Exogenous iodide ameliorates perchlorate-induced thyroid phenotypes in threespine stickleback

Abstract

Perchlorate is a ubiquitous environmental contaminant that has widespread endocrine disrupting effects in vertebrates, including threespine stickleback (Gasterosteus aculeatus). The target of perchlorate is thyroid tissue where it induces changes in the organization, activation, and morphology of thyroid follicles and surrounding tissues. To test the hypothesis that some phenotypes of perchlorate toxicity are not mediated by thyroid hormone, we chronically exposed stickleback beginning at fertilization to perchlorate (10, 30, 100ppm) or control water with and without supplementation of either iodide or thyroxine (T₄). Stickleback were sampled across a one-year timespan to identify potential differences in responses to treatment combinations before and after sexual maturation. We found that most thyroid histomorphological phenotypes induced by perchlorate (follicle proliferation, reduced follicle area (adults only), colloid depletion, thyrocyte hypertrophy (subadults only)) were significantly ameliorated by exogenous iodide supplementation. In contrast, treatment with exogenous T₄ did not correct any of the thyroid-specific histopathologies induced by perchlorate. Whole-body thyroid hormone concentrations were not significantly affected by perchlorate exposure; however, supplementation with iodide and T₄ significantly increased T₄ concentrations. This study also revealed an increased erythrocyte area in the thyroid region of perchlorate-exposed adults, while lipid droplet number increased in perchlorate-exposed subadults. Increased erythrocyte area was ameliorated by both iodide and T₄, while neither supplement was able to correct lipid droplet number. Our finding on lipid droplets indicates that exposure to perchlorate in early development may have obesogenic effects.

Keywords: Endocrine disruption; Histopathology; NIS; Obesogen; Sodium-iodide symporter; Thyroxine.

Figure 1

Representative histological image from an adult stickleback depicting the targeted region for measuring parameters related to the thyroid follicles, blood vessels, and lipid droplets. LD=Lipid Droplet, BV=Blood Vessel, CA=Cartilage, CO=Colloid, SM=Skeletal Muscle, TC=Thyrocyte, TF=Thyroid Follicle. Scale bar is 100 μm.

Figure 2

Figure 2A–D: Iodide supplementation ameliorates most perchlorate-induced effects on thyroid histomorphology. Mean follicle number (A), follicle area (B), colloid area (C), and thyrocyte height (D) for subault and adult stickleback exposed to perchlorate alone or co-exposed to perchlorate and iodide or T₄. Asterisks (* P<0.05, ** P<0.01, *** P<0.001) denote signficant main effects and/or interactions identified in one- and two-way ANOVAs.

Figure 3

Figure 3A–D: Iodide supplementation ameloriates perchlorate-induced thyroid histopathologies while T₄ does not. Representative histological images for targeted thyroid-specific phenotypes in each of the following treatments: 0 ppm perchlorate + no supplement (A), 100 ppm perchlorate + no supplement (B), 100 ppm perchlorate + iodide (C), 100 ppm perchlorate + T₄ (D). Concentration of perchlorate and supplement type are noted on the lower left and lower right of each image panel, respectively. Scale bar is 100 μm.

Figure 4

Figure 4A–C: Perchlorate increased erythrocyte area in adults, but not subadults. Mean vessel number (A), vessel cross-sectional area (B), and total erythrocyte area (C) for subault and adult stickleback exposed to perchlorate alone or co-exposed to perchlorate and iodide or T₄. Asterisks (* P<0.05, ** P<0.01) denote signficant main effects and/or interactions identified in one- and two-way ANOVAs.

Figure 5

Figure 5A–B: Perchlorate increased lipid droplet number in subadults, but not adults. Mean lipid droplet number (A) and individual droplet area (B) for subault and adult stickleback exposed to perchlorate alone or co-exposed to perchlorate and iodide or T₄. Asterisks (* P<0.05) denote signficant main effects and/or interactions identified in one- and two-way ANOVAs.

Figure 6

Figure 6A–B: Representative histological images of lipid droplet accumulation surrounding the thyroid follicles in a perchlorate-treated adult (A) compared to a control adult (B). Neither supplementation with iodide nor T₄ were able to rescue this perchlorate-induced phenotype. Scale bar is 125 μm.

Figure 7

Figure 7A–B: Supplementation with iodide and T₄ increased whole-body thyroid hormone concentrations. Mean whole-body T₄ (A) and T₃ (B) concentrations in stickleback following exposure to perchlorate alone or co-exposure to perchlorate and iodide or T₄. Asterisks (* P<0.05, ** P<0.01, *** P<0.001) denote signficant main effects and/or interactions identified in one- and two-way ANOVAs.