ToxSci Advance Access originally published online on June 13, 2006
Toxicological Sciences 2006 94(1):139-152; doi:10.1093/toxsci/kfl037
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Novel Retinoid Targets in the Mouse Limb during Organogenesis
Department of Pharmacology and Therapeutics, McGill University, Montréal, Québec, Canada H3G 1Y6
1To whom correspondence should be addressed at Department of Pharmacology and Therapeutics, McGill University, 3655 Promenade Sir-William-Osler, Montréal, Québec, Canada H3G 1Y6. Fax: (514) 398-7120. E-mail: barbara.hales{at}mcgill.ca.
Received March 30, 2006; accepted June 8, 2006
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ABSTRACT |
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Bioactive retinoids are potent limb teratogens, upregulating apoptosis, decreasing chondrogenesis, and producing limb-reduction defects. To target the origins of these effects, we examined gene expression changes in the developing murine limb after 3 h of culture with teratogenic concentrations of vitamin A. Embryonic day 12 CD-1 limbs were cultured in the absence or presence of vitamin A (retinol acetate) at 1.25 and 62.5µM (n = 5). Total RNA was used to probe Atlas 1.2 cDNA arrays. Eighty-one genes were significantly upregulated by retinol exposure; among these were key limb development signaling molecules, extracellular matrix and adhesion proteins, oncogenes, and a large number of transcriptional regulators, including Eya2, Id3, Snail, and Hes1. To relate these expression changes to teratogenic outcome, the response of these four genes was assessed after culture with vitamin A and retinoid receptor antagonists that are able to rescue retinoid-induced malformations; expression levels were correlated with limb malformations. Lastly, pathways analysis revealed that a large number of the genes significantly affected by retinoid treatment are functionally linked through direct interactions. Several regulatory gene cascades emerged relevant to morphogenesis, cell-fate, and chondrogenesis; moreover, members of these cascades crosstalk with one other. These results indicate that retinoids act in a coordinated fashion to disrupt development at multiple levels. In sum, this work proposes several unifying mechanisms for retinoid-induced limb malformations, identifies novel retinoid targets, and highlights Eya2, Id3, Snail, and Hes1 as potential key teratogenic effectors.
Key Words: limb development; apoptosis; chondrogenesis; retinoid receptor antagonists; gene expression; teratogenicity.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Vitamin A (retinol) and its metabolites, particularly the major bioactive metabolite, retinoic acid, are essential endogenous regulators of morphogenesis at physiological levels, but in excess are teratogenic to practically every developing organ system (Geelen, 1979
). Due to their diverse application, the use of retinoids and their derivatives is blossoming; exposure remains high in spite of cautionary measures and pregnancy prevention programs (Honein et al., 2001). Exposed fetuses have a 25-fold increased risk of congenital malformation. In both animals and humans, the pattern of anomalies induced depends on the developmental stage at the time of exposure (Lammer et al., 1985; Shenefelt, 1972).
The limb is a useful experimental model for studying how teratogens disturb the pathways of normal embryogenesis. During limb development, the undifferentiated cells of the limb bud mesenchyme are imparted with a specific three-dimensional positional identity, subsequently differentiating into tendon, cartilage, and muscle in the proper pattern. All the while the limb is elongating and limb shape is refined by selective programmed cell death by apoptosis (for review see, Francis-West and Tickle, 1996). Retinoids are crucial mediators of normal limb development at several stages (Mic et al., 2004; Niederreither et al., 2002), but in excess they interfere with the signals orchestrating these processes in time and space (for review see, Lee et al., 2004).
In animal studies, exposure to excessive levels of retinoids during mid-organogenesis results in reductive limb defects (Kochhar, 1973). Numerous studies have shown that retinoids cause changes in cell proliferation (Kochhar, 1985) and cell adhesion (Kwasigroch and Kochhar, 1980), upregulate apoptosis (Zakeri and Ahuja, 1994), and decrease chondrogenesis in the developing limb bud (Kwasigroch et al., 1986), but the molecular mediators underpinning these alterations have yet to be clearly defined.
