonic development, and its deletion in mice disrupts chondrogenesis. PBX protein levels are also increased in mouse embryonal carcinoma P19 cells during retinoic acid (RA)-induced differentiation. To elucidate the role of PBX proteins in this process, we stably overexpressed PBX1b antisense mRNA in P19 cells (PBX1b-AS cells). PBX1b-AS cells did not differentiate to neuronal or endodermal cells following treatment with RA suggesting PBX proteins are required for both processes. Furthermore we demonstrated that PBX proteins regulate the RA-dependent induction in the mRNA levels of bone morphogenetic protein 4 (BMP4) and Decorin (DCN) in P19 cells using both PBX1b-AS cells and PBX1 small interfering RNA. Chromatin immunoprecipitation assays further demonstrated that PBX proteins directly bind to the promoter of Bmp4 and Dcn in vivo in a RA-dependent fashion. In addition, type I and type II BMP receptor mRNA levels were also increased in P19 cells following RA treatment; however, this was PBX-independent. Taken together these data demonstrate that PBX proteins are required for RA-induced differentiation of P19 cells and that PBX proteins regulate the expression of BMP4 and DCN during this differentiation process.

Retinoic acid regulates the expression of PBX1, PBX2, and PBX3 in P19 cells both transcriptionally and post-translationally

Pre-B cell leukemia transcription factors (PBXs) are important co-factors for the transcriptional regulation mediated by a number of Hox proteins during embryonic development. It was previously shown that the expression of several Pbx genes is elevated in mouse embryo limb buds and embryonal carcinoma P19 cells upon retinoic acid (RA) treatment although the mechanism of this induction is not well understood. In this report, we demonstrate that PBX1a, PBX1b, PBX2, and PBX3 mRNAs and PBX1/2/3 proteins are induced during endodermal and neuronal differentiation of P19 cells in a RAR-dependent subtype-unspecific manner following RA treatment. The increases in both PBX1 mRNA and PBX3 mRNA levels are secondary responses to RA treatment requiring new proteins synthesis while the increase in PBX2 mRNA is a primary response. The RA-dependent increases in PBX1 mRNA, PBX2 mRNA, and PBX3 mRNA levels are likely to be transcriptionally regulated since the stability of these mRNAs does not change. In addition, the half-lives of PBX1/2/3 proteins are significantly extended by RA treatment. Two possible mechanisms could contribute to the stabilization of PBX proteins: PBX proteins associate with RA-dependent increased levels of MEIS proteins, and RA may decrease the proteasome dependent degradation of PBX proteins.

Axial skeletal and hox expression domain alterations induced by retinoic acid, valproic acid, and bromoxynil during murine development

Retinoic acid (RA) alters the developmental fate of the axial skeletal anlagen. "Anteriorizations" or "posteriorizations," the assumption of characteristics of embryonic areas normally anterior or posterior to the affected tissues, are correlated with altered embryonal expression domains of Hox genes after in utero RA treatment. These "homeotic" changes have been hypothesized to result from alterations of a "Hox cod" which imparts positional identity in the axial skeleton. To investigate whether such developmental alterations were specific to RA, or were a more general response to xenobiotic exposure, CD-1 pregnant mice were exposed to RA, valproic acid (VA), or bromoxynil (Br) during organogenesis. Additionally, the expression domains of two Hox genes, Hoxa7 and Hoxa10, were examined in gestation day (GD) 12.5 embryos obtained from control, RA, VA, or Br, treated gravid dams exposed on GD 6, 7, or 8. The anterior expression boundary of Hoxa7 is at the level of the C7/T1 vertebrae and that of Hoxa10 is at L6/S1. Compound-induced changes in the incidence of skeletal variants were observed. These included supernumerary cervical ribs (CSNR) lateral to C7, 8 vertebrosternal ribs, supernumerary lumbar ribs (LSNR) lateral to L1, extra presacral vertebrae, and the induction of vertebral and/or rib malformations. RA and VA administration on GD 6 caused posteriorization in the cervico-thoracic region (CSNR) while GD 8 exposure to any of the three compounds resulted in anteriorizations in the thoraco-lumbar area (LSNR and an increase in the number of presacral vertebrae). These effects occurred across regions of the axial skeleton. Analysis of gene expression demonstrated changes in the anterior boundaries of Hoxa7 expression domains in embryos treated on GD 6 and 8 with RA. VA and Br did not induce any statistically significant alterations in Hoxa7 and none of the compounds caused alterations in Hoxa10 expression domains. The studies indicate that RA GD 6 treatment-induced Hoxa7 shifts were rostral (posteriorization) while the RA-induced GD 8 anterior expression boundary shift was caudal (anteriorization), correlating with the axial skeletal changes noted. These data suggest that xenobiotic compounds such as VA and Br may induce similar axial skeletal changes by affecting different components of the developmental processes involved in the patterning of the axial skeleton.

