ToxSci Advance Access originally published online on February 14, 2008
Toxicological Sciences 2008 102(2):413-424; doi:10.1093/toxsci/kfn010
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Glucocorticoids Alter Craniofacial Development and Increase Expression and Activity of Matrix Metalloproteinases in Developing Zebrafish (Danio rerio)
Joint Graduate Program in Toxicology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901
1 To whom correspondence should be addressed at Department of Biochemistry and Microbiology, 76 Lipman Drive, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901. Fax: (732) 932-8965. E-mail: lawhite{at}aesop.rutgers.edu.
Received October 25, 2007; accepted January 10, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Teratogenic effects are observed following long-term administration of glucocorticoids, although short-term glucocorticoid therapy is still utilized to reduce fetal mortality, respiratory distress syndrome, and intraventricular hemorrhage in preterm infants. However, the mechanism of glucocorticoid-induced teratogenicity is unknown. We hypothesize that glucocorticoid-induced teratogenesis is mediated through the glucocorticoid receptor (GR) and results from altering the expression and activity of the matrix metalloproteinases (MMPs). During embryogenesis, degradation of the extracellular matrix to allow for proper cellular migration and tissue organization is a tightly regulated process requiring appropriate temporal and spatial expression and activity of the MMPs. Studies have demonstrated that MMP gene expression can be either inhibited or induced by glucocorticoids in a variety of model systems. Using the zebrafish (Danio rerio) as a model of development, the data presented here demonstrate that embryonic exposure to the glucocorticoids dexamethasone or hydrocortisone increased expression of two gelatinases, MMP-2 (
1.5-fold) and MMP-9 (7.6- to 9.0-fold), at 72 h postfertilization (hpf). Further, gelatinase activity was increased approximately threefold at 72 hpf following glucocorticoid treatment, and changes in craniofacial morphogenesis were also observed. Cotreatment of zebrafish embryos with each glucocorticoid and the GR antagonist RU486 resulted in attenuation of glucocorticoid-induced increases in MMP expression (52–84% decrease) and activity (41–94% decrease). Furthermore, the abnormal craniofacial phenotype observed following glucocorticoid exposure was less severe following RU486 cotreatment. These studies demonstrate that in the embryonic zebrafish, dexamethasone, and hydrocortisone alter expression and activity of MMP-2 and -9, and suggest that these increases may be mediated through the GR. Key Words: zebrafish; glucocorticoids; matrix metalloproteinases; RU486; development.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Glucocorticoids play a central role in vertebrate physiology and are involved in numerous regulatory mechanisms associated with development, bone replication, bone differentiation, apoptosis, metabolism, circadian cell cycle rhythmicity, and the stress response, among others (Canalis and Delany, 2002
; Dickmeis et al., 2007). Glucocorticoids act as immunosuppressive and anti-inflammatory agents, and are therefore widely utilized to treat autoimmune and inflammatory disorders, transplant rejection, and lymphoproliferative diseases (Almawi et al., 2002). These compounds are also potent teratogens whose antenatal use has been linked to fetal growth restriction and cleft palate (Abbott, 1995; Mandl et al., 2006), and intrauterine programming of metabolic, neuroendocrine and cardiovascular disorders in adult life (Seckl, 2004). The exact mechanism by which glucocorticoids exert their teratogenic effects is unknown, although recent work in the zebrafish has demonstrated that these compounds are capable of upregulating matrix metalloproteinase-13 (MMP-13) (Hillegass et al., 2007). Further, glucocorticoids have been shown to stimulate MMP-9 expression through induction of soluble glucocorticoid-induced tumor necrosis factor receptor in murine macrophages (Lee et al., 2003, 2004). These results sit in direct opposition to what has been found in mammalian cell models in vitro, where MMP expression is inhibited by exposure to glucocorticoids (Canalis and Delany, 2002; Chakraborti et al., 2003; Vincenti et al., 1996). Despite contradictory findings, these studies suggest MMPs may be pivotal in eliciting glucocorticoid-induced teratogenicity and support further examination of the mechanism.
Extracellular matrix (ECM) remodeling is essential for a number of physiological processes including embryonic development, reproduction, tissue resorption, wound healing, and apoptosis (Brinckerhoff and Matrisian, 2002; Hulboy et al., 1997; Nagase and Woessner, 1999). Central to these processes are MMPs, a group of over twenty zinc-dependent endopeptidases responsible for precise and regulated ECM degradation. Dysregulation and excessive expression of MMPs have been tied to a number of pathological disorders such as osteo- and rheumatoid arthritis, emphysema, multiple sclerosis, bacterial meningitis, and tumor invasion and metastasis (D'Armiento et al., 1992; Folgueras et al., 2004; Hendrix et al., 2003; Leppert et al., 2001; Rundhaug, 2005). Recent work using zebrafish has focused on the role of MMPs during embryonic development. Our laboratory and others have shown that MMP-2, MMP-9, MMP-13, and membrane-type MMP-
and -β are required for normal zebrafish embryogenesis (Hillegass et al., 2007, unpublished data; Zhang et al., 2003a, b).
