Increased susceptibility to retinoid-induced teratogenesis in TGF-beta2 knockout mice

Folic acid rescue of ATRA-induced cleft palate by restoring the TGF-β signal and inhibiting apoptosis

BACKGROUND

Cleft palate is a frequent congenital malformation with a heterogeneous etiology, for which folic acid (FA) supplementation has a protective effect. To gain more insight into the molecular pathways affected by FA, TGF-β signaling and apoptosis in mouse embryonic palatal mesenchymal (MEPM) cells of all-trans retinoic acid (ATRA)-induced cleft palate in organ culture were tested.

METHODS

C57BL/6J mice embryonic palates were explanted on embryonic day 14 and cultured in DMEM/F12 medium with or without ATRA or FA for 72 h. The palatal fusion was examined by light microscopy. Immunohistochemistry was used to detect TGFβ3/TGF receptor II and caspase 9 in MEPM cells. TUNEL was used to detect apoptosis.

RESULTS

Similar to development in vivo, palatal development and fusion were normal in control medium. ATRA inhibited palatal development and induced cleft palate, which can be rescued by FA. A higher apoptosis rate and caspase-9 in MEPM cells were detected in the ATRA group than in the control or the ATRA+FA group. Compared with the control or the ATRA+FA group, ATRA had little effect on TGF-β3 in MEPM cells but significantly inhibited TGF-β receptor II.

CONCLUSIONS

Folic acid can rescue the cultured palates to continue developing and fusing that were inhibited and resulted in cleft palate by ATRA. Apoptosis and TGFβ signaling in MEPM cells were involved in folic acid rescued ATRA-induced cleft palate.

Biological mechanisms in palatogenesis and cleft palate

Clefts of the palate are common birth defects requiring extensive treatment. They appear to be caused by multiple genetic and environmental factors during palatogenesis. This may result in local changes in growth factors, extracellular matrix (ECM), and cell adhesion molecules. Several clefting factors have been implicated by studies in mouse models, while some of these have also been confirmed by genetic screening in humans. Here, we discuss several knockout mouse models to examine the role of specific genes in cleft formation. The cleft is ultimately caused by interference with shelf elevation, attachment, or fusion. Shelf elevation is brought about by mesenchymal proliferation and changes in the ECM induced by growth factors such as TGF-betas. Crucial ECM molecules are collagens, proteoglycans, and glycosaminoglycans. Shelf attachment depends on specific differentiation of the epithelium involving TGF-beta3, sonic hedgehog, and WNT signaling, and correct expression of epithelial adhesion molecules such as E-cadherin. The final fusion requires epithelial apoptosis and epithelium-to-mesenchyme transformation regulated by TGF-beta and WNT proteins. Other factors may interact with these signaling pathways and contribute to clefting. Normalization of the biological mechanisms regulating palatogenesis in susceptible fetuses is expected to contribute to cleft prevention.

The expanding role for retinoid signaling in heart development

The importance of retinoid signaling during cardiac development has long been appreciated, but recently has become a rapidly expanding field of research. Experiments performed over 50 years ago showed that too much or too little maternal intake of vitamin A proved detrimental for embryos, resulting in a cadre of predictable cardiac developmental defects. Germline and conditional knockout mice have revealed which molecular players in the vitamin A signaling cascade are potentially responsible for regulating specific developmental events, and many of these molecules have been temporally and spatially characterized. It is evident that intact and controlled retinoid signaling is necessary for each stage of cardiac development to proceed normally, including cardiac lineage determination, heart tube formation, looping, epicardium formation, ventricular maturation, chamber and outflow tract septation, and coronary arteriogenesis. This review summarizes many of the significant milestones in this field and particular attention is given to recently uncovered cross-talk between retinoid signaling and other developmentally significant pathways. It is our hope that this review of the role of retinoid signaling during formation, remodeling, and maturation of the developing heart will serve as a tool for future discoveries.

Recent advances in understanding transforming growth factor beta regulation of orofacial development

Members of the transforming growth factor (TGF) family have emerged as critical contributors to the choreography of cellular and tissue interactions underlying morphogenesis of the orofacial region. The TGFs beta, and their downstream effector molecules, the Smads, play a pivotal role in normal as well as abnormal development of first branchial arch structures. Components of the TGFbeta signal transduction machinery are discussed in relation to regulation of transcription, cell division and tissue differentiation in developing orofacial tissue, as evidence for a functional linkage between the TGFbeta and retinoic acid signal transduction pathways during orofacial development.

Gene–nutrient interactions: importance of folates and retinoids during early embryogenesis

The role that nutritional factors play in mammalian development has received renewed attention over the past two decades, as the scientific literature exploded with reports of retinoid compounds disrupting craniofacial development, and with other reports that folic acid supplementation in the periconceptional period can protect embryos from highly significant malformations. As was often the case, the situation became far more complicated, as the interaction between nutritional factors with selected genes was recognized. In this review, we attempt to summarize a complex clinical and experimental literature of nutritional factors, their biological transport mechanisms, and the impact that they have during early embryogenesis. Although not exhaustive, our goal was to provide an overview of important gene-nutrient interactions and a framework for their investigation.