The Cdx1 homeodomain protein: an integrator of posterior signaling in the mouse

The vertebrate Cdx genes (Cdx1 Cdx2 and Cdx4 in the mouse) encode homeodomain transcription factors related to the Drosophila caudal gene. The vertebrate Cdx gene products have been implicated in the development of the posterior embryo. In particular, loss- and gain-of-function experiments suggest that Cdx members are direct regulators of Hox genes and likely impart posterior information, in part, through this mechanism. Several signaling molecules, notably retinoic acid (RA*) and members of the Wnt (wingless) and fibroblast growth factor (FGF) families, are also implicated in patterning of the posterior vertebrate embryo. Interestingly, recent work indicates that members of the Cdx family are targets of Wnt, RA and FGF signaling, suggesting that Cdx factors act to convey the activity of these signaling molecules to Hox genes. This article will briefly review Cdx expression and function, with particular emphasis on vertebrate model systems.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Vitamin A deficiency in the late gastrula stage rat embryo results in a one to two vertebral anteriorization that extends throughout the axial skeleton

Vitamin A and its metabolites are known to be involved in patterning the vertebrate embryo. Study of the effect of vitamin A on axial skeletal patterning has been hindered by the fact that deficient embryos do not survive past midgestation. In this study, pregnant vitamin A-deficient rats were maintained on a purified diet containing limiting amounts of all-trans retinoic acid (12 microg atRA/g diet) and given a daily oral bolus dose of retinol starting at embryonic day 0.5, 8.25, 8.5, 8.75, 9.25, 9.5, 9.75, or 10.5. Embryos were recovered at E21.5 for analysis of the skeleton and at earlier times for analysis of select mRNAs. Normal axial skeletal development and patterning were observed in embryos from pregnant animals receiving retinol starting on or before E8.75. Delay of retinol supplementation to E9.5 or later resulted in a marked increase in both occurrence and severity of skeletal malformations, extending from the craniocervical to sacral regions. Embryos from the groups receiving retinol starting at E9.5 and E9.75 had one-vertebral anterior transformations of the cervical, thoracic, lumbar, and sacral vertebrae. Few embryos survived in the E10.5 group, but these embryos yielded the most severe and extensive anteriorization events. The skeletal alterations seen in vitamin A deficiency are associated with posterior shifts in the mesodermal expression of Hoxa-4, Hoxb-3, Hoxd-3, Hoxd-4, and Hoxa-9 mRNAs, whereas the anterior domains of Hoxb-4 and Cdx2 expression are unaltered. This work defines a critical window of development in the late gastrula-stage embryo when vitamin A is essential for normal axial skeletal patterning and shows that vitamin A deficiency causes anterior homeotic transformations extending from the cervical to lumbosacral regions.

The role of a retinoic acid response element in establishing the anterior neural expression border of Hoxd4 transgenes

The zebrafish hoxd4a locus was compared to its murine ortholog, Hoxd4. The sequence of regulatory elements, including a DR5 type retinoic acid response element (RARE) required for Hoxd4 neural enhancer activity, are highly conserved. Additionally, zebrafish and mouse neural enhancers function identically in transgenic mouse embryos. We tested whether sequence conservation reflects functional importance by altering the spacing and sequence of the RARE in the Hoxd4 neural enhancer. Stabilizing receptor-DNA interactions did not anteriorize transgene expression. By contrast, conversion of the RARE from a DR5 to a DR2 type element decreased receptor-DNA stability and posteriorized expression. Hence, the setting of the Hox anterior expression border is not a simple function of the affinity of retinoid receptors for their cognate element.

