The retinol signaling pathway in mouse pluripotent P19 cells

Stereological analysis and transcriptome profiling of testicular injury induced by di-(2-ethylhexyl) phthalate in prepubertal rats

Di-(2-ethylhexyl) phthalate (DEHP) is the most common phthalate that can affect the male reproductive system. DEHP exposure at the prepubertal stage could lead to the injury of immature testes, but the mechanism has not been fully clarified. In the present study, we elucidated the possible underlying mechanism of DEHP-induced prepubertal testicular injury through stereological analysis and transcriptome profiling. Compared with the control group, the DEHP-treated rats had lower body weight gain and decreased testicular weight and organ coefficient. Moreover, lower serum levels of testosterone and LH were observed in the DEHP group, in contrast to the increased FSH level. Additionally, the serum level of estradiol had no significant difference after DEHP exposure. Stereological analysis showed significant reduction in volumes of most testicular structures, especially in the seminiferous tubule and seminiferous epithelium, along with a vast decrease of spermatogenic cells and obvious structural damages with substantial pathological signs (germ cracks, cytoplasmic vacuolization, sloughing, multinucleated giant cell formation, chromatolysis desquamation and dissolution, pyknosis of nuclei) in the seminiferous tubule upon DEHP exposure at the prepubertal stage. Furthermore, transcriptome profiling identified 5548 differentially expressed genes (DEGs) upon DEHP exposure. Pathway enrichment analysis revealed several crucial signaling pathways related to retinol metabolism, oxidative phosphorylation, steroid hormone biosynthesis, and cell adhesion molecules (CAMs). In addition, seven DEGs selected from RNA-seq data were validated by quantitative real-time polymerase chain reaction (qRT-PCR), and the results showed the same trends as the RNA-seq results. In conclusion, the above findings provide basic morphological data and lay a foundation for systematic research on transcriptome profiling in prepubertal testicular injury induced by DEHP.

Variability in retinoid-like activity of extracellular compound mixtures produced by wide spectra of phytoplankton species and contributing metabolites

Some phytoplankton species were shown to produce teratogenic retinoids. This study assessed the variability in the extracellular production of compounds with retinoid-like activity for 50 independent cultivations of wide spectra of species including 12 cyanobacteria (15 strains) and 4 algae of different orders. Extracellular retinoid-like activity was detected for repeated cultivations of six cyanobacteria. The results were consistent for some species including Microcystis aeruginosa and Aphanizomenon gracile. The detected retinoid-like activities ranged from below the limit of quantification of 16 ng/L to over 6 µg all-trans retinoic acid (ATRA) equivalent/L. Nontargeted virtual fractionation together with suspect screening approach enabled to identify some retinoid-like compounds in exudates, including ATRA, 9/13-cis retinoic acid, all-trans 5,6-epoxy retinoic acid, 4keto-ATRA, 4keto-retinal, 4hydroxy-ATRA, and retinal. Most of them were for the first time repeatedly detected in exudates of all studied algae (at ng/L levels) and cyanobacteria. Their relative potencies ranged from 0.018 (retinal) to 1 compared to ATRA. They accounted for less than 0.1-50% of total detected retinoid-like activity. The high detected activities and concentrations of retinoids in some samples and their direct accessibility from exudates document potential risk of developmental toxicity for organisms in proximity of massive water blooms.

Field cyanobacterial blooms producing retinoid compounds cause teratogenicity in zebrafish embryos

Cyanobacteria routinely release potentially harmful bioactive compounds into the aquatic environment. Several recent studies suggested a potential link between the teratogenicity of effects caused by cyanobacteria and production of retinoids. To investigate this relationship, we analysed the teratogenicity of field-collected cyanobacterial bloom samples by means of an in vivo zebrafish embryo test, an in vitro reporter gene bioassay and by the chemical analysis of retinoids. Extracts of biomass from cyanobacterial blooms with the dominance of Microcystis aeruginosa and Aphanizomenon klebahnii were collected from water bodies in the Czech Republic and showed significant retinoid-like activity in vitro, as well as high degrees of teratogenicity in vivo. Chemical analysis was then used to identify a set of retinoids in ng per gram of dry weight concentration range. Subsequent fractionation and bioassay-based characterization identified two fractions with significant in vitro retinoid-like activity. Moreover, in most of the retinoids eluted from these fractions, teratogenicity with malformations typical for retinoid signalling disruption was observed in zebrafish embryos after exposure to the total extracts and these in vitro effective fractions. The zebrafish embryo test proved to be a sensitive toxicity indicator of the biomass extracts, as the teratogenic effects occurred at even lower concentrations than those expected from the activity detected in vitro. In fact, teratogenicity with retinoid-like activity was detected at concentrations that are commonly found in biomasses and even in bulk water surrounding cyanobacterial blooms. Overall, these results provide evidence of a link between retinoid-like activity, teratogenicity and the retinoids produced by cyanobacterial water blooms in the surrounding environment.

