Reference Gene Validation for Quantitative Real-time PCR Studies in Amphibian Kidney-derived A6 Epithelial Cells

Impaired spermatogenesis and associated endocrine effects of azole fungicides in peripubertal Xenopus tropicalis

Early life exposure to endocrine disrupting chemicals (EDCs) has been suggested to adversely affect reproductive health in humans and wildlife. Here, we characterize endocrine and adverse effects on the reproductive system after juvenile exposure to propiconazole (PROP) or imazalil (IMZ), two common azole fungicides with complex endocrine modes of action. Using the frog Xenopus tropicalis, two short-term (2-weeks) studies were conducted. I: Juveniles (2 weeks post metamorphosis (PM)) were exposed to 0, 17 or 178 µg PROP/L. II: Juveniles (6 weeks PM) were exposed to 0, 1, 12 or 154 µg IMZ/L. Histological analysis of the gonads revealed an increase in the number of dark spermatogonial stem cells (SSCs)/testis area, and in the ratio secondary spermatogonia: dark SSCs were increased in all IMZ groups compared to control. Key genes in gametogenesis, retinoic acid and sex steroid pathways were also analysed in the gonads. Testicular levels of 3β-hsd, ddx4 were increased and cyp19 and id4 levels were decreased in the IMZ groups. In PROP exposed males, increased testicular aldh1a2 levels were detected, but no histological effects observed. Although no effects on ovarian histology were detected, ovarian levels of esr1, rsbn1 were increased in PROP groups, and esr1 levels were decreased in IMZ groups. In conclusion, juvenile azole exposure disrupted testicular expression of key genes in retinoic acid (PROP) and sex steroid pathways and in gametogenesis (IMZ). Our results further show that exposure to environmental concentrations of IMZ disrupted spermatogenesis in the juvenile testis, which is a cause for concern as it may lead to impaired fertility. Testicular levels of id4, ddx4 and the id4:ddx4 ratio were associated with the number of dark SSCs and secondary spermatogonia suggesting that they may serve as a molecular markers for disrupted spermatogenesis.

cyp21a2 Knockout Tadpoles Survive Metamorphosis Despite Low Corticosterone

Corticosteroids are so vital for organ maturation that reduced corticosteroid signaling during postembryonic development causes death in terrestrial vertebrates. Indeed, death occurs at metamorphosis in frogs lacking proopiomelanocortin (pomc) or the glucocorticoid receptor (GR; nr3c1). Some residual corticosteroids exist in pomc mutants to activate the wild-type (WT) GR and mineralocorticoid receptor (MR), and the elevated corticosteroids in GR mutants may activate MR. Thus, we expected a more severe developmental phenotype in tadpoles with inactivation of 21-hydroxylase, which should eliminate all interrenal corticosteroid biosynthesis. Using CRISPR/Cas9 in Xenopus tropicalis, we produced an 11-base pair deletion in cyp21a2, the gene encoding 21-hydroxylase. Growth and development were delayed in cyp21a2 mutant tadpoles, but unlike the other frog models, they survived metamorphosis. Consistent with an absence of 21-hydroxylase, mutant tadpoles had a 95% reduction of aldosterone in tail tissue, but they retained some corticosterone (∼40% of WT siblings), an amount, however, too low for survival in pomc mutants. Decreased corticosteroid signaling was evidenced by reduced expression of corticosteroid-response gene, klf9, and by impaired negative feedback in the hypothalamus-pituitary-interrenal axis with higher messenger RNA expression levels of crh, pomc, star, and cyp11b2 and an approximately 30-fold increase in tail content of progesterone. In vitro tail-tip culture showed that progesterone can transactivate the frog GR. The inadequate activation of GR by corticosterone in cyp21a2 mutants was likely compensated for by sufficient corticosteroid signaling from other GR ligands to allow survival through the developmental transition from aquatic to terrestrial life.

Hypothesis-driven dragging of transcriptomic data to analyze proven targeted pathways in Rhinella arenarum larvae exposed to organophosphorus pesticides

Transcriptional analysis of the network of transcription regulators and target pathways in exposed organisms may be a hard task when their genome remains unknown. The development of hundreds of qPCR assays, including primer design and normalization of the results with the appropriate housekeeping genes, seems an unreachable task. Alternatively, we took advantage of a whole transcriptome study on Rhinella arenarum larvae exposed to the organophosphorus pesticides azinphos-methyl and chlorpyrifos to evaluate the transcriptional effects on a priori selected groups of genes. This approach allowed us to evaluate the effects on hypothesis-selected pathways such as target esterases, detoxifying enzymes, polyamine metabolism and signaling, and regulatory pathways modulating them. We could then compare the responses at the transcriptional level with previously described effects at the enzymatic or metabolic levels to obtain global insight into toxicity-response mechanisms. The effects of both pesticides on the transcript levels of these pathways could be considered moderate, while chlorpyrifos-induced responses were more potent and earlier than those elicited by azinphos-methyl. Finally, we inferred a prevailing downregulation effect of pesticides on signaling pathways and transcription factor transcripts encoding products that modulate/control the polyamine and antioxidant response pathways. We also tested and selected potential housekeeping genes based on those reported for other species. These results allow us to conduct future confirmatory studies on pesticide modulation of gene expression in toad larvae.

Reference gene screening of Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans for quantitative real-time PCR studies

Real-time quantitative PCR studies largely depend on reference genes for the normalization of gene expression. Stable reference genes should be accurately selected in order to obtain reliable results. We here present a study screening commonly used reference genes (TEF1F, α-centractin, Ctsyn1, GAPDH, R6046, APRT and TUB) in the chytrid fungi Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal), which cause the lethal amphibian skin disease chytridiomycosis. We evaluated the stability of the reference gene candidates during different growth stages of the fungi, using different statistical software packages: ΔCT, BestKeeper, GeNorm, NormFinder and RefFinder. In order to reflect the in vivo situation, the stability of the candidates was assessed when taking all growth stages into account. Using an ex-vivo approach, we tested whether the expression of GAPDH, TUB, R6046 and APRT (Bd) and GAPDH, TUB, R6046 and α-centractin (Bsal) remained stable when these fungi came in contact with host tissue. Finally, their role as in vivo reference genes was examined in skin tissue of experimentally infected midwife toads (Alytes obstetricans) (Bd) and fire salamanders (Salamandra salamandra) (Bsal). Summarized, the present study provides guidance for selecting appropriate reference genes when analyzing expression patterns of these fungal organisms during different growth stages and in Bd- or Bsal-infected tissues.