SELENOMETHIONINE TOXICITY IN MODEL, NON-MODEL, AND AT-RISK FISHES
Date
2024-05-07
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-0552-6741
Type
Thesis
Degree Level
Doctoral
Abstract
Selenium (Se) is an atomic element and essential nutrient for almost all forms of life that can be toxic at exposures that minimally exceed those that are required. Toxicity risks are exacerbated for oviparous (egg-laying) vertebrates dependent upon Se-inundated aquatic environments, wherein naturally or anthropogenically elevated Se concentrations can bioconcentrate in primary producers and be maternally transferred to eggs, causing developmental toxicity and recruitment impairments in exposed populations. Appreciable research has characterized Se risks to some species at risk of exposure and identified fishes as the primary receptors of concern for Se toxicity. However, despite extensive research, hazard assessments for aquatic environments are complicated by the complex biogeochemistry of Se, its unconfirmed molecular toxicity mechanism(s), and the need to account for maternal transfer exposures. These challenges represent barriers to expedient hazard assessment for unstudied species that are difficult or impossible to overcome using traditional toxicological approaches.
To date, regulatory toxicology and risk assessment has relied on traditional approaches (e.g., deformity and mortality assessments) that require substantial time and financial investments and appreciable amounts of animal experimentation. However, developed and developing New Approach Methodologies (NAMs; e.g., high throughput approaches, mechanistic models, and in vitro techniques) have the capacity to expedite biological discovery at reduced cost and with decreased need for animal experimentation. Therefore, the purpose of this thesis was to employ traditional approaches and NAMs to characterize Se toxicity across phylogenetically distant fishes to facilitate Se (as selenomethionine; SeMet) toxicity hazard assessment across ecologically, culturally, and economically important fish species.
To address these objectives, five experiments were conducted that are summarized across three research chapters (thesis chapters 2-4) using environmentally relevant Se (as SeMet) exposures. Research presented in chapter 2 used traditional apical approaches (meristic, deformity, and mortality assessments) and early life stage (ELS) bioassays to characterize Se sensitivity across phylogenetically distant fishes and the value of microinjections as an analogue for maternally transferred Se exposures. Chapter 3 evaluated Se toxicity in SeMet microinjected ELS fathead minnow (FHM) and dietary SeMet exposed adult male FHM using apical, biochemical (thiobarbituric acid reactive substances; TBARS), and species-specific proteomic and transcriptomic NAM analyses with the intent to identify molecular toxicity markers common across life stages. Chapter 4 applied taxa-level whole transcriptome and 5-day post hatch apical analyses across four phylogenetically distant ELS fishes exposed to microinjected SeMet to elucidate conserved transcriptomic SeMet toxicity markers and pathways.
Chapter 2 identified spinal deformity, followed by edema, as the most sensitive cross-species apical outcomes induced by SeMet microinjections. Microinjected SeMet was shown to be more potent than maternally transferred Se, supporting the exclusion of microinjection data during guideline derivation and its inclusion in mechanistic studies. No apical or hepatic biochemical indications of Se toxicity were observed in adult male FHM in Chapter 3, results mirrored by negligible mechanistic toxicity insights from their hepatic TBARS, proteomic, and transcriptomic analyses. ELS FHM proteomic analyses identified dysregulated vitellogenin variants, and their transcriptomic analyses suggested that SeMet microinjection-associated deformity and mortality include mitochondrial, inflammatory, and/or immunological toxicity mechanisms. Though results for adult male FHM were less robust, they did provide support for the results observed in ELS FHM. Shorter-term cross-species ELS adverse apical outcomes observed in Chapter 4 corroborated those observed under longer-term exposures in Chapter 2, and transcriptomic analyses identified that the toxicity mechanisms observed in Chapter 3 may be highly conserved across species.
Though the mechanistic insights into the toxicity of Se generated by this research require further validation, they comport with previously observed cross-species evidence of Se toxicity and mechanisms that underlie human pathologies analogous to those observed across SeMet exposed fishes. Therefore, the Se toxicity mechanisms observed in this research may inform Se toxicity hazard assessment for unstudied species and support the use of NAMs in mechanistic assessments of toxicants lacking adequate toxicological characterization. It is recommended that additional and targeted research be used to validate these hypotheses to facilitate Se toxicity hazard assessment across fishes, and to continue to develop NAMs as practical tools in toxicological research.
Description
Keywords
Selenium, fish, microinjection, transcriptomics, apical
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Toxicology Centre
Program
Toxicology