TBR2 antagonizes retinoic acid dependent neuronal differentiation by repressing Zfp423 during corticogenesis

During cerebral cortex development, neural progenitors are required to elaborate a variety of cell differentiation signals to which they are continuously exposed. RA acid is a potent inducer of neuronal differentiation as it was found to influence cortical development. We report herein that TBR2, a transcription factor specific to Intermediate (Basal) Neural Progenitors (INPs), represses activation of the RA responsive element and expression of RA target genes in cell lines. This repressive action on RA signaling was functionally confirmed by the decrease of RA-mediated neuronal differentiation in neural stem cells stably overexpressing TBR2. In vivo mapping of RA activity in the developing cortex indicated that RA activity is detected in radial glial cells and subsequently downregulated in INPs, revealing a fine cell-type specific regulation of its signaling. Thus, TBR2 might be a molecular player in opposing RA signaling in INPs. Interestingly, this negative regulation is achieved at least in part by directly repressing the critical nuclear RA co-factor ZFP423. Indeed, we found ZFP423 to be expressed in the developing cortex and promote RA-dependent neuronal differentiation. These data indicate that TBR2 contributes to suppressing RA signaling in INPs, thereby enabling them to re-enter the cell cycle and delay neuronal differentiation.

Expression and Regulation of Tal2 during Neuronal Differentiation in P19 Cells

T-cell acute lymphocytic leukemia 2 (Tal2) is a gene encoding a member of the basic helix-loop-helix transcription factor family, which is essential for the normal development of the mouse brain. We found that Tal2 was induced during neural differentiation in P19 cells, which are pluripotent mouse embryonal carcinoma cells that differentiate into the neural lineage upon both exposure to all-trans retinoic acid (atRA) and the formation of cell aggregation. Tal2 expression during neural differentiation in P19 cells was detected within 3 h after induction with atRA and retinoic acid receptor α (RARα). The atRA-RARα complex is known to bind to a characteristic retinoic acid response element (RARE) located in the promoter of target genes. We found a RARE-like element in the intron of Tal2. We also found a TATA-box-like element in the 5' region. The TATA-box-like element functioned as a core promoter, and TATA- box binding protein bound to this element upstream of Tal2 in P19 cells. The RARE-like element responded to atRA signaling that activated the transcription, and RARα was bound to this element in the intron of Tal2 in P19 cells. Furthermore, the interaction between these elements on Tal2 was confirmed in a chromatin immunoprecipitation assay. Because the neural differentiation of P19 cells mimics in part the development of the nervous system, P19 cells are useful for studying the mechanism underlying the role of Tal2 in neural differentiation. Further work is underway to clarify the function of Tal2 in neural differentiation using the differentiation system of P19 cells.

Corexit-EC9527A Disrupts Retinol Signaling and Neuronal Differentiation in P19 Embryonal Pluripotent Cells

Corexit-EC9500A and Corexit-EC9527A are two chemical dispersants that have been used to remediate the impact of the 2010 Deepwater Horizon oil spill. Both dispersants are composed primarily of organic solvents and surfactants and act by emulsifying the crude oil to facilitate biodegradation. The potential adverse effect of the Corexit chemicals on mammalian embryonic development remains largely unknown. Retinol (vitamin A) signaling, mediated by all-trans retinoic acid (RA), is essential for neural tube formation and the development of many organs in the embryo. The physiological levels of RA in cells and tissues are maintained by the retinol signaling pathway (RSP), which controls the biosynthesis of RA from dietary retinol and the catabolism of RA to polar metabolites for removal. RA is a potent activating ligand for the RAR/RXR nuclear receptors. Through RA and the receptors, the RSP modulates the expression of many developmental genes; interference with the RSP is potentially teratogenic. In this study the mouse P19 embryonal pluripotent cell, which contains a functional RSP, was used to evaluate the effects of the Corexit dispersants on retinol signaling and associated neuronal differentiation. The results showed that Corexit-EC9500A was more cytotoxic than Corexit-EC9527A to P19 cells. At non-cytotoxic doses, Corexit-EC9527A inhibited retinol-induced expression of the Hoxa1 gene, which encodes a transcription factor for the regulation of body patterning in the embryo. Such inhibition was seen in the retinol- and retinal- induced, but not RA-induced, Hoxa1 up-regulation, indicating that the Corexit chemicals primarily inhibit RA biosynthesis from retinal. In addition, Corexit-EC9527A suppressed retinol-induced P19 cell differentiation into neuronal cells, indicating potential neurotoxic effect of the chemicals under the tested conditions. The surfactant ingredient, dioctyl sodium sulfosuccinate (DOSS), may be a major contributor to the observed effect of Corexit-EC9527A in the cell.