Most glucocorticoid-associated effects are mediated through the glucocorticoid receptor (GR), which belongs to the nuclear receptor superfamily and acts as a ligand-dependent transcription factor (Evans, 2005). A single GR has been identified in zebrafish thus far, although other related teleosts have two distinct GR genes (Bury et al., 2003; Greenwood et al., 2003). The teleost GR is unique from the mammalian GR in that the DNA-binding domain contains nine additional residues between the two zinc fingers. These nine amino acid inserts are remarkably conserved among teleostean fish species (Stolte et al., 2006; Terova et al., 2005) and appear to be the result of alternative splicing (Stolte et al., 2006). It has been suggested that because these residues promote greater DNA affinity in the GR, they could have been selected to serve the large spectrum of cortisol functions in fish (Lethimonier et al., 2002). Such GR splice variants have been characterized in the rainbow trout (Bury et al., 2003) and Burtons' mouthbrooder (Greenwood et al., 2003; Takeo et al., 1996) thus far, and it is believed this could result in separate biological functions for each receptor variant (Prunet et al., 2006).
The purpose of these studies is to examine the effects of the glucocorticoids dexamethasone and hydrocortisone on MMP-2 and MMP-9 expression and activity and to establish a causal link between activation of the GR and changes in MMP-2, MMP-9, and MMP-13 levels. Further we wish to better characterize the craniofacial defects known to result from exposure of embryonic zebrafish to these glucocorticoids. The gelatinases MMP-2 (gelatinase A) and MMP-9 (gelatinase B) readily degrade gelatins (denatured collagens) and intact collagen type IV, and the collagenase MMP-13 (collagenase-3) cleaves native interstitial collagens I, II, and III (Chakraborti et al., 2003). These particular MMPs were selected because they have been shown to be vital for normal development in both zebrafish and mice (Inada et al., 2004; Itoh et al., 1997; Mosig et al., 2007; Stickens et al., 2004; Vu et al., 1998). The data presented in this paper demonstrate that dexamethasone and hydrocortisone cause increases in MMP-2 and MMP-9 expression and activity in the zebrafish embryo at 72 h postfertilization (hpf), with resultant changes in craniofacial morphogenesis. Cotreatment with glucocorticoids and the GR antagonist RU486 results in attenuation of the increases in MMP-2, MMP-9, and MMP-13 expression and activity normally observed following glucocorticoid treatment, as well as a partial rescue of the abnormal craniofacial phenotype. These results further demonstrate that in the embryonic zebrafish, dexamethasone, and hydrocortisone alter expression and activity of MMPs, including MMP-2 and -9, and suggest that these increases may be mediated through the GR.
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MATERIALS AND METHODS |
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Zebrafish strains and husbandry.
The AB strain of zebrafish (Danio rerio), obtained from the Zebrafish International Resource Center, was used for all of the experiments described. Zebrafish were maintained and bred in an Aquatic Habitats recirculation system according to a husbandry protocol approved by the Rutgers University Animal Care and Facilities Committee.
Collection and treatment of zebrafish embryos.
Embryos were collected from breeding stocks of zebrafish and treated starting at 3 hpf. Treatments consisted of continuous (i.e., without renewal) exposure through 24, 48, 72, or 96 hpf to 100 mg/l of dexamethasone (254.81µM equivalent) or hydrocortisone (275.88µM equivalent) alone or in conjunction with 100–250nM mifepristone (RU486) in embryo medium (Westerfield, 2000). This dose of dexamethasone or hydrocortisone has been shown in our laboratory to be effective in inducing MMP-13 levels in developing zebrafish (Hillegass et al., 2007). Dexamethasone and hydrocortisone were dissolved in dimethylformamide (DMF) prior to being diluted in embryo medium to the experimental concentrations. In order to generate experimental concentrations of RU486, a 1mM stock of RU486 was first made by dissolving in 100% ethanol, and a subsequent 100µM working stock was generated by further dilution into ethanol. Final concentrations of RU486 were achieved by diluting this working stock in embryo medium. All RU486 stocks were stored at –20°C between uses. Controls, including a no treatment control of embryo medium alone and a solvent control consisting of ethanol and/or DMF alone were run concurrently with each treatment. When cotreatments of dexamethasone or hydrocortisone and RU486 were performed, a RU486 control consisting of the appropriate concentration of RU486 alone was included. The no treatment control embryos were monitored throughout the course of each study to confirm embryo viability. Data from any treatment in which > 10% of the solvent or no treatment control embryos exhibited developmental abnormalities were not considered for analysis. All chemicals and solvents were purchased from Sigma-Aldrich (St Louis, MO) and possessed purities 
98%.
Real-time reverse transcription–PCR.