The role of retinoids and retinoic acid receptors in normal hematopoiesis

The dramatic therapeutic activity of all-trans retinoic acid (ATRA) in inducing terminal granulocytic differentiation of the malignant promyelocytes that characterize human acute promyelocytic leukemia (APL) has led to numerous studies assessing the role of retinoids and the retinoic acid receptors (RARs) in the regulation of normal hematopoiesis. Studies with knock out mice indicate that retinoic acid receptor activity is not essential for normal hematopoiesis, but both in vitro and in vivo studies indicate that these receptors may be important modifiers/regulators of different myeloid precursors/ progenitors including the primitive transplantable stem cell. A number of target genes have been identified that are either directly or indirectly regulated by RA receptors and which likely play important roles in the retinoid-mediated regulation of myelopoiesis. Several in vitro models of hematopoiesis suggest that the transcriptional activity of RA receptors is developmentally regulated during different stages of myelopoiesis. This regulation might involve non-ligand mediated molecular events that alter the interaction of RA receptors with transcriptional corepressor complexes. Moreover, the interaction of RA receptors with other families of transcription factors expressed in different hematopoietic lineages might also account for differential RA receptor activity at different stages of myelopoiesis.

Embryonic stem cells provide a powerful and versatile model system

Embryonic stem (ES) cells are pluripotent stem cells that differentiate both in vitro and in vivo into cell types derived from each of the three embryonic germ layers. ES cells and their close relatives, embryonal carcinoma (EC) cells and embryonic germ (EG) cells, have been used extensively as model systems for studying early mammalian development. This work has led to important insights into the mechanisms that control embryogenesis at the molecular and cellular levels. This chapter focuses on the use of ES cells as an in vitro model system for studying cellular differentiation and reviews several areas where important progress has been made. Impressive progress has been made in the isolation and characterization of ES cells from many species, including humans. Significant progress has also been made in the development of culture conditions that help direct the differentiation of ES cells to specific cell types that form during myogenesis, angiogenesis, hematopoiesis, neurogenesis, and cardiogenesis. The ability to inactivate virtually any gene in ES cells by gene targeting has vastly improved our understanding of the roles played by specific genes at the cellular and organismic levels. Moreover, ES cells and EC cells have been used widely to investigate how specific genes are turned on and turned off in the course of differentiation. In this connection, DNA array technology has been used to identify genes regulated when ES cells differentiate. The final section of this chapter discusses how work with ES cells is shaping our understanding of stem cells, mammalian development, and cell replacement therapy.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Analysis of two distinct retinoic acid response elements in the homeobox gene Hoxb1 in transgenic mice

Expression of vertebrate Hox genes is regulated by retinoids such as retinoic acid (RA) in cell culture and in early embryonic development. Retinoic acid response elements (RAREs) have been identified in Hox gene regulatory regions, suggesting that endogenous retinoids may be involved in the direct control of Hox gene patterning functions. Previously, two RAREs located 3' of the murine Hoxb1 gene, a DR(2) RARE and a DR(5) RARE, have been shown to regulate Hoxb1 mRNA expression in the neural epithelium and the foregut region, respectively; the foregut develops into the esophagus, liver, pancreas, lungs, and stomach. We have now examined the functional roles of these two types of 3' RAREs in regulating Hoxb1 expression at different stages of gestation, from embryonic day 7.5 to 13.5, in transgenic mice carrying specific RARE mutations. We demonstrate that the DR(5) RARE is required for the regulation of Hoxb-1 transgene region-specific expression in the gut and extraembryonic tissues, as well as for the RA-induced anteriorization of Hoxb-1 transgene expression in the gut. In contrast, expression of the Hoxb1 transgene in the neural epithelium requires only the DR(2) RARE. By in situ hybridization, we have identified a new site of Hoxb1 expression in the developing forelimbs at approximately day 12.5, and we show that, in transgenic embryos, expression in the forelimb buds requires that either the DR(2) or the DR(5) RARE is functional. Attainment of a high level of Hoxb1 transgene expression in other regions, such as in rhombomere 4 (r4) and in the somites, requires that both the DR(2) and DR(5) RAREs are functional. In addition, our transgenic data indicate that the Hoxb1 gene is expressed in other tissues such as the hernia gut, genital eminence, and lung. Our analysis shows that endogenous retinoids act through individual DR(2) and DR(5) RAREs to regulate Hoxb1 expression in different regions of the embryo and that functional redundancy between these DR(2) and DR(5) RAREs does not exist with respect to neural epithelium and the gut Hoxb1 expression.