Retinoids mediate most of their biological activity by binding to a heterodimer consisting of two subclasses of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) (Mangelsdorf et al., 1995). Receptor heterodimers exist bound to DNA retinoic acid response elements (RAREs) present in target gene regulatory regions; ligand-binding switches the heterodimer from a transcriptional repressor mode to transactivation, by releasing corepressors and recruiting coactivators and basic transcriptional machinery (for review see, Bastien and Rochette-Egly, 2004). More than 500 genes have been identified as retinoid-regulated in some way (Balmer and Blomhoff, 2002); recent studies have begun to link retinoid-inducible gene expression with specific teratogenic outcomes in the embryo (Qin et al., 2002; Williams et al., 2004).
Retinol induces dose-dependent decreases in limb outgrowth and chondrogenesis, and upregulates apoptosis in susceptible regions of the limb during organogenesis (Ali-Khan and Hales, 2006). To investigate how retinoids mediate their teratogenic effects during limb development, we have analyzed gene expression changes after vitamin A (all-trans retinol acetate; retinol) treatment of the mid-organogenesis mouse limb in vitro. To target early changes, exposure was limited to 3 h. In order to relate these changes in gene expression to teratogenic outcomes, we cultured limbs with retinoid receptor antagonists that have been previously shown to rescue limb bud apoptosis and morphology (Ali-Khan and Hales, 2006), as well as retinol, and assessed the expression of four transcriptional regulators, Id3, Snai1, Hes1, and Eya2 by real time qRT-PCR.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Limb bud cultures and drug treatments.
Limb bud cultures were done as previously described (Ali-Khan and Hales, 2003
). Briefly, embryos were dissected from timed-pregnant ED12 CD1 mice, forelimbs were excised and cultured. All-trans retinol acetate (Sigma, St Louis, MI) dissolved in 100% ethanol was added to the culture medium. For full investigation of the teratogenicity of retinol, we selected 1.25µM (1 IU) and 62.5µM (50 IU) for use in this study. Limb malformations are relatively mild at 1.25µM (1 IU); after 6 days in culture, treated limbs show some decrease in chondrogenesis, reduced limb outgrowth, and dysmorphogenic skeletal elements (shortened ulna and radius, and some missing, fused or indistinct phalanges and carpalia). In contrast, after culture for 6 days with 62.5µM (50 IU) retinol, treated limbs show severe defects; limb outgrowth and chondrogenesis are seriously retarded (Ali-Khan and Hales, 2003). The total volume of ethanol in vitro did not exceed 0.5% vol/vol.
BMS 189453 (BMS 453), a competitive pan-RAR antagonist, a gift from Bristol-Myers Squibb (Wallingford, CT) and HX603, a pan-RXR antagonist, a gift from Dr H. Kagechika, University of Tokyo, were dissolved in DMSO and added to the designated cultures with a final percentage of DMSO of 0.05 or less. Stock solutions of BMS 453 and HX603 were stored, protected from light, at – 20°C between usages. A control study examining the effects of DMSO and ethanol, together and separately, on limb development showed that ethanol at the concentrations used in this work had no significant effect on limb development (assessed using the Limb Morphogenetic Differentiation Scoring System developed by Neubert and Barrach, 1977), or quantification of limb area or percent cartilage. DMSO produced small but significant reductions in limb score and limb area of 3% (p = 0.04) and 2% (p = 0.02), respectively; there was no statistically significant interaction between the two vehicles when they were added to cultures together (for details see, Ali-Khan and Hales, 2006).
All animal studies complied with the guidelines set by the Canadian Council on Animal Care. Five separate replicates were completed for cDNA Atlas Array and another five separate replicates for real-time qRT-PCR.
RNA extraction.
Limbs were removed after 3 h of culture, rinsed with PBS, and stored at – 20°C in RNAlater RNA Stabilization Reagent (Qiagen, Mississauga, ON, Canada). Total RNA from the homogenized limbs was extracted using the RNeasy Mini Kit (Qiagen) for cDNA Atlas Arrays, or using the RNeasy Micro Kit (Qiagen) for real-time qRT-PCR, following the manufacturer's guidelines. The extracted total RNAs were assessed for quality and quantity by formaldehyde agarose gel electrophoresis and spectrophotometric ultraviolet absorbance at 260/280 nm, respectively.
Probe preparation and hybridization to cDNA Atlas Arrays.