Embryos collected for RNA isolation (n = 50–100) were snap frozen in 1.5-ml microcentrifuge tubes using liquid nitrogen and stored at –80°C. Total RNA was isolated from embryos using TRIzol (Invitrogen Carlsbad, CA) and DNase-treated (DNA-free kit, Ambion Austin, TX) to remove genomic DNA contamination. Total RNA yields were typically 1–2 µg/µl. Reverse transcription was performed on 1 µg aliquots of total RNA to produce complimentary DNA (cDNA) for real-time reverse transcription–PCR (RT-PCR) (quantitative RT-PCR; qRT-PCR) using an iScript cDNA Synthesis Kit (BioRad Hercules, CA). Real-time RT-PCR reactions were performed in triplicate using BioRad iQ SYBR Green Supermix, and cDNA amplification was performed for 40 cycles on a BioRad iCycler equipped with an iCycler iQ Detection System. Primers to zebrafish β-actin, MMP-2, MMP-9, and MMP-13 were used in amplification reactions. For β-actin, the forward primer was 5'-CGAGCAGGAGATGGGAACC-3' and the reverse primer was 5'-CAACGGAAACGCTCATTGC-3' giving a product size of 102 base pairs (bp). For MMP-2, the forward primer was 5'-AGCTTTGACGATGACCGCAAATGG -3' and the reverse primer was 5'-GCCAATGGCTTGTCTGTTGGTTCT-3' giving a product size of 224 bp. For MMP-9, the forward primer was 5'-AACCACCGCAGACTATGACAAGGA-3' and the reverse primer was 5'-GTGCTTCATTGCTGTTCCCGTCAA-3' giving a product size of 89 bp. For MMP-13, the forward primer was 5'-ATGGTGCAAGGCTATCCCAAGAGT-3' and the reverse primer was 5'-GCCTGTTGTTGGAGCCAAACTCAA-3' giving a product size of 289 bp. Real-time threshold cycle data were normalized to β-actin, which served as a loading control, and standard curves generated for MMP-2, MMP-9, and MMP-13 were used to quantify messenger RNA (mRNA) expression.
In situ hybridization.
The MMP-2 RNA probe was generated using a construct generously donated by Dr Robert Tanguay (Oregon State University) consisting of full-length MMP-2 cDNA expressed in pCR-Blunt II-TOPO. The plasmid was linearized using PstI, and SP6 and T7 RNA polymerase were used to generate antisense and sense, respectively, digoxigenin (DIG)-labeled RNA probes (DIG RNA Labeling Kit-SP6/T7, Roche Indianapolis, IN). The MMP-9 RNA probe was generated from a cDNA clone encoding a 318-bp portion of the MMP-9 gene amplified using the following primers: 5'-TTTGAGCTCTACAGTCTGTTTCTGGTGG-3' (forward primer; the italicized portion designates a SacI restriction site) and 5'-ATAGGATCCGGCGTCAAACTCCTT-3' (reverse primer; the italicized portion designates a BamHI restriction site). To generate this portion of the MMP-9 gene, total RNA from untreated 72 hpf embryos was isolated, DNase-treated, made into cDNA as described previously and polymerase-amplified via PCR. The 318-bp PCR product was digested using SacI and BamHI restriction enzymes and subsequently cloned into PSPT18 (Roche). The PSPT18-MMP9 construct was linearized with BamHI, and SP6 and T7 RNA polymerase were used to create sense and antisense probes, respectively.
Embryos to be used for in situ hybridization were grown in 0.003% (0.033 mg/ml in embryo medium) phenylthiourea (Sigma) to inhibit formation of pigmentation. Prior to initiation of staining, embryos were dechorionated, euthanatized using an overdose of MS-222 (Sigma), and fixed overnight in BT-fix (4% sucrose, 4% paraformaldehyde, 0.1M sodium phosphate, 0.15mM calcium chloride, titrated to pH 7.3) (Westerfield, 2000). The in situ hybridization protocol followed was a slight modification of that described by (Oxtoby and Jowett, 1993). Following staining, the embryos were cleared of nonspecific staining by being transferred into methanol for 10 min, soaked in isopropanol for 10 min, and then placed in 1,2,3,4-tetrahydronaphthalene (Sigma-Aldrich) for visualization.
In vitro zymography.
