Deiodinase 2 inhibition leading to increased mortality via reduced anterior swim bladder inflation

Abstract

All key events in this AOP are plausibly applicable to both sexes. Sex differences are not often investigated in tests using early life stages of fish. For zebrafish and fathead minnow, it is currently unclear whether sex-related differences are important in determining the magnitude of the changes across the sequence of events in this AOP. Different fish species have different sex determination and differentiation strategies. Zebrafish do not have identifiable heteromorphic sex chromosomes and sex is determined by multiple genes and influenced by the environment . Zebrafish are undifferentiated gonochorists since both sexes initially develop an immature ovary (Maack and Segner, 2003). Immature ovary development progresses until approximately the onset of the third week. Later, in female fish immature ovaries continue to develop further, while male fish undergo transformation of ovaries into testes. Final transformation into testes varies among male individuals, however finishes usually around 6 weeks post fertilization. Since the anterior chamber inflates around 21 days post fertilization in zebrafish, sex differences are expected to play a minor role in the current AOP. Fathead minnow gonad differentiation also occurs during larval development. Fathead minnows utilize a XY sex determination strategy and markers can be used to genotype sex in life stages where the sex is not yet clearly defined morphologically . Ovarian differentiation starts at 10 dph followed by rapid development . At 25 dph germ cells of all stages up to the primary oocytes stage were present and at 120 dph, vitellogenic oocytes were present. The germ cells (spermatogonia) of the developing testes only entered meiosis around 90-120 dph. Mature testes with spermatozoa are present around 150 dph. Since the anterior chamber inflates around 14 days post fertilization (9 dph) in fathead minnows, sex differences are expected to play a minor role in the current AOP. Essentiality of the Key EventsOverall, the confidence in the supporting data for essentiality of KEs within the AOP is moderate. There is evidence from deiodinase knockdowns showing the link with reduced posterior chamber inflation and the essentiality for downstream effects, but anterior chamber inflation was not studied. There is additional indirect evidence that reduced thyroid hormone synthesis causes reduced anterior swim bladder inflation: Chopra et al. (2019) showed that knockdown of dual oxidase, important for thyroid hormone synthesis, reduced anterior swim bladder inflation. It should be noted that dual oxidase also plays a role in oxidative stress. There is also evidence that alleviation of the effect on anterior chamber inflation reduces the effect on swimming performance.

New approach methods to improve human health risk assessment of thyroid hormone system disruption–a PARC project

“…This includes the AOP initiated by inhibition of thyroperoxidase (TPO) and leading to adverse neurodevelopmental outcomes in mammals ( Crofton et al, 2019 ) and the AOP leading from inhibition of the Na+/I- symporter (NIS) to learning and memory impairment in mammals ( Rolaki et al, 2019 ). Five AOPs linking TPO and deiodinase inhibition to impaired swim bladder inflation in fish have recently been endorsed ( Vergauwen et al, 2022a ; Vergauwen et al, 2022b ; Vergauwen et al, 2022c ; Vergauwen et al, 2022d ; Vergauwen et al, 2022e ). A multitude of other AOPs describing TH system disruption with applicability to different taxa are in varying stages of development.…”