RNA samples were used to probe cDNA arrays (Atlas Mouse 1.2 Array, Clontech Laboratories, Inc, Palo Alto, CA) according to the manufacturer's instructions. RNA (5 µg) was reverse transcribed using Moloney-murine-leukemia virus reverse transcriptase and radiolabeled with [
-32P]dATP (Amersham Pharmacia Biotech, Baie d'Urfé, PQ, Canada; 10 µCi/µl). Labeled cDNAs were purified from unincorporated 32P nucleotides by filtering through CHROMA SPIN-200 DEPC-H2O columns (Clontech). After prehybridization, the probe was added to nylon cDNA arrays spotted with 1176 cDNAs; hybridization was allowed to occur overnight with continuous agitation at 68°C. After 18 h, the arrays were washed three times for 30 min with washing solution 1 (2x sodium chloride/sodium citrate (SSC), 1% SDS) and once for 30 min with washing solution 2 (0.1x SSC, 0.5% SDS) with continuous agitation at 68°C. After a final wash with 2x SSC at room temperature, the membranes were wrapped and exposed to a phosphorimager plate for 24 h. Five arrays, each the result of a separate independent culture experiment, were completed for each treatment.
Analysis of gene expression.
Arrays were visualized after scanning with a phosphorimager (Storm; Molecular Dynamics, Sunnyvale, CA), and images were imported to AtlasImage software (version 2.0; Clontech) for quantification. The raw data for each gene (intensity minus the background) generated by AtlasImage software were imported into GeneSpring (version 7.2; Agilent Technologies, Palo Alto, CA) for further analysis. For each individual array, the gene detection threshold was set at a raw signal intensity of 2x the background intensity on that array. To minimize experimental variation, expression values were normalized to the median of all measurements on that individual array (standard experiment-to-experiment normalization). Statistical comparisons were made of retinol-treated versus control limbs by Student's t-test (p 
0.05) for genes that had passed a 
1.5-fold change filter in at least three out of five replicates. For K-means cluster analysis, "gene-to-gene normalization" in addition to "experiment-to-experiment normalization" was applied to the data; the gene expression value for each gene was normalized relative to the median of all values taken for that gene in each treatment. This allowed for clearer visualization of cluster patterns with all genes being closer on the vertical axis . Genes were only included in the group of genes upregulated from below threshold (Table 5), if they were significantly upregulated by 1.5-fold or more from their relative intensity value in control (0µM) (p 0.05); if the control value for these was below the cutoff threshold for expression, the p value is not shown in Table 5. All other genes not expressed in control but expressed after exposure to 1.25 or 62.5µM retinol are listed in the supplementary data. Volcano plots were generated using Genespring GX 7.3. Pathways Assist 3.0 software (Stratagene, La Jolla, CA) was used to visualize direct relationships between genes significantly affected by retinol exposure. Objects were limited to proteins and pathways. Controls were limited to promoter binding, direct regulation, expression, and regulation only. The starting group for analysis was all genes significantly upregulated by retinol (i.e., Tables 2–5).
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Real-time qRT-PCR.
RNA was diluted to working concentrations of 10 ng/ml and QuantiTect One-Step SYBR Green RT-PCR (Qiagen) was completed using the Roche LightCycler (Roche Diagnostics, Laval, QC, Canada) according to the manufacturer's instructions. PCR thermal cycling parameters were: 95°C for 15 min (1 cycle), 94°C for 15 s, 55°C for 30 s, and 72°C for 20 s (for 35–45 cycles, depending on the primers). Embryonic hindlimb tissues were used to make 1, 25, 50, and 100 ng/ml RNA stocks for standard curves for quantification. RT-PCR primers (Table 1) were designed with Primer3 software (http://frodo.wi.mit.edu/cgi-bin/primer3/primer3_www.cgi) and synthesized at the Sheldon Biotechnology Centre (McGill University, Montréal, QC, Canada); expression was normalized against 18S rRNA, and melting curve analyses were done following each PCR to determine the output and detection quality (i.e., formation of primer-dimers). Each treatment consisted of RNA from five separate culture experiments and each RNA sample was measured in duplicate.
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Statistical analysis for qRT-PCR.