Lysates were prepared from 72 hpf zebrafish embryos (n = 30–50) as described by (Crawford and Pilgrim, 2005). Lysate protein concentration was determined using a Modified Lowry Protein Assay Kit (Pierce Rockford, IL) to ensure that each reaction contained equal amounts of total protein. In vitro zymography reactions were set up in 96-well plates as follows: 1 µl of lysate (corresponding to approximately 0.2–14 µg total protein), 10 µl of 1 mg/ml fluoresceinated type I or type IV DQ Collagen (Molecular Probes Carlsbad, CA), and lysis buffer (150mM NaCl, 10mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 2mM dithiotreitol, 0.1% Triton X-100, pH 8.0) to bring the volume to 200 µl. Ethylenediaminetetraacetic acid (EDTA) (1mM), GM6001 (100µM; Calbiochem Gibbstown, NJ), and MMP-2/MMP-9 inhibitor II (100µM; Calbiochem; Tamura et al., 1998) were added to these reactions as inhibitors. When inhibitors were utilized, reactions consisting of lysate, lysis buffer, and inhibitor only were allowed to preincubate overnight prior to addition of fluoresceinated substrate. Following preincubation, reactions were incubated for 24-72 h prior to measurement. Fluorescein isothiocyanate fluorescence was measured using a Perkin Elmer (Waltham, MA) HTS 7000 Plus Bio Assay Reader set for excitation at 492 nm and emission detection at 535 nm. All reactions were run in at least triplicate and were corrected for background fluorescence.
Alcian blue staining.
Embryos to be used for Alcian blue staining were grown in 0.003% phenylthiourea. Embryos were euthanatized using an overdose of MS-222, fixed overnight at 4°C in 4% paraformaldehyde, and then transferred to 70% ethanol to allow dehydration of the tissue for at least 24 h. Prior to staining, embryos were bleached in 30% hydrogen peroxide for approximately 2 h and then rinsed twice with 0.1% Tween-20 in phosphate buffered saline. Staining was performed overnight at room temperature using 0.1% Alcian blue 8GX (Sigma-Aldrich) that had been filtered through a 2 µm syringe filter. Following staining, embryos were placed in acidified ethanol for 2–4 h to allow for clearing of nonspecific staining. Finally embryos were washed in increasing concentrations of glycerol (15 min each of 20%, 50%, 80%, and 100%) and visualized as described previously.
Statistical analysis.
Statistical analysis was performed using the SigmaStat v1.0 computer software package (Jandel Scientific San Rafael, CA). Data were evaluated by one-way analysis of variance (ANOVA), and Dunnett's test or the Bonferroni t-test was used as the multiple comparison method. The probability level for statistical significance was p < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Exposure to Dexamethasone or Hydrocortisone Causes an Induction of MMP-2 and MMP-9 mRNA in Zebrafish Embryos
Previous studies from our laboratory show that zebrafish embryos respond to dexamethasone and hydrocortisone in a dose-dependent manner, and that a dose of 100 mg/l is required to generate responses in both MMP-13 mRNA expression level and phenotype (Hillegass et al., 2007
). Embryos exposed to 100 mg/l dexamethasone or hydrocortisone demonstrate a significant (p < 0.05) increase in MMP-2 and MMP-9 mRNA at 72 hpf measured via qRT-PCR (Fig. 1). Dexamethasone- and hydrocortisone-treated embryos exhibit a 1.6- and 1.5-fold induction in MMP-2 mRNA over solvent-control treated embryos, respectively. A 9.0- and 7.6-fold induction in MMP-9 mRNA is also observed in dexamethasone- and hydrocortisone-treated embryos, respectively. In situ hybridization of 72 hpf embryos using either a MMP-2–specific or MMP-9–specific mRNA probe confirms the glucocorticoid-induced increase in expression observed via qRT-PCR (Fig. 2). Sense probes for each of these MMPs yield essentially no signal and confirm the specificity of the antisense probes (data not shown). For MMP-2, solvent control embryos express transcript throughout their entire body, with the exception of the yolk sac and yolk sac extension. Slightly higher levels of MMP-2 expression appear to be localized rostrally and distinct areas of expression are observed in the neurocranial cartilage, midbrain, heart, and anterior kidney. Following treatment with either dexamethasone or hydrocortisone, increased transcript levels are seen in both the medial and lateral aspects of the trunk, the anterior kidney, the heart, and several areas associated with the head specifically. These include the neurocranial cartilage, pharyngeal cartilage, and midbrain. MMP-9 transcript in solvent control embryos is also localized rostrally, although some expression is seen in medial trunk superior to the yolk sac extension. Glucocorticoid-treated embryos exhibit increased MMP-9 expression levels throughout their entire trunk and head. Given the intensity of signal present in the embryo head following treatment with dexamethasone or hydrocortisone, it is difficult to distinguish explicit areas of increased expression.
