Gibson LA et al. (2016), Life cycle exposure of the frog Silurana tropic...

XB-ART-51298

Gen Comp Endocrinol 2016 Aug 01;234:133-41. doi: 10.1016/j.ygcen.2015.09.015.

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Life cycle exposure of the frog Silurana tropicalis to arsenate: Steroid- and thyroid hormone-related genes are differently altered throughout development.

Gibson LA , Koch I , Reimer KJ , Cullen WR , Langlois VS .

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Arsenic contaminates water surface and groundwater worldwide. Several studies have suggested that arsenic acts as an endocrine disruptor in mammalian and non-mammalian species, although its chronic effect during development remains largely unknown. To address this question, life cycle exposures to 0, 0.3 and 0.8ppm of arsenate (pentavalent arsenic; As(V)) were performed in the Western clawed frog (Silurana tropicalis) from the gastrulae stage (developmental stage Nieuwkoop-Faber; NF12) until metamorphosis (NF66). Tissue samples were collected at the beginning of feeding (NF46; whole body), sexual development (NF56; liver), and at metamorphosis completion (NF66; liver and gonadal mesonephros complex). Real-time RT-PCR analysis quantified decreases in mRNA levels of genes related to estrogen- (estrogen receptor alpha and aromatase), androgen- (androgen receptor and steroid 5-alpha-reductase type 2), and cholesterol metabolism- (steroidogenic acute regulatory protein) at stage NF46. Similarly, arsenate decreased steroid 5-alpha-reductase type 2 expression in stage NF56 livers, but transcript increases were observed for both estrogen receptor alpha and steroidogenic acute regulatory protein at this stage. Given the changes observed in the expression of genes essential for proper sexual development, gonadal histological analysis was carried out in stage NF66 animals. Arsenate treatments did not alter sex ratio or produce testicular oocytes. On the other hand, arsenate interfered with thyroid hormone-related transcripts at NF66. Specifically, thyroid hormone receptor beta and deiodinase type 2 mRNA levels were significantly reduced after arsenate treatment in the gonadal mesonephros complex. This reduction in thyroid hormone-related gene expression, however, was not accompanied by any morphological changes measured. In summary, environmentally relevant concentrations of As(V) altered steroidogenesis-, sex steroid signaling- and thyroid hormone-related gene expression, although transcriptional changes varied among tissues and developmental stages.

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Species referenced: Xenopus
Genes referenced: actb ar cyp19a1 dio2 odc1 rpl8 srd5a2 thra thrb

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	Fig. 1. Transcriptional effects of arsenate (AsV) treatments on S. tropicalis early development. Embryos were exposed from NF12 to NF46 to 0, 0.3, and 0.8 ppm AsV and the expression of the thyroid hormone, trb (A) and dio2 (B); and steroid, ar (C); srd5a2 (D); erα (E); cyp19 (F); and star (G)-related genes was quantified by qPCR. Gene expression data are presented as log10 fold changes relative to control and are normalized to the average of rpl8 and actb. Pooled-sample (10 larvae per sample) points are presented along with the mean. Asterisks indicate significant statistical difference between the control and the treatments (p < 0.05).
	Fig. 2. Effects of arsenate (AsV) treatments on the expression of thyroid hormone, trb (A) and dio2 (B); and steroid, ar (C); srd5a2 (D); erα (E); and star (F)-related genes in the liver of stage NF56 tadpoles. Embryos were exposed from NF12 to NF56 to 0, 0.3, and 0.8 ppm AsV. Gene expression data are presented as log10 fold changes relative to control and are normalized to the average of rpl8 and actb. Pooled-sample (2 livers per sample) points are presented along with the mean. Asterisks indicate significant statistical difference between the control and the treatments (p < 0.05).
	Fig. 3. Transcriptional changes induced by arsenate (AsV) treatments on the expression of thyroid hormone, trb (A) and dio2 (B); and steroid, ar (C); srd5a2 (D); erα (E); and star (F)-related genes in the liver of stage NF66 froglets. Embryos were exposed from NF12 to NF66 to 0, 0.3, and 0.8 ppm AsV. Gene expression data are presented as log10 fold changes relative to control and are normalized to the average of odc. Sample points are presented along with the mean. Asterisk indicates significant statistical difference the two treatments only (p < 0.05).
	Fig. 4. The effects of arsenate (AsV) treatments on the expression of thyroid hormone, trb (A) and dio2 (B); and steroid, ar (C); srd5a2 (D); erα (E); cyp19 (F); and star (G)-related genes in the GMC of stage NF66 froglets. Embryos were exposed from NF12 to NF66 to 0, 0.3, and 0.8 ppm AsV. Gene expression data are presented as log10 fold changes relative to control and are normalized to the average of rpl8. Sample points are presented along with the mean. Asterisks indicate significant statistical difference between the control and the treatments (p < 0.05).