RARgamma and Cdx1 interactions in vertebral patterning

Exogenous retinoic acid (RA) can evoke vertebral homeosis when administered during late gastrulation. These vertebral transformations correlate with alterations of the rostral limit of Hox gene expression in the prevertebrae, suggesting that retinoid signaling regulates the combinatorial expression of Hox genes dictating vertebral identity. Conversely, loss of certain RA receptors (RARs) results in anterior homeotic transformations principally affecting the cervical region. Despite these observations, the relationship between retinoid signaling, somitic Hox expression, and vertebral patterning is poorly understood. The members of the murine Cdx family (Cdx1, Cdx2, and Cdx4) are the homologues of Drosophila caudal and encode homeobox-containing transcription factors. Cdx1 homozygous null mutants exhibit anterior homeotic transformations, some of which are reminiscent of those in RARgamma null offspring. In Cdx1 mutants, these transformations occur concomitant with posteriorized prevertebral expression of certain Hox genes. Cdx1 has recently been demonstrated to be a direct RA target, suggesting an indirect means by which retinoid signaling may impact vertebral patterning. To further investigate this relationship, a complete allelic series of Cdx1-RARgamma mutants was generated and the skeletal phenotype assessed either following normal gestation or after administration of RA. Synergistic interactions between these null alleles were observed in compound mutants, and the full effects of exogenous RA on vertebral morphogenesis required Cdx1. These findings are consistent with a role for RA upstream of Cdx1 as regards axial patterning. However, exogenous RA attenuated several defects inherent to Cdx1 null mutants. This finding, together with the increased phenotypic severity of RARgamma-Cdx1 double null mutants relative to single nulls, suggests that these pathways also function in parallel, likely by converging on common targets.

Multiple pathways governing Cdx1 expression during murine development

Cdx1 encodes a mammalian homeobox gene involved in vertebral patterning. Retinoic acid (RA) is likewise implicated in vertebral patterning. We have previously shown that Cdx1 is a direct retinoid target gene, suggesting that Cdx1 may convey some of the effects of retinoid signaling. However, RA appears to be essential for only early stages of Cdx1 expression, and therefore other factors must be involved in maintaining later stages of expression. Based on function and pattern of expression, Wnt family members, in particular Wnt3a, are candidates for regulation of expression of Cdx1. Consistent with this, we confirm prior results which demonstrated that Cdx1 can be directly regulated by Wnt signaling, and identify functional LEF/TCF response motifs essential for this response. We also find that Cdx1 expression is markedly attenuated in a stage- and tissue-specific fashion in the Wnt3a hypomorph vestigial tail, and present data demonstrating that Wnt3a and RA synergize strongly to activate Cdx1. Finally, we show that Cdx1 positively regulates its own expression. These data prompt a model whereby retinoid and Wnt signaling function directly and synergistically to initiate Cdx1 expression in the caudal embryo. Expression is then maintained, at least in part, by an autoregulatory mechanism at later stages.