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Gelatinase Activity is Increased in Embryos Treated with Dexamethasone or Hydrocortisone
In vitro zymography is a quantitative assay that has been shown to be an effective means of determining MMP activity in lysates isolated from zebrafish embryos (Crawford and Pilgrim, 2005
). Fluorescein-conjugated type IV collagen serves as a substrate for MMP-2 and MMP-9, and its degradation is measured by the release of the fluorescein, which is indicative of gelatinase activity. Approximately threefold higher gelatinase activity is observed in lysates isolated from 72 hpf zebrafish embryos treated with either dexamethasone or hydrocortisone (Fig. 3A). These data confirm that increased transcript levels detected via qRT-PCR and in situ hybridization result in increased protein levels and subsequent enzyme activity. Given the broad substrate specificity of many of the MMPs, the inclusion of several inhibitors was implemented as a means of ascribing the gelatinase activity observed to MMP-2 and MMP-9. In order to increase the effectiveness of the inhibitors, less lysate was added per reaction; hence, the amount of fluorescein-conjugated type IV collagen substrate cleavage was less, resulting in lower relative fluorescence units overall. However, glucocorticoid-induced increases in gelatinase activity were observed once again (data not shown). The divalent cation chelator EDTA (1mM) causes a complete loss of gelatinase activity in all treatment groups. This indicates that the activity observed is due to a metalloenzyme because these proteins require Zn2+ for proper three-dimensional structure and Ca2+ for stability and expression of catalytic activity (Bode et al., 1999; Nagase and Woessner, 1999) (Fig. 3B). Inclusion of GM6001, a potent, broad-spectrum inhibitor of MMPs, causes a significant (p < 0.05) decrease in type IV collagen degradation by lysates isolated from both dexamethasone-treated (47% inhibition) and hydrocortisone-treated (91% inhibition) embryos. Similarly, use of an inhibitor specific for MMP-2 and MMP-9 (MMP-2/MMP-9 Inhibitor II) results in a 34% decrease in activity by dexamethasone-treated embryo lysates and a 62% decrease in activity by hydrocortisone-treated embryo lysates. All inhibitors were also effective in decreasing activity in solvent control lysates (data not shown). These data suggest that the activity observed can be attributed, at least in part, to the gelatinases MMP-2 and MMP-9.
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RU486 Prevents Increases in MMP mRNA and Activity following Exposure to Dexamethasone or Hydrocortisone
It is generally accepted that the effects of glucocorticoids in fish are mediated through the GR (Prunet et al., 2006
); therefore, we utilized the GR antagonist RU486 to determine whether or not we could inhibit glucocorticoid-induced increases in MMP expression. This compound has been used successfully to study GR-related functions in other teleost models such as osmoregulation, urea excretion, and metabolism (DiBattista et al., 2006; Marshall et al., 2005; Rodela and Wright, 2006; Veillette et al., 1995, 2007a). Exposure of the embryos to dexamethasone or hydrocortisone through 72 hpf again causes an increase in MMP-2, MMP-9, and MMP-13 mRNA (Figs. 4A–C, respectively). Cotreatment of embryos with glucocorticoid and RU486, however, causes a significant (p < 0.05) decrease in MMP-2, MMP-9, and MMP-13 mRNA levels compared with embryos treated with glucocorticoid alone. In fact, MMP mRNA levels following cotreatment are comparable to those observed in solvent control and RU486 control embryos.
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In vitro zymography assays examining type I collagen (MMP-13) and type IV collagen (MMP-2 and MMP-9) degradation confirm the results achieved using qRT-PCR. Specifically, an increase in type I (Fig. 5A) and type IV (Fig. 5B) collagen degradation is observed in lysates isolated from embryos treated with dexamethasone or hydrocortisone. This increase in activity is attenuated in lysates isolated from zebrafish embryos cotreated with glucocorticoid and RU486. These data suggest that dexamethasone and hydrocortisone are acting through the GR, and that a link exists between activation of the GR and MMP expression and activity.
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Dexamethasone and Hydrocortisone Cause Aberrant Craniofacial Cartilage Development
In order to better characterize the changes known to occur in the craniofacial region following exposure to dexamethasone or hydrocortisone (Hillegass et al., 2007
), Alcian blue staining was performed. Alcian blue is a copper-containing phthalocyanine dye that stains glycosaminoglycans, a central component of cartilage and bone (Terry et al., 2000). Following staining procedures, morphometric measurements were carried out on the craniofacial cartilage of 96 hpf embryos. These consisted of measuring intercranial distance (ID), lower jaw length (LJL), and ceratohyal cartilage length (CCL). ID measurements were performed by measuring the distance between the medial edge of each eye. LJL and CCL measurements were performed by measuring the distance from a reference line drawn between the posterior edges of the hyosymplectic cartilage to the anterior edge of the Meckel's cartilage and ceratohyal cartilage, respectively. At 72 hpf, all treatment groups exhibit a well-developed neurocranial cartilage architecture (Fig. 6A). In the solvent control embryos, partially developed pharyngeal cartilages are observable, with the hyosymplectic, palatoquadrate, and ceratohyal cartilages being the most apparent. Partially developed hyosymplectic and palatoquadrate cartilages are also observed in dexamethasone- and hydrocortisone-treated embryos, although the level of chondrification of these cartilage types is appreciably less. Alcian blue staining of 96 hpf embryos reveals a more well-developed cartilaginous head skeleton, with all treatments possessing neurocranial, pharyngeal, and ceratobranchial cartilage types (Fig. 6B). However, morphometric measurements of ID, LJL, and CCL reveal quantifiable differences between treatment groups (Fig. 6C). In particular, dexamethasone-treated embryos possess significantly (p < 0.05) smaller CCLs, and hydrocortisone-treated embryos have both significantly (p < 0.05) decreased CCLs and LJLs. In addition, a noticeable increase in the angle between the ethmoid plate and Meckel's cartilage is seen in glucocorticoid-treated embryos. Taken collectively, these data indicate that dexamethasone and hydrocortisone are capable of causing quantifiable changes in craniofacial parameters during zebrafish embryogenesis, possibly through a mechanism involving impairment of the anterior growth and migration of the pharyngeal cartilages.