Disruption of Retinol (Vitamin A) Signaling by Phthalate Esters: SAR and Mechanism Studies

A spectrum of reproductive system anomalies (cryptorchidism, hypospadias, dysgenesis of Wolffian duct-derived tissues and prostate, and reduced sperm production) in male rats exposed in utero to phthalate esters (PEs) are thought to be caused by PE inhibition of fetal testosterone production. Recently, dibutyl and dipentyl phthalate (DBuP, DPnP) were shown to disrupt the retinol signaling pathway (RSP) in mouse pluripotent P19 embryonal carcinoma cells in vitro. The RSP regulates the synthesis and cellular levels of retinoic acid (RA), the active metabolite of retinol (vitamin A). In this new study, a total of 26 di- and mono-esters were screened to identify additional phthalate structures that disrupt the RSP and explore their mechanisms of action. The most potent PEs, those causing > 50% inhibition, contained aryl and cycloalkane groups or C4-C6 alkyl ester chains and were the same PEs reported to cause malformations in utero. They shared similar lipid solubility; logP values were between 4 and 6 and, except for PEs with butyl and phenyl groups, were stable for prolonged periods in culture. Mono- and cognate di-esters varied in ability to disrupt the RSP; e.g., DEHP was inactive but its monoester was active while DBuP was active yet its monoester was inactive. DBuP and dibenzyl phthalate both disrupted the synthesis of RA from retinol but not the ability of RA to activate gene transcription. Both PEs also disrupted the RSP in C3H10T1/2 multipotent mesenchymal stem cells. Based on this in vitro study showing that some PEs disrupt retinol signaling and previous in vivo studies that vitamin A/RA deficiency and PEs both cause strikingly similar anomalies in the male rat reproductive system, we propose that PE-mediated inhibition of testosterone and RA synthesis in utero are both causes of malformations in male rat offspring.

Identification of compounds that modulate retinol signaling using a cell-based qHTS assay

In vertebrates, the retinol (vitamin A) signaling pathway (RSP) controls the biosynthesis and catabolism of all-trans retinoic acid (atRA), which regulates transcription of genes essential for embryonic development. Chemicals that interfere with the RSP to cause abnormal intracellular levels of atRA are potential developmental toxicants. To assess chemicals for the ability to interfere with retinol signaling, we have developed a cell-based RARE (Retinoic Acid Response Element) reporter gene assay to identify RSP disruptors. To validate this assay in a quantitative high-throughput screening (qHTS) platform, we screened the Library of Pharmacologically Active Compounds (LOPAC) in both agonist and antagonist modes. The screens detected known RSP agonists, demonstrating assay reliability, and also identified novel RSP agonists including kenpaullone, niclosamide, PD98059 and SU4312, and RSP antagonists including Bay 11-7085, LY294002, 3,4-Methylenedioxy-β-nitrostyrene, and topoisomerase inhibitors (camptothecin, topotecan, amsacrine hydrochloride, and idarubicin). When evaluated in the P19 pluripotent cell, these compounds were found to affect the expression of the Hoxa1 gene that is essential for embryo body patterning. These results show that the RARE assay is an effective qHTS approach for screening large compound libraries to identify chemicals that have the potential to adversely affect embryonic development through interference with retinol signaling.