Hindbrain patterning involves graded responses to retinoic acid signalling

Several recent studies have shown that retinoic acid signalling is required for correct patterning of the hindbrain. However, the data from these studies are disparate and the precise role of retinoic acid signalling in patterning the anteroposterior axis of the neural tube remains uncertain. To help clarify this issue, we have cultured a staged series of chick embryos in the presence of an antagonist to the all three retinoic acid receptors. Our data indicate that retinoic acid is the transforming signal involved in the expansion of posterior hindbrain structures. We find that the hindbrain region of the neural tube down to the level of the sixth somite acquires the identity of rhombomere 4 when retinoic acid signalling is blocked. Specification of future rhombomere boundaries has a retinoic acid dependency between stage 5 and stage 10(+) that is lost progressively in an anterior-to-posterior sequence. Furthermore, the application of various concentrations of antagonist shows that successively more posterior rhombomere boundaries require progressively higher concentration of endogenous retinoic acid for their correct positioning, a result that strengthens the hypothesis that a complex retinoid gradient acts to pattern the posterior hindbrain. Our dissection of early retinoic acid functions allows us to re-interpret the wide disparity of hindbrain phenotypes previously observed in various models of retinoic acid deficiency.

Abnormal development of the sinuatrial venous valve and posterior hindbrain may contribute to late fetal resorption of vitamin A-deficient rat embryos

BACKGROUND

Normal embryonic development and survival in utero is dependent on an adequate supply of vitamin A. Embryos from vitamin A-deficient (VAD) pregnant rats fed an inadequate amount of all-trans retinoic acid (atRA; 12 microg per g of diet or approximately 230 microg per rat per day) exhibit severe developmental abnormalities of the anterior cardinal vein and hindbrain by embryonic day (E) 12.5 and die shortly thereafter.

METHODS

In the present study, we sought to determine whether supplementation of VAD-RA supported (12 microg per g of diet) pregnant rats with retinol (ROL) at the late-gastrula (presomite or rat E9.5) or early somite stages (E10.5), or provision of higher levels of atRA throughout this period could prevent abnormalities in the developing cardiovascular and nervous systems.

RESULTS

A newly described defect in the sinuatrial venus valve along with enlarged anterior cardinal veins and nervous system abnormalities and the later death of embryos are prevented by supplementing pregnant animals with ROL on the morning of E9.5. If ROL supplementation is delayed by 1 day (E10.5), most embryos are abnormal and die by E18.5. Supplementation of VAD rats with atRA (250 microg per g of diet) between E8.5 and E10.5 also prevents the cardiovascular and nervous system abnormalities and a significant number of these embryos survive to parturition. Thus, high levels of atRA can obviate the need for ROL between E9.5 and E10.5.

CONCLUSIONS

These results support an essential role for retinoid signaling between the late gastrula and early somite stages in the rat embryo for normal morphogenesis of the primitive heart tube and the posterior hindbrain. Further, these results suggest that embryonic death occurring at midgestation in the VAD rat may be linked to the abnormal development of one or both of these embryonic structures.

Murine hoxd4 expression in the CNS requires multiple elements including a retinoic acid response element

We have identified a retinoic acid response element (RARE) within a neural enhancer located 3' to the Hoxd4 gene. This RARE is required for the initiation and maintenance of Hoxd4 transgene expression in neurectoderm, and for full anteriorized expression upon retinoic acid (RA) treatment. Mutations within the sequence TTTTCTG, located 2 bp downstream of the RARE, posteriorized transgene activity. However, the onset of transgene expression and its response to RA were indistinguishable from wild type. While the TTTTCTG motif resembles a CDX binding site, human CDX1 protein did not interact with this element in vitro. Three additional regions were also shown to control transgene expression in neurectoderm, establishing that multiple elements constitute the Hoxd4 neural enhancer.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Regulation of granulopoiesis by transcription factors and cytokine signals

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

Vitamin A deficiency results in the dose-dependent acquisition of anterior character and shortening of the caudal hindbrain of the rat embryo