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RU486 Partially Rescues Abnormal Craniofacial Phenotypes Observed following Treatment with Dexamethasone or Hydrocortisone
As demonstrated previously, RU486 is capable of preventing glucocorticoid-induced increases in MMP mRNA and activity levels. Given these data, we proposed that RU486 would also be able to rescue the abnormal craniofacial phenotypes known to result from exposure to dexamethasone or hydrocortisone. Alcian blue staining of 96 hpf embryos demonstrates that a partial rescue in glucocorticoid-induced phenotype can be achieved by cotreatment with RU486 (Fig. 7A). Embryos cotreated with either dexamethasone or hydrocortisone and RU486 possess LJLs and CCLs closer to the controls compared with their counterparts treated with dexamethasone or hydrocortisone alone. Further, the angle between the ethmoid plate and Meckel's cartilage in these embryos is similar to that observed in the controls. However, morphometric measurements reveal that statistical significance between cotreated and glucocorticoid-only groups is not achieved even though a general trend implying phenotype rescue is apparent.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Despite being known teratogens, glucocorticoids are one of the most commonly prescribed drugs worldwide due to their anti-inflammatory and immunosuppressant properties (Bello and Garrett, 1999
). Short-term antenatal glucocorticoid therapy is also generally implemented as a means of reducing fetal mortality, respiratory distress syndrome and intraventricular hemorrhage in preterm infants (Sloboda et al., 2005). Several medical conditions during pregnancy, such as asthma, systemic lupus erythematosus, allografts, and inflammatory bowel disease require long-term administration of glucocorticoids (Pacheco et al., 2007). Well-described deleterious side effects from long-term glucocorticoid administration during pregnancy include growth restriction in fetuses (Seckl, 1994) and intrauterine programming of cardiovascular, metabolic, and neuroendocrine disorders in adult life (Seckl, 2004). Also, retrospective studies of women receiving pharmacologic doses of glucocorticoids during pregnancy reveal instances of cleft palate, hydrocephalus, gastroschisis, stillbirths, and premature births (AHFS, 2000). Because the exact mechanism by which glucocorticoids elicit teratogenicity is unknown, the studies described here focused on the role of MMPs and the ability of glucocorticoids to modulate levels of these metalloenzymes during embryogenesis.
Glucocorticoids are key endocrine factors in teleosts, where they are involved in regulation of numerous physiological functions related to metabolism, growth, reproduction, osmoregulation, respiration, and the immune and stress responses (Prunet et al., 2006). The major circulating glucocorticoid in teleosts is cortisol (i.e., hydrocortisone) (Prunet et al., 2006; Vijayan et al., 2003), and was selected for use in our studies. The second glucocorticoid examined in these studies, dexamethasone, is a more potent GR agonist as demonstrated by in vitro experiments in rainbow trout, during which dexamethasone activated both variants of the GR at lower concentrations than cortisol (Bury et al., 2003). In addition, other reports demonstrate that dexamethasone was able to elicit a stronger reporter gene activation than similar concentrations of cortisol (Greenwood et al., 2003; Takeo et al., 1996).
Data presented here demonstrated that expression of MMP-2 and MMP-9 were increased at 72 hpf following glucocorticoid treatment compared with solvent controls as determined by qRT-PCR and in situ hybridization. Recent data in murine macrophages show that glucocorticoids stimulate MMP-9 expression through induction of soluble glucocorticoid-induced tumor necrosis factor receptor (Lee et al., 2003, 2004). Also, earlier studies conducted in our laboratory have shown that dexamethasone and hydrocortisone are able to induce MMP-13 levels during zebrafish embryogenesis (Hillegass et al., 2007). Whereas MMP-13 transcript levels were previously demonstrated to increase at both 24 and 72 hpf (Hillegass et al., 2007), MMP-2 and MMP-9 transcript levels increased at 72 hpf only. This difference may be due to variable sensitivity of the different MMP promoters to glucocorticoids. Alternatively, these differences may be a reflection of the different sensitivities of the changing cell populations present during embryonic development. Additional qRT-PCR data presented here suggest that base levels of MMP-2 mRNA were approximately 12-fold to 46-fold higher than MMP-9 mRNA at equivalent time points. This finding was supported by mRNA localization studies using in situ hybridization, which revealed MMP-2 mRNA was widespread throughout the entire embryo, whereas MMP-9 mRNA expression was restricted to the head and medial portion of the embryo trunk. Similar differences in expression of these gelatinases have been observed previously (Hillegass et al., unpublished data), and it has been proposed that the widespread expression of MMP-2 could be explained by the fact that the promoter of this gene has several characteristics of a constitutive (housekeeping) promoter (Chakraborti et al., 2003; Huhtala et al., 1990). Induction of MMP-9 was more prevalent as indicated by 7.6- to 9-fold higher levels of MMP-9 mRNA versus controls, compared with approximately 1.5-fold higher MMP-2 mRNA. This was substantiated by in situ hybridization, which showed increased MMP-9 mRNA following glucocorticoid treatment to the point where individual areas of signal could not be identified due to overall intensity. MMP-2 mRNA signal also increased following treatment with dexamethasone or hydrocortisone, but to a lesser degree. These findings oppose those reported for several in vitro mammalian cell models, which show that glucocorticoid treatment results in a downregulation of MMPs (Vincenti et al., 1996). However, these studies dealt with a single cell type rather than a whole animal model as described here, and are therefore not directly comparable.