Expression data were analyzed using two-way ANOVA (group 1 = retinol, group 2 = BMS 453 or HX603) and the post hoc Holm-Sidak multiple comparison test (Sigmastat Statistical Software, SPSS Inc, Chicago, IL). Data are expressed as means ± SEM. The minimum level of significance is p < 0.05 for all tests.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Global Gene Expression Changes Induced by Retinol in the Developing Limb
In control limbs (0µM, no exogenously added retinol), 478 (40%) of the 1176 genes present on the array were expressed, attesting to the transcriptionally active nature of this developing tissue (Fig. 1). In limbs exposed to 1.25 or 62.5µM retinol, 31 and 38% of the genes were expressed, respectively. The majority of detected genes were expressed in all three treatment groups (352); however, some genes were exclusively expressed in only one treatment or in two treatments. The list of all the genes expressed in control limbs (supplementary Table 1), and those that were exclusively expressed in one or two treatment groups (supplementary Tables 2 and 3), can be found in the supplementary data.
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K-Means Cluster Analysis after Retinol Treatment
To further characterize the response of the developing limbs to retinoid exposure, K-means cluster analysis was used to visualize the overall trends in gene expression. The starting group for analysis was all detected genes (518). Four distinct expression profiles emerged. Figures 2A, 2C, 2E, and 2G depict the average expression profile for the genes within each set, while Figures 2B, 2D, 2F, and 2H show the functional breakdown of each set. The genes in this study were categorized according to the functional classifications supplied with the Clontech Atlas Mouse 1.2 Arrays.
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Set 1 (53 genes) included genes that show a dose-dependent decline in expression from control (0µM) to 62.5µM retinol (Figs. 2A and 2B). These may represent transcripts that are involved in the normal functioning of the developing limb or whose decline contributes to dysmorphogenesis. Interestingly, Set 2 (126 genes) included genes that were downregulated at 1.25µM retinol, but were increased in expression at 62.5µM retinol (Figs. 2C and 2D). This group may represent genes that respond to a relatively smaller retinoid surge, 1.25µM, by decreasing in expression, but then under more severe stress are upregulated. Set 3 (220 genes) (Figs. 2E and 2F) and Set 4 (119 genes) (Figs. 2G and 2H) both included genes that were upregulated by retinoid exposure. The former showed relatively smaller changes in expression between control (0µM) and 1.25µM, but striking induction between 1.25 and 62.5µM retinol, while genes in Set 4 showed more progressive incline from control (0µM) to 62.5µM retinol. Some of the genes in Sets 3 and 4, which showed a dose-dependent response, may be directly regulated by retinoids through RAREs. Strikingly, most of the genes affected by retinol exposure belonged to the same functional groups in all four cluster analysis sets (Figs. 2B, 2D, 2F, and 2H). Gene lists, including accession numbers and the primary functional classifications for each of the four sets, are provided in the supplementary data. "Transcription factors," "transcriptional regulators," "growth factors, cytokines, chemokines and their receptors, " "intracellular signaling," and "cell-cycle-related" transcripts dominated each cluster set. "Oncogenes and tumor suppressors" were also highly featured in the sets that were generally upregulated by retinol, Sets 2, 3, and 4.
Statistical Analysis of Gene Expression Changes after Retinol Treatment
To select for genes that are more important in initiating limb malformations and are potentially directly regulated by retinoids, the data were filtered for statistically significant expression changes of 1.5-fold or more between treatment groups (Figs. 3A and 3B). In Figure 3, volcano plots show fold change plotted against statistical significance. Comparison of the fold change between control (0µM) and 1.25µM retinol (Fig. 3A) and control (0µM) and 62.5µM retinol (Fig. 3B) clearly shows that although genes were both up and downregulated at 1.25µM retinol, the overriding response to 62.5µM retinol was upregulation. None of the genes on the array were downregulated with statistical significance by the retinol treatments. Only one gene was significantly upregulated between control (0µM) and 1.25µM retinol; Eya2, a transcription factor involved in eye development (1.9-fold) (Fig. 3A) (Table 2), but 50 genes were significantly upregulated between control (0µM) and 62.5µM retinol (Fig. 3B) (Table 3) and 24 genes were significantly upregulated between 1.25 and 62.5µM retinol (Table 4). Fifteen genes were induced from below threshold (Table 5). The majority of these upregulated genes fell into Set 3 or Set 4 of the K-means cluster analysis sets (see final column in Tables 2–4
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Significant Upregulation between Control (0µM) and 62.5µM Retinol
Among the genes upregulated between control (0µM) and 62.5µM retinol (Table 3) are several well-characterized retinoid targets, including the oncogene Nmyc1, the RARE-regulated T-box transcription factor Tbx2 (Boskovic and Niles, 2004
; Wada et al., 1992), and several genes that are involved in the retinoid system/nuclear receptor signaling: RXR (Rxra), the LIF receptor (Lifr) (Lane et al., 1999), and calmodulin (Calm1), which is able to regulate the activity of the nuclear receptor coactivator CBP (Chawla et al., 1998). Many other genes of interest, some of which have been previously linked to apoptosis or chondrogenesis in the developing limb, were over-expressed as a consequence of retinol exposure. Members of fundamental limb development signaling pathways were a key affected group; multiple genes from the Wnt, BMP, Notch, and FGF families were upregulated, as was as the cell surface receptor syndecan 1 (Sdc1)(see Fig. 4).