The developing nervous system is particularly vulnerable to vitamin A deficiency. Retinoid has been proposed to be a posteriorizing factor during hindbrain development, although direct evidence in the mammalian embryo is lacking. In the present study, pregnant vitamin A-deficient (VAD) rats were fed purified diets containing varying levels of all-trans-retinoic acid (atRA; 0, 0.5, 1.5, 6, 12, 25, 50, 125, or 250 microg/g diet) or were supplemented with retinol. Hindbrain development was studied from embryonic day 10 to 12.5 ( approximately 6 to 40 somites). Normal morphogenesis was observed in all embryos from groups fed 250 microg atRA/g diet or retinol. The most caudal region of the hindbrain was the most sensitive to retinoid insufficiency, as evidenced by a loss of the hypoglossal nerve (cranial nerve XII) in embryos from the 125 microg atRA/g diet group. Further reduction of atRA to 50 microg/g diet led to the loss of cranial nerves IX, X, XI, and XII and associated sensory ganglia IX and X in all embryos as well as the loss of hindbrain segmentation caudal to the rhombomere (r) 3/4 border in a subset of embryos. Dysmorphic orthotopic otic vesicles or immature otic-like vesicles in both orthotopic and caudally ectopic locations were also observed. As the level of atRA was reduced, a loss of caudal hindbrain segmentation was observed in all embryos and the incidence of otic vesicle abnormalities increased. Perturbations in hindbrain segmentation, cranial nerve formation, and otic vesicle development were associated with abnormal patterning of the posterior hindbrain. Embryos from VAD dams fed between 0.5 and 50 microg atRA/g diet exhibited Hoxb-1 protein expression along the entire neural tube caudal to the r3/r4 border at a time when it should be restricted to r4. Krox-20 protein expression was expanded in r3 but absent or reduced in presumptive r5. Hoxd-4 mRNA expression was absent in the posterior hindbrain, and the rostral limit of Hoxb-5 protein expression in the neural tube was anteriorized, suggesting that the most posterior hindbrain region (r7/r8) had been deleted and/or improperly patterned. Thus, when limiting amounts of atRA are provided to VAD dams, the caudal portion of the hindbrain is shortened and possesses r4/r5-like characteristics, with this region finally exhibiting r4-like gene expression when retinoid is restricted even more severely. Thus, regions of the anterior hindbrain (i.e., r3 and r4) appear to be greatly expanded, whereas the posterior hindbrain (r5-r8) is reduced or absent. This work shows that retinoid plays a critical role in patterning, segmentation, and neurogenesis of the caudal hindbrain and serves as an essential posteriorizing signal for this region of the central nervous system in the mammal.

Characterization of cis-elements required for the transcriptional activation of the rae28/mph1 gene in F9 cells

The rae28/mph1 gene is the mouse homologue of the Drosophila polyhomeotic gene, which plays a crucial role in the maintenance of the transcriptional repression state of Hox genes. Expression of the rae28/mph1 gene is induced during retinoic acid (RA)-mediated differentiation of embryonal carcinoma F9 cells. By transient-transfection experiments, we identified a pair of inverted differentiation response sequences (DRS(s)) in the 5' flanking region. Each of the DRS(s) contained the consensus sequence [5'-CCTCCCCXCXGCCCCCTCCXCXC-3'], which is also conserved in the human counterpart of the rae28/mph1 gene. Electrophoretic mobility shift assay and DNase I foot printing with nuclear extracts derived from F9 cells demonstrated the presence of novel DNA-binding factors which specifically interact with DRS(s). Nucleotide substitutions in the 3' DRS abrogated the factor binding and the transcriptional activation, suggesting that DRS(s) and DRS-binding factors play an important role in the transcriptional regulation of the rae28/mph1 gene.

The HNF-3alpha transcription factor is a primary target for retinoic acid action

We have previously demonstrated that gene expression of the hepatocyte nuclear factor 3alpha (HNF-3alpha) transcription factor is activated during retinoic-acid-induced differentiation of F9 embryonal carcinoma cells (A. Jacob et al. (1994). Nucleic Acids Res. 22, 2126-2133). We have extended these studies and now show that HNF-3alpha mRNA is induced approximately 6 h after addition of retinoic acid to the cells, peaks at 1 day postdifferentiation, and then declines to undetectable levels. Furthermore, HNF-3alpha induction occurs in the absence of de novo protein synthesis, suggesting that it is a primary target for retinoic acid action. In order to corroborate this hypothesis, we have mapped the cis-acting HNF-3alpha promoter site that mediates the retinoic acid response. DNA sequence analysis indicates that the HNF-3alpha promoter contains an authentic retinoic acid response element (RARE) of the DR5 class. As expected, this element is able to confer retinoic acid responsiveness to a heterologous promoter. In addition, the HNF-3alpha-specific RARE is able to interact with various retinoic acid receptor heterodimers of the RAR/RXR type. Since HNF-3alpha is induced early during mammalian neurogenesis, our data shed new light on the connection between retinoic-acid-mediated HNF-3alpha activation and establishment of the neuronal phenotype.