Our data show that glucocorticoid-induced MMP-2 and MMP-9 expression is followed by an increase in gelatinase activity as determined by in vitro zymography. Fluoresceinated type IV collagen was used for this assay, which serves as substrate for both MMP-2 and MMP-9. Utilization of a MMP-2/MMP-9–specific N-sulfonylamino acid inhibitor (Tamura et al., 1998) resulted in a 34% and 62% decrease in activity by dexamethasone- and hydrocortisone-treated embryo lysates, respectively, constituting a partial inhibition of type IV collagenase activity. The fact that only a partial inhibition was achieved may be representative of the decreased effectiveness of this inhibitor against zebrafish MMPs, or may indicate that other MMPs present in these lysates have gelatinolytic activity. Type IV collagens are also substrates for MMP-3, MMP-7, MMP-10, MMP-12, and MMP-26 (Chakraborti et al., 2003). Of these, only homologs to MMP-3 and MMP-7 have been identified in zebrafish, although a full characterization of these genes has yet to be conducted. Although there may be some contribution of other potentially unidentified zebrafish type IV collagenases in these assays, the ability of the MMP-2/MMP-9–specific inhibitor to at least partially inhibit type IV collagenase activity demonstrated that the increases in activity observed following glucocorticoid treatment were most likely the result of MMP-2 and MMP-9.
Our laboratory has previously reported a number of developmental abnormalities and localized lesions in zebrafish embryos resulting from exposure to dexamethasone and hydrocortisone (Hillegass et al., 2007). These abnormalities included changes in the normal head–trunk angle, truncated body axis, changes in the shape and size of the yolk sac and yolk sac extension, altered somitogenesis, and abnormal craniofacial morphogenesis. In order to better characterize the changes known to occur in the craniofacial region during zebrafish embryogenesis following exposure to dexamethasone and hydrocortisone, Alcian blue staining was performed. Our data show that dexamethasone and hydrocortisone were capable of altering parameters associated with both mandibular and hyoid arches, suggesting impairment of the anterior growth and migration of the pharyngeal cartilages. Specifically, dexamethasone-treated embryos possessed significantly (p < 0.05) smaller CCLs, and hydrocortisone-treated embryos had both significantly (p < 0.05) decreased CCLs and LJLs. An increase in the angle between the ethmoid plate and Meckel's cartilage was also observed in glucocorticoid-treated embryos. These differences were discernable starting at 72 hpf, suggesting a progressive effect rather than cartilage degradation or dedifferentiation. No differences in ID were observed between control and glucocorticoid-treated embryos. Zebrafish eyes are derived from a single field of cells originating in the anterior neural plate, so any alterations in ID would imply changes in neural plate growth. However, because no differences were observed following treatment despite the prevalence of MMP-2 and MMP-9 in the head mesenchyme, perhaps these MMPs play minor roles in neural plate growth. In our laboratory, previous exposures of zebrafish embryos to identical concentrations of dexamethasone (100 mg/l) caused significant (p < 0.05) increases in intercranial (interocular) distance at 72 hpf (Hillegass et al., 2007). This may indicate that changes in ID following glucocorticoid treatment are variable. Interestingly, the changes observed in the neurocranial and pharyngeal cartilages following glucocorticoid treatment were similar to those observed in zebrafish embryos following knockdown of MMP-2, MMP-9, and MMP-13 using antisense morpholino oligonucleotides (Hillegass et al., unpublished data). Because the ECM plays critical roles in both neural crest cell migration (Henderson and Copp, 1997; Perris, 1997; Perris and Perissinotto, 2000) and cartilage morphogenesis (Kimmel et al., 1998, 2001), it appears that dysregulation of the MMPs responsible for remodeling the ECM contributes to abnormal craniofacial patterning.