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A number of the most highly upregulated transcripts on the array were transcription-related genes. Many of these are developmentally important; basic Helix-Loop-Helix and homeobox transcription factors, as well as repressors, were highly represented. Also related to morphogenesis were genes involved in protein dynamics and cell-extracellular matrix (ECM) attachment. Proteases (Ctsd, Ctsb, Mmp2) and the inhibitor Timp3 (see Fig. 4), as well as the major ECM components, glycoproteins laminin
5 (Lama5), nidogen (Nid), and the integral membrane protein endoglin (Eng) were all affected. Stress-response and oncogenes related to the regulation of cell-fate decision-making and apoptosis were also affected by retinol exposure in the limbs; these included Jund1, Nmyc1, Rad50, and Dnajc3, as well as cyclin D1 (Ccnd1) and the transcription factor E2F3 (E2f3) (see Fig. 4). A number of intracellular signaling mediators, in addition to the members of major signaling pathways mentioned above were affected, including three Map kinases (see Fig. 4), and the phosphoinositol kinase Pik3r1. Finally, several functionally unclassified and housekeeping genes were upregulated, including phospholipase A2 (Ywhaz), the most highly upregulated gene on the array (4.1-fold) (see Fig. 4).
Significant Upregulation between 1.25 and 62.5µM Retinol
A number of genes in this group (Table 4) were also present in the previous list (Table 3), while the remainder were part of the cluster analysis Set 2 that were downregulated at 1.25µM. Some may be mediators of stress-management, cell cycle exit and apoptosis, and do not necessarily represent a retinoid-specific response. Accordingly, several cyclins and cell cycle regulating kinases were represented; three were members of Set 2. The apoptosis-inducers survival of motor neuron (Smn) and presenilin (Pse) (Psen2) (see Fig. 4) as well as two stress response genes (Hsp105 and Ei24) were upregulated. Interestingly, presenilin is also involved in Notch and Wnt family signal transduction; two other noncanonical Wnt family members were also highly upregulated (Fzd3 and Fzd6).
Genes Induced from below the Detection Threshold
Genes in this group (Table 5) were either downregulated to below the threshold of detection at 1.25µM retinol, before being upregulated at 62.5µM (Set 2) or induced de novo by the retinol treatments (Sets 3 and 4). It is tempting to speculate that these latter genes may be especially important in triggering or executing retinoid-mediated apoptosis and teratogenesis. There were many genes of interest in this category. The developmental transcription factor distal-less 3 (Dlx3 [Set 3]) and transcriptional regulator c-Ets2 (Ets2 [Set 3]) (see Fig. 4) were highly induced. Stress-response proteins were affected (Hspa14, Sqstm1 [both Set 2]), as was the death receptor intracellular transducer Pea15 (Set 4).