In Vivo Imaging of Retinoic Acid Receptor Activity using a Sodium/Iodide Symporter and Luciferase Dual Imaging Reporter Gene

Retinoic acids are natural derivatives of vitamin A, and play important roles in modulating tumor cell growth by regulating differentiation, thus suggesting the potential use of these derivatives in cancer therapy and prevention. To visualize the intranuclear responses of functional retinoic acid receptors, we have developed a dual-imaging reporter gene system based on the use of sodium/iodide symporter (NIS) and luciferase in cancer cell lines. NIS and luciferase genes were linked with an internal ribosome entry site, and placed under the control of an artificial cis-acting retinoic acid responsive element (pRARE/NL). After retinoic acid treatment, I-125 uptake by pRARE/NL transfected cells was found to have increased by up to about five times that of nontreated cells. The bioluminescence intensity of pRARE/NL transfected cells showed dose-dependency. In vivo luciferase images showed higher intensity in retinoic acid treated SK-RARE/NL tumors, and scintigraphic images of SK-RARE/NL tumors showed increased Tc-99m uptake after retinoic acid treatment. The NIS/luciferase imaging reporter system was sufficiently sensitive to allow the visualization of intranuclear retinoic acid receptor activity. This cis-enhancer imaging reporter system may be useful in vitro and in vivo for the evaluation of retinoic acid responses in such areas as cellular differentiation and chemoprevention.

Molecular fingerprinting of TGFß-treated embryonic maxillary mesenchymal cells

The transforming growth factor-beta (TGF(beta)) family represents a class of signaling molecules that plays a central role in normal embryonic development, specifically in development of the craniofacial region. Members of this family are vital to development of the secondary palate where they regulate maxillary and palate mesenchymal cell proliferation and extracellular matrix synthesis. The function of this growth factor family is particularly critical in that perturbation of either process results in a cleft of the palate. While the cellular and phenotypic effects of TGF(beta) on embryonic craniofacial tissue have been extensively cataloged, the specific genes that function as downstream mediators of TGF(beta) in maxillary/palatal development are poorly defined. Gene expression arrays offer the ability to conduct a rapid, simultaneous assessment of hundreds to thousands of differentially expressed genes in a single study. Inasmuch as the downstream sequelae of TGF(beta) action are only partially defined, a complementary DNA (cDNA) expression array technology (Clontech's Atlas Mouse cDNA Expression Arrays), was utilized to delineate a profile of differentially expressed genes from TGF(beta)-treated primary cultures of murine embryonic maxillary mesenchymal cells. Hybridization of a membrane-based cDNA array (1178 genes) was performed with 32P-labeled cDNA probes synthesized from RNA isolated from either TGF(beta)-treated or vehicle-treated embryonic maxillary mesenchymal cells. Resultant phosphorimages were subject to AtlasImage analysis in order to determine differences in gene expression between control and TGF(beta)-treated maxillary mesenchymal cells. Of the 1178 arrayed genes, 552 (47%) demonstrated detectable levels of expression. Steady state levels of 22 genes were up-regulated, while those of 8 other genes were down-regulated, by a factor of twofold or greater in response to TGF(beta). Affected genes could be grouped into three general functional categories: transcription factors and general DNA-binding proteins; growth factors/signaling molecules; and extracellular matrix and related proteins. The extent of hybridization of each gene was evaluated by comparison with the abundant, constitutively expressed mRNAs: ubiquitin, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ornithine decarboxylase (ODC), cytoplasmic beta-actin and 40S ribosomal protein. No detectable changes were observed in the expression levels of these genes in-response to TGF(beta) treatment. Gene expression profiling results were verified by Real-Time quantitative polymerase chain reaction. Utilization of cDNA microarray technology has enabled us to delineate a preliminary transcriptional map of TGF(beta) responsiveness in embryonic maxillary mesenchymal cells. The profile of differentially expressed genes offers revealing insights into potential molecular regulatory mechanisms employed by TGF(beta) in orchestrating craniofacial ontogeny.

Cross-Talk Between Interleukin-6 and Transforming Growth Factor-β3Regulates Extracellular Matrix Production by Human Fibroblasts from Subjects with Non-Syndromic Cleft Lip and Palate

BACKGROUND

Transforming growth factor-beta (TGF-beta) interference with interleukin 6 (IL-6) activity and the role of the latter in early human embryonic development prompted us to examine the effects IL-6 on matrix synthesis and the effects of TGF-beta3 on IL-6 expression human cleft lip and palate (CLP) fibroblasts.

METHODS

Collagen and glycosaminoglycan (GAG) synthesis were determined by radiolabeled precursors and biglycan expression by Northern blotting before and after adding IL-6. The effects of TGF-beta3 on IL-6 production were assayed by evaluating IL-6 transcript by Northern blotting and IL-6 protein secretion by enzyme-linked immunosorbent assay.

RESULTS

The results showed that IL-6 elicited an inhibitory effect on collagen and GAG levels in CLP fibroblasts by lowering hyaluronan and dermatan sulfate secretion. IL-6 up-regulated biglycan expression, but less strongly than TGF-beta3. TGF-beta3 significantly down-regulated IL-6 transcript and secretion in CLP fibroblasts.

CONCLUSIONS

These data suggest the increase in matrix components that characterize the CLP fibroblast phenotype might be due to a concerted TGF-beta3-IL-6 action. We hypothesize changes in cross-talk between TGF-beta3 and IL-6 signal transduction pathways are involved in the induction of cleft palate.