RU486 is a GR receptor antagonist that binds directly to the GR, eliciting a transconformational change in the DNA-binding domain and a subsequent inability to bind to the glucocorticoid response element (GRE) in the promoter region of target genes (Cadepond et al., 1997; Mahajan and London, 1997). RU486 has also been shown to competitively displace cortisol from high affinity binding sites (Pottinger, 1990). We chose to use RU486 to abrogate GR function rather than an antisense morpholino oligonucleotide given the potential existence of multiple GR isoforms in the zebrafish. One published study has used morpholino knockdown of the GR to prevent glucocorticoid-induced blockage of tailfin regeneration in zebrafish larvae (Mathew et al., 2007). However, it is apparent the question of the actual number of zebrafish GRs is debatable given recent opposing studies suggesting that zebrafish have either one (Alsop and Vijayan, 2007) or two (Schaaf et al., 2007) GR isoforms. Further, if multiple isoforms do exist, performing morpholino knockdown of one particular GR transcript may not necessarily change receptor function if other GR isoforms are still available to interact with glucocorticoids. We utilized RU486 because it is a chemical inhibitor likely to block all GR activity regardless of isoform, and has been used in a similar capacity in numerous published reports (DiBattista et al., 2006; Marshall et al., 2005; Rodela and Wright, 2006; Veillette et al., 1995, 2007a,b). Morpholino knockdown studies may be more appropriate when the zebrafish GR is better characterized.
Our data demonstrate that cotreatment of zebrafish embryos with dexamethasone or hydrocortisone and RU486 inhibited the glucocorticoid-associated induction of MMP transcript shown to occur at 72 hpf. RU486 caused a 52–84% reduction in MMP expression depending on the MMP type and treatment, with an average reduction of 63% across all MMPs. RU486 was also able to prevent glucocorticoid-associated increases in type I or type IV collagen degradation, indicating that the ability of this compound to block transcription results in a reduction in active protein as well. RU486 caused a 41–89% reduction in type I collagen degradation and a 60–94% reduction in type IV collagen degradation depending on the treatment. Finally, RU486 was able to decrease the severity of craniofacial changes that typically occur following glucocorticoid treatment, namely changes in LJL, CCL, and in the angle between the ethmoid plate and Meckel's cartilage. Morphometric measurements revealed that statistical significance between cotreated and glucocorticoid-only groups was not achieved, although a general trend implying phenotype rescue was apparent. This may have simply been a function of increased variability due to treatment with multiple compounds (indeed, the standard deviation among treatment groups is noticeably greater). Regardless, these data imply that RU486 is an effective GR antagonist in this model system and serves as a valuable tool in examining glucocorticoid-induced changes in MMP expression and activity.
Collectively, our data demonstrate that the glucocorticoids dexamethasone and hydrocortisone are capable of activating MMP-2 and MMP-9 expression and activity at 72 hpf. Developmentally, this induction results in altered craniofacial morphogenesis related to migration and growth of pharyngeal cartilages. This was essentially a recapitulation of the deleterious changes in the human palate (cleft palate) known to result from gestational exposure to glucocorticoids. Therefore, the teratogenic effects resulting from prolonged treatment with glucocorticoids during gestation may stem from the ability of these compounds to cause dysregulation of MMPs, subsequently leading to altered craniofacial cartilage migration and growth. Use of the GR antagonist RU486 resulted in attenuation of the increases in MMP expression and activity normally observed following glucocorticoid treatment, as well as a partial rescue in the abnormal craniofacial phenotype. These results suggest that in the embryonic zebrafish, dexamethasone, and hydrocortisone function through the GR, and that activation of this receptor can modulate expression of MMPs. However, further study is required to determine mechanistically how the GR and MMPs are interacting. Given the extensive amount of literature available regarding the promoter regions of the various mammalian MMPs, it does not seem likely that zebrafish MMP promoters contain GREs, although luciferase reporter studies could be conducted to confirm this absence. As a better understanding of the true complement of MMPs present and active in zebrafish becomes available, so will an appreciation of the complexity of their regulation. In terms of regulation by glucocorticoids specifically, this will be aided by a better characterization of both the number and differential expression patterns of various GR isoforms in the zebrafish. Nonetheless, the data presented here identify MMPs as a potential in utero therapeutic target for the prevention of glucocorticoid-induced teratogenic effects.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Zebrafish International Resource Center (ZIRC) is supported by the National Institutes of Health—National Center for Research Resources (NIH-NCRR) (RR12546); National Institute of Environmental Health Sciences (NIEHS) training grant (5T32ESO7148); the NIEHS sponsored UMDNJ Center for Environmental Exposures and Disease (NIEHS P30ES005022); and the New Jersey Agricultural Experiment Station (NJAES), Cook College, Rutgers University.
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ACKNOWLEDGMENTS |
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We wish to acknowledge Dr Richard Peterson for providing the in situ hybridization protocol and Dr Vicky Sutherland for providing the Alcian blue staining protocol used during these studies. We would also like to thank Dr Robert Tanguay for the zebrafish MMP-2 construct, and Dr Herbert Lowndes for supplying the microinjection equipment.
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