Differential distribution of retinoic acid synthesis in the chicken embryo as determined by immunolocalization of the retinoic acid synthetic enzyme, RALDH-2

Retinaldehyde dehydrogenase type 2 (RALDH-2) is a major retinoic acid generating enzyme in the early embryo. Here we report the immunolocalization of this enzyme (RALDH-2-IR) in stage 6-29 chicken embryos; we also show that tissues that exhibit strong RALDH-2-IR in the embryo contain RALDH-2 and synthesize retinoic acid. RALDH-2-IR indicates dynamic and discrete patterns of retinoic acid synthesis in the embryo, particularly within the somitic mesoderm, lateral mesoderm, kidney, heart, and spinal motor neurons. Prior to somitogenesis, RALDH-2-IR is present in the paraxial mesoderm with a rostral boundary at the level of the presumptive first somite; as the somites form, they exhibit strong RALDH-2-IR. Cervical presomitic mesoderm exhibits RALDH-2-IR but thoracic presomitic mesoderm does not. Neural crest cells do not express detectable levels of RALDH-2, but migrating crest cells are associated with RALDH-2 expressing mesoderm. The developing limb mesoderm expresses little RALDH-2-IR; however, RALDH-2-IR is strongly expressed in tissues adjacent to the limb. The most lateral, earliest-projecting motor neurons at all levels of the spinal cord exhibit RALDH-2-IR. Subsequently, many additional motor neurons in the brachial and lumbar cord regions express RALDH-2-IR. Motor neuronal expression of RALDH-2-IR is present in the growing axons as they extend to the periphery, indicating a potential role of retinoic acid in nerve influences on peripheral differentiation. With the exception of a transient expression in the facial/vestibulocochlear nucleus, cranial motor neurons do not express detectable levels of RALDH-2-IR.

RARbeta mediates the response of Hoxd4 and Hoxb4 to exogenous retinoic acid

One action of retinoids in development is the regulation of Hox gene expression. Hoxd4 and Hoxb4 expression in the embryonic hindbrain is anteriorized by retinoic acid (RA) treatment of mid-gestation mouse embryos. Here we demonstrate that retinoic acid receptor beta (Rarb) deficient mice present only a partial anteriorization of either Hoxd4 or Hoxb4 in response to RA treatment. Our results strongly suggest that RARbeta is a mediator of their RA-response, and reveal anteroposterior polarity within a single rhombomere (r). Additionally, we generated mice doubly mutated for Hoxd4 and Rarb in an attempt to identify common morphogenetic pathways between these two genes. We conclude that there are no synergistic interactions between Hoxd4 and Rarb in the specification of the cervical vertebrae.

Retinoids and mammalian development

All vertebrate embryos require retinoic acid (RA) for fulfilment of the developmental program encoded in the genome. In mammals, maternal homeostatic mechanisms minimize variation of retinoid levels reaching the embryo. Retinol is transported as a complex with retinol-binding protein (RBP): transplacental transfer of retinol and its uptake by the embryonic tissues involves binding to an RBP receptor at the cell surface. Embryonic tissues in which this receptor is present also contain the retinol-binding protein CRBP I and the enzymes involved in RA synthesis; the same tissues are particularly vulnerable to vitamin A deficiency. In the nucleus, the RA signal is transduced by binding to a heterodimeric pair of retinoid receptors (RAR/RXR). In general, the receptors show functional plasticity, disruption of one RAR or RXR gene having minor or no effects on embryogenesis. However, genetic studies indicate that RXR alpha is essential for normal development of the heart and eye. Excess RA causes abnormalities of many systems; altered susceptibility to RA excess in mice lacking RAR gamma or RXR alpha suggests that the teratogenic signal is transduced through different receptors compared with physiological RA function in the same tissue.