Interactions between Thryoid Hormone and Dioxin Signaling in the frog Xenopus laevis

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is a persistent organic pollutant that elicits toxic effects in vertebrates, including developmental defects, cancer, endocrine disruption, and death. TCDD toxicity results from binding to the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor. High levels of dioxin disrupt the thyroid system in several species. However, a clear understanding of the underlying mechanism of dioxin-induced thyroid hormone (TH) disruption is lacking. Amphibian metamorphosis (the development of tadpoles into frogs) represents a well-established model system to assess thyroid hormone-driven vertebrate development. In this study we used the African clawed frog, Xenopus laevis, as a model to study TH disruption by dioxin, examining the expression of target genes of thyroid hormone receptor (TR) and AHR in XLK-WG cells as well as tail resorption during metamorphosis. Expression of Cytochrome P450 1A6 (CYP1A6) mRNA, a well-characterized AHR target, was induced at least 300-fold by 100 nM TCDD in XLK-WG cells. The primary TH target gene, Krüppel-Like Factor 9 (KLF9) was induced 5-fold by 50 nM TH and approximately 2-fold by dioxin. Upon co-exposure to TH and TCDD, CYP1A6 was induced at least 500-fold, while KLF9 was induced 11-fold. Increased target gene induction following co-exposure of XLK-WG cells to TH and TCDD occurred in the absence of serum in culture media. Therefore, this phenomenon did not involve competitive displacement from serum binding proteins and resulting changes in bioavailability of these compounds. Enhanced target gene expression was sensitive to pharmacological inhibitors of both TR and AHR, demonstrating the requirement for both receptors. In experiments with the candidate endogenous AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ; 10 nM), CYP1A6 and KLF9 induction were not augmented by co-exposure to 50 nM TH compared to either compound alone. To pair molecular data with a morphological endpoint, we used tadpole tail explants to determine the effect of TCDD and TH co-exposure on the rate of tail resorption. We found that the decrease in tail area was, at best, modestly accelerated by co-exposure to TH and TCDD compared to TH alone. These findings suggest that molecular biomarkers may represent a more sensitive indicator than tail resorption for examining thyroid-disrupting effects of TCDD and other xenobiotics

A predictive density for semiparametric scale and location models

COMMD1 deficiency results in defective copper homeostasis, but the mechanism for this has remained elusive. Here we report that COMMD1 is directly linked to early endosomes through its interaction with a protein complex containing CCDC22, CCDC93, and C16orf62. This COMMD/CCDC22/CCDC93 (CCC) complex interacts with the multisubunit WASH complex, an evolutionarily conserved system, which is required for endosomal deposition of F-actin and cargo trafficking in conjunction with the retromer. Interactions between the WASH complex subunit FAM21, and the carboxyl-terminal ends of CCDC22 and CCDC93 are responsible for CCC complex recruitment to endosomes. We show that depletion of CCC complex components leads to lack of copper-dependent movement of the copper transporter ATP7A from endosomes, resulting in intracellular copper accumulation and modest alterations in copper homeostasis in humans with CCDC22 mutations. This work provides a mechanistic explanation for the role of COMMD1 in copper homeostasis and uncovers additional genes involved in the regulation of copper transporter recycling.Christine A. Phillips-Krawczak, Amika Singla, Petro Starokadomskyy, Zhihui Deng, Douglas G. Osbornea, Haiying Li, Christopher J. Dick, Timothy S. Gomez, Megan Koenecke, Jin-San Zhang, Haiming Dai, Luis F. Sifuentes-Dominguez, Linda N. Geng, Scott H. Kaufmann, Marco Y. Hein, Mathew Wallis, Julie McGaughran, Jozef Gecz, Bart van de Sluis, Daniel D. Billadeau and Ezra Burstei