Pathways Analysis
Next we employed Pathways Assist software and manual searches to visualize known regulatory relationships between the genes that were significantly affected by retinol in our study. The linkages mapped in Figure 4 show only primary interactions between the depicted genes. This analysis highlighted the fact that a large number of these genes are functionally linked, directly influencing either the expression or regulation of one another. Several regulatory cascades and cohorts of genes emerged that are pertinent to morphogenesis, cell fate decision-making, patterning, and chondrogenesis; moreover, these cohorts crosstalk with one other. For example, the bone morphogenetic protein Bmp4 appears as a major node of activity. Regulating and being regulated by it are the developmental signaling family members Bmp7, Fgfr3, Sdc1, Acvr2b, the homeobox transcription factors Msx1, Msx2, and Mapk14, all of which have been reported to be integral to the regulation of apoptosis and/or chondrogenesis in the limb bud and/or in developing craniofacial structures (Albertson et al., 2005; Dupe et al., 1999; Rodriguez-Leon et al., 1999; Zuzarte-Luis et al., 2004). Through not only Msx1 but also Mapk14, this cohort influences a cell cycle and proliferation-regulating group made up of the cyclins Ccnd1, Ccne1, and the transcription factors Jund1, Ets1, and E2f3. Going a step further, E2f3 regulates Nmyc1, which upregulates Id3. We hypothesize that this latter group and another that feeds back to Ccnd1, the Notch1, Notch2, and Hes1 group, may regulate chondrogenic differentiation in the limb (Light et al., 2005; Vasiliauskas et al., 2003; Watanabe et al., 2003); Notch1 and Notch2 are also mediators of limb bud apoptosis (Pan et al., 2005). Another cohort, Crk, Crkas (Bcar1), Cbl, and the adjacent Timp3 and Mmp2, are involved with cell adhesion and ECM remodeling, crucial aspects of morphogenesis (Abraham et al., 2005; Huang et al., 2002). Taken together, these results lead to the hypothesis that retinoid excess acts in a coordinated fashion, targeting genes at multiple levels of signaling networks. Crosstalk between signaling cascades may propagate these effects, disturbing cellular processes that are crucial to proper morphogenesis.
Array Verification by qRT-PCR
A search for key molecular mediators of retinoid-induced malformation must link changes in gene expression to abnormal phenotype. To move toward this goal, we selected four genes of particular interest from those significantly upregulated by the teratogenic retinol treatments. Two were involved in our pathways analysis, the transcriptional repressors Id3 and Hes1. Additionally, we included the repressor Snail and the transcription factor Eya2, and verified the retinoid responsiveness of all four by qRT-PCR.
Echoing the microarray analysis (Fig. 5), all four genes showed dose-dependent upregulation with retinol treatment (Fig. 6); this upregulation was significant between control (0µM) and 62.5µM retinol. Previous work shows that the pan-RAR and pan-RXR antagonists, BMS 453, and HX603, respectively, revert vitamin A–induced limb defects (including decreased chondrogenesis and upregulated apoptosis), back to the control phenotype. Optimal limb bud rescue in the presence of 1.25µM retinol occurred with 0.5µM BMS 453 or 12µM HX603. These antagonists produced more modest improvements at 62.5µM retinol (Ali-Khan and Hales, 2006). Likewise, in the present work, over-expression of Id3, Snai1, Hes1, and Eya2 was reduced to close to control values at 0.5µM BMS 453 (Figs. 6A, 6C, 6E, and 6G) or 12µM HX603 (Figs. 6B, 6D, 6F, and 6H). Both BMS 453 and HX603 were able to reduce retinol-induced over-expression of Id3, Snai1, Hes1, and Eya2 with similar success, indicating that these four genes are indeed regulated by retinoid signaling through the RAR-RXR heterodimer at some level; whether this is direct, mediated through an RARE, or indirect, mediated through intermediates, remains to be determined. In sum, gene expression analysis in the presence of the antagonists verifies that Id3, Snai1, Hes1, and Eya2 are retinoid-responsive and are upregulated as a prequel to aberrant retinol-induced apoptosis and limb malformations. Their return to baseline expression levels is correlated with reductions in ectopic apoptosis and improvements in limb outgrowth, and chondrogenesis, suggesting that Id3, Snai1, Hes1, and Eya2 are indeed molecular mediators of retinol-induced teratogenesis.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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The goal of this study was to uncover potential molecular mechanisms of retinol-induced limb malformation. Exposure to teratogenic concentrations of retinol during mid-organogenesis, normally a period of tight transcriptional control, shifts the expression profiles of large numbers of genes in the developing limb. Despite the relatively short exposure period, 3 h, and the dilution effect of using whole limb homogenates in this study, many of these gene expression changes were statistically significant.
