6PPD,\nNot 6PPD-Quinone,\nInduced Serious Zebrafish\nEye Damage by Disrupting the Thyroid Signaling Pathway

N-(1,3-Dimethylbutyl)-<i>N</i>′-phenyl-1,4-phenylenediamine\n(6PPD) and its oxidation product 6PPD-quinone (6PPDQ) showed different\nacute toxicities and bioaccumulation potencies in fish. In this study,\nwe compared the thyroid disrupting effects of 6PPD and 6PPDQ through <i>in vitro</i>, <i>in silico</i>, and <i>in vivo</i> assays. Interestingly, although 6PPD and 6PPDQ showed similar docking\naffinities with thyroid hormone receptor (TR) isoforms and GH3 cell\ninhibition effects, the thyroid signaling pathway, eye development,\nphototactic behaviors, and cell density in the retinal layer in the\nlarval zebrafish were significantly affected only following 6PPD\nexposure. Further investigation demonstrates that 6PPD can act as\na TR antagonist to reduce the opsin protein abundance and inhibit\nthe cone photoreceptor cell proliferation, which finally alters the\nretinal layer structure and causes microphthalmus in zebrafish. Especially,\nunder environmental relevant concentration exposure, 6PPD induced\nalterations of <i>trβ</i>, <i>opn1lw1</i>, <i>opn1mw1</i>, <i>rpe65a</i>, <i>nr2e3</i> gene expressions although no significant eye histopathological change\nwas observed. This study illustrates for the first time the more serious\nvisual system impairment of 6PPD compared to 6PPDQ, with thyroid signaling\ndisruption being a contributing factor, while other important toxic\ntargets still require further research.