Impact of preconditioning with retinoic acid during early development on morphological and functional characteristics of human induced pluripotent stem cell-derived neurons

Human induced pluripotent stem cells (hiPSCs) are a suitable tool to study basic molecular and cellular mechanisms of neurodevelopment. The directed differentiation of hiPSCs via the generation of a self-renewable neuronal precursor cell line allows the standardization of defined differentiation protocols. Here, we have investigated whether preconditioning with retinoic acid during early neural induction impacts on morphological and functional characteristics of the neuronal culture after terminal differentiation. For this purpose we have analyzed neuronal and glial cell markers, neuronal outgrowth, soma size, depolarization-induced distal shifts of the axon initial segment as well as glutamate-evoked calcium influx. Retinoic acid preconditioning led to a higher yield of neurons vs. glia cells and longer axons than unconditioned controls. In contrast, glutamatergic activation and depolarization induced structural plasticity were unchanged. Our results show that the treatment of neuroectodermal cells with retinoic acid during early development, i.e. during the neurulation phase, increases the yield of neuronal phenotypes, but does not impact on the functionality of terminally differentiated neuronal cells.

Actions of β-apo-carotenoids in differentiating cells: differential effects in P19 cells and 3T3-L1 adipocytes

β-Apo-carotenoids, including β-apo-13-carotenone and β-apo-14'-carotenal, are potent retinoic acid receptor (RAR) antagonists in transactivation assays. We asked how these influence RAR-dependent processes in living cells. Initially, we explored the effects of β-apo-13-carotenone and β-apo-14'-carotenal on P19 cells, a mouse embryonal carcinoma cell line that differentiates into neurons when treated with all-trans-retinoic acid. Treatment of P19 cells with either compound failed to block all-trans-retinoic acid induced differentiation. Liquid chromatography tandem mass spectrometry studies, however, established that neither of these β-apo-carotenoids accumulates in P19 cells. All-trans-retinoic acid accumulated to high levels in P19 cells. This suggests that the uptake and metabolism of β-apo-carotenoids by some cells does not involve the same processes used for retinoids and that these may be cell type specific. We also investigated the effects of two β-apo-carotenoids on 3T3-L1 adipocyte marker gene expression during adipocyte differentiation. Treatment of 3T3-L1 adipocytes with either β-apo-13-carotenone or β-apo-10'-carotenoic acid, which lacks RAR antagonist activity, stimulated adipocyte marker gene expression. Neither blocked the inhibitory effects of a relatively large dose of exogenous all-trans-retinoic acid on adipocyte differentiation. Our data suggest that in addition to acting as transcriptional antagonists, some β-apo-carotenoids act through other mechanisms to influence 3T3-L1 adipocyte differentiation.

Transcriptional regulation of Tal2 gene by all-trans retinoic acid (atRA) in P19 cells

TAL2 is a transcription factor required in the normal development of mouse brain. In a previous study, we demonstrated that the expression of Tal2 gene is induced by the complex of all-trans retinoic acid (atRA) and retinoic acid receptor α (RARα) in mouse embryonal carcinoma P19 cells. atRA is also known to be important in inducing P19 cells to differentiate into the neural lineage. Therefore, we believe that the function of TAL2 in neural differentiation may be clarified by utilizing P19 cells. As the atRA-RARα complex induced the expression of Tal2, we focused on the regulatory region that is involved in its transcription. The atRA-RARα complex occupies a characteristic retinoic acid response element (RARE) located in the promoter of target genes. Therefore, we searched for RARE on the mouse Tal2 and found that a RARE-like element was located in the intron. We also found that a TATA-box-like element was located in the 5'-region of Tal2. Involvement between transcriptional activity and the TATA-box-like element was confirmed in the luciferase assay, and TATA-box binding protein was bound to this element upstream of Tal2 in P19 cells. atRA signaling activated the transcription through the RARE-like element, and RARα was bound to this element on Tal2 in P19 cells. In addition, the interaction between these elements on Tal2 was shown in the chromatin immunoprecipitation assay. These results suggest that the transcription of Tal2 is coordinately mediated by two distal regulatory elements.

Tal2 expression is induced by all-trans retinoic acid in P19 cells prior to acquisition of neural fate

TAL2 is a member of the basic helix-loop-helix family and is essential for the normal development of the mouse brain. However, the function of TAL2 during brain development is unclear. P19 cells are pluripotent mouse embryonal carcinoma cells that adopt neural fates upon exposure to all-trans retinoic acid (atRA) and culture in suspension. We found that the expression of Tal2 gene was induced in P19 cells after addition of atRA in suspension culture. Tal2 expression was detected within 3 h after the induction, and had nearly returned to basal levels by 24 h. When GFP-tagged TAL2 (GFP-TAL2) was expressed in P19 cells, we observed GFP-TAL2 in the nucleus. Moreover, we showed that atRA and retinoic acid receptor α regulated Tal2 expression. These results demonstrate for the first time that atRA induces Tal2 expression in P19 cells, and suggest that TAL2 commits to the acquisition of neural fate in brain development.