The majority of the responder genes in this study did so in a dose-dependent fashion (Set 4 and some genes in Set 3; Tables 2–5; Fig. 5). These were the ones of most interest to us; many can be linked to the discrete endpoints that are obvious after retinol treatment: decreased chondrogenesis and outgrowth, and increased apoptosis. Most were expressed in control limbs (0µM retinol), implying roles in normal development, where endogenous retinol levels should contribute to their basal expression. Interestingly, a large number of the gene expression changes we noted impinged on apoptosis and chondrogenesis, morphogenesis and cell motility, often through an ability to alter cell-cell and matrix-cell adhesion, or to respond to or transduce such changes.
Impaired Condensation
Multiple groups have pointed to disturbances in the regulation of chondrogenesis as a key pathological event leading to limb-reduction defects after retinoid exposure (Cho et al., 2003; Kochhar, 1973; Weston et al., 2003). Our results highlight several stages in this process that may be targeted by retinol. Cell condensation is a crucial early step in morphogenesis and tissue formation, including establishment of the limb cartilage anlagen. Cell attachment to other cells, and importantly to the ECM, organizes developing tissues, regulates differentiation and survival, and influences growth factor signaling (for review see, Lonai, 2003). Retinoid exposure altered levels of the key ECM/cell adhesion genes, Lama5, Nid, and Eng, and also Sdc1, which play an important role in epithelial-mesenchymal interactions and chondrogenesis, and facilitate fibroblast growth factor signaling by presenting these molecules to their receptors (Hall and Miyake, 2000; Mundhenke et al., 2002). We hypothesize that changes in these gene products may not only alter tissue architecture and hinder chondrogenic differentiation but also deprive developing tissues of key growth or survival factors.
Many other genes involved in cell adhesion were significantly affected by retinol exposure. Alterations in cell surface attachment can activate downstream mediators including kinases, phosphatases, and GTPases (Shum et al., 2003); several of these were upregulated by retinol, some from below threshold (Rangap1 and Ptpri; Table 5). The adaptor Cbl and the transducers Crk and Crkas are known to regulate cell adhesion and motility (Huang et al., 2002; Scaife et al., 2003), while Calm1, Msx1, Msx2, and Snail family Zn finger repressors (Snai1 and Snai2) can all mediate adhesion and cell-sorting through the regulation of cadherins (Lincecum et al., 1998). The latter five have defined roles, not only in chondrogenesis (for review see, Hall and Miyake, 2000; Mototani et al., 2005; Seki et al., 2003) but also in apoptosis in the developing limb and/or craniofacial regions (Dowd et al., 1991; Montero et al., 2001; Zuzarte-Luis et al., 2004). The NF
B inhibitor Nfkbia, in addition to other mechanisms involved in inducing limb malformation, also affects both skeletal and craniofacial development through the regulation of epidermal-mesodermal interactions (Sil et al., 2004). Taken together, the abnormal expression of these gene products could disrupt cell-cell and cell-matrix attachment at multiple levels providing several routes for alterations in limb morphology, chondrogenesis, and apoptosis (see Fig. 4).
Impaired Differentiation: Id3, Hes1, and Snai1
Cell adhesion is intimately related to the timing of differentiation. The transcriptional repressors we have highlighted in this study may interfere with chondrogenesis at this switch point; Id3, Hes1, and Snai1 (as well as Snai2) are markers of undifferentiated states. Snai1 and Snai2, expressed in condensed precartilage mesenchyme but not in chondrocytes (Nieto et al., 1992), repress expression of the key cartilage ECM components, collagen type2A and aggreccan (Seki et al., 2003). Similarly, Id3 expression remains high in both the amphibian limb (Shimizu-Nishikawa et al., 1999) and neural crest cells under the influence of Nmyc1 (see Fig. 4), maintaining multipotency until the appropriate time for differentiation (Light et al., 2005). A similar regulatory mechanism might exist in the limb. In humans, Id3 has been functionally linked to proliferation and differentiation in cartilage (Asp et al., 1998). Similarly, the Notch effector Hes1 (see Fig. 4) may determine skeletal element length by regulating the differentiation rate of proliferating chondrocytes; misexpression of the Hes1 homolog in the developing chick results in shortened limbs (Vasiliauskas et al., 2003). Thus, we hypothesize that retinol-mediated over-expression of Snai1, Id3, and Hes1 may perturb the differentiation program of pre-chondrocytes, providing another mechanism that could underlie the decreased cartilage deposition seen in treated limbs.
Upregulated Apoptosis
Dramatic upregulation of apoptosis is another key event preceding retinoid-induced limb malformations (Ali-Khan and Hales, 2006; Alles and Sulik, 1989; Zakeri and Ahuja, 1994). While this effect is most probably multigenic, Eya2 is an excellent candidate for a key mediator decisive in triggering this process. Recent work confirms that Eya2 is retinoid-responsive and mediates apoptosis in the eye (Matt et al., 2005). Interestingly, when over-expressed, it was able to initiate cytochrome-c efflux from the mitochondria, a key cell survival decision point (Clark et al., 2002). Cytochrome-c is also released during retinoid-induced apoptosis in the limb (Ali-Khan and Hales, 2003). Hence, in our system, we suggest that Eya2 may mediate both normal developmental apoptosis, as well as the aberrant induction seen after over-expression by retinol. Verification of the functional roles for the genes in this study will be hastened by localization of their transcripts in the developing limb.
Gene Regulation
Transcriptional regulators, secreted growth factors, and signaling transducers, all families of genes capable of propagating widespread downstream change, were highly affected by the retinol exposures. Expression profiles in response to 1.25 and 62.5µM retinol were markedly different, while they were both up and downregulated by 1.25µM retinol (about 10-fold endogenous levels [Horton and Maden, 1995]), suggesting a more specific regulatory response, the vast majority of genes were over-expressed in response to 62.5µM retinol. There were no genes that were downregulated with statistical significance in this study. Timing, as well as different regulatory mechanisms may account for these observations. Several genes can be directly repressed by retinoids through liganded receptors (Kirfel et al., 1997), but for the most part repression is achieved through the induction of intermediates (Balmer and Blomhoff, 2002), most of which have not been identified; the transcriptional repressors we have highlighted in this work are candidates. Alternatively, for some genes, decreased expression may result from antagonism of other transcription factors (Caelles et al., 1997), or squelching of mutually utilized cofactors (Kamei et al., 1996). Similarly, induction of the genes on our lists may occur through upregulation of intermediates (or upstream cascade members; see Fig. 4), through direct RARE-mediated induction of the gene itself, or through less-specific mechanisms. Some of the genes that were induced at 62.5µM retinol, a concentration that induced widespread apoptosis by 24 h, are likely to be nonspecific players, responding to the high concentrations of exogenous retinol. All these influences, however, must factor into retinoid-induced teratogenesis.
Teratogenicity of Retinoids
Interestingly, some of the gene expression changes we noted after retinoid treatment of the developing limb were similar to those reported after exposure of cranial neural crest cells (CNCCs), to excess retinoic acid (Williams et al., 2004). In that report, isolated CNCCs in culture were treated for 6–48 h. We target more immediate effects with a tighter 3 h exposure window and offer a dose-response perspective on expression changes. Additionally, the variety of cell-types present in the developing limb allowed us to detect potential alterations in chondroprogenitors, and in undifferentiated mesoderm susceptible to retinoid-induced apoptosis. Teratogen-mediated apoptosis and altered chondrogenesis are common endpoints after exposure to developmental toxicants; elucidation of the pathways mediating such events should contribute to our understanding of normal and abnormal development in the limb and other structures.
In summary, these studies provide a first critical step in an integrated approach to defining the molecular basis for the teratogenic action of retinoids in the developing limb. Eighty-one retinoid-responsive genes were identified including novel retinoid targets, a number of which have defined functions in morphogenesis, apoptosis, and chondrogenesis. This is the first demonstration that the transcriptional regulators Snai1, Id3, Hes1, and Eya2 are induced during vitamin A–mediated limb malformations. We propose several potential mechanisms for retinoid-induced limb defects, as well as opening new avenues for the elucidation of retinoid function in the developing embryo and in other systems.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Supplementary Tables 1–4 are available online at http://toxsci.oxfordjournals.org/.
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ACKNOWLEDGMENTS |
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This work was supported by the Canadian Institutes for Health Research. We are grateful to Amar V. Singh and the Knudsen Laboratory at the Birth Defects Centre, Systems Analysis Laboratory, University of Louisville, Kentucky, for the Pathways Assist analysis, and to Dr Bernard Robaire for critical assessment of the manuscript. We thank Bristol Myers Squibb and Dr Hiroyuki Kagechika for their gifts of BMS 189453 and HX603, respectively.
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