Retinoid machinery in distinct neural stem cell populations with different retinoid responsiveness

Retinoic acid (RA) is present at sites of neurogenesis in both the embryonic and adult brain. While it is widely accepted that RA signaling is involved in the regulation of neural stem cell differentiation, little is known about vitamin A utilization and biosynthesis of active retinoids in the neurogenic niches, or about the details of retinoid metabolism in neural stem cells and differentiating progenies. Here we provide data on retinoid responsiveness and RA production of distinct neural stem cell/neural progenitor populations. In addition, we demonstrate differentiation-related changes in the expression of genes encoding proteins of the retinoid machinery, including components responsible for uptake (Stra6) and storage (Lrat) of vitamin A, transport of retinoids (Rbp4, CrbpI, CrabpI-II), synthesis (Rdh10, Raldh1-4), degradation of RA (Cyp26a1-c1) and RA signaling (Rarα,β,γ, Rxrα,β,γ). We show that both early embryonic neuroectodermal (NE-4C) stem cells and late embryonic or adult derived radial glia like progenitors (RGl cells) are capable to produce bioactive retinoids but respond differently to retinoid signals. However, while neuronal differentiation of RGl cells can not be induced by RA, neuron formation by NE-4C cells is initiated by both RA and RA-precursors (retinol or retinyl acetate). The data indicate that endogenous RA production, at least in some neural stem cell populations, may result in autocrine regulation of neuronal differentiation.

A screen for disruptors of the retinol (vitamin A) signaling pathway

The pathway through which retinol (vitamin A) is converted to its active metabolite, all-trans-retinoic acid (atRA), and subsequent receptor-mediated regulation of gene transcription by atRA is essential for all mammal life stages. This pathway is required for normal embryonic development and maintenance of cellular phenotype in adult organisms; chemicals that cause even minor interference with its normal function are potential developmental and adult toxicants. A short-term (24 h) in vitro mode-of-action screen for detecting chemicals that disrupt this essential pathway is described. It uses the mouse pluripotent P19 stem cell in a 96-well format, RT-qPCR gene-expression assay that does not require RNA purification to detect chemicals that interfere with retinol-induced Hoxa1 gene expression, a target of retinol signaling in mammals. A total of 21 chemicals were screened at a single 45 μM concentration. Four chemicals known to disrupt the pathway in the rodent embryo (citral, disulfiram, and two rodent teratogens, nitrofen and bisdiamine) all significantly inhibited Hoxa1 upregulation by retinol. An additional four of seven chemicals with varying degrees of structural similarity to known disruptors or to the retinoid side chain, but not previously known to disrupt the pathway, were positive in the screen. The xenoestrogens, diethylstilbestrol, bisphenol A, 4-n-nonylphenol, and genistein and the phthalate esters, dibutyl phthalate and dipentyl phthalate, but not diethylhexyl phthalate, also significantly disrupted the pathway. Of the 21 chemicals tested, diethylstilbestrol was the only chemical that showed evidence in the MTT assay that cytotoxicity may have contributed to disruption of the pathway.

Software for quantitative proteomic analysis using stable isotope labeling and data independent acquisition

Many software tools have been developed for analyzing stable isotope labeling (SIL)-based quantitative proteomic data using data dependent acquisition (DDA). However, programs for analyzing SIL-based quantitative proteomics data obtained with data independent acquisition (DIA) have yet to be reported. Here, we demonstrated the development of a new software for analyzing SIL data using the DIA method. Performance of the DIA on SYNAPT G2MS was evaluated using SIL-labeled complex proteome mixtures with known heavy/light ratios (H/L = 1:1, 1:5, and 1:10) and compared with the DDA on linear ion trap (LTQ)-Orbitrap MS. The DIA displays relatively high quantitation accuracy for peptides cross all intensity regions, while the DDA shows an intensity dependent distribution of H/L ratios. For the three proteome mixtures, the number of detected SIL-peptide pairs and dynamic range of protein intensities using DIA drop stepwise, whereas no significant changes in these aspects using DDA were observed. The new software was applied to investigate the proteome difference between mouse embryonic fibroblasts (MEFs) and MEF-derived induced pluripotent stem cells (iPSCs) using (16)O/(18)O labeling. Our study expanded the capacities of our UNiquant software pipeline and provided valuable insight into the performance of the two cutting-edge MS platforms for SIL-based quantitative proteomic analysis today.