Abstract
Untargeted lipidomics has previously been applied to the study of daphnids and the discovery of biomarkers that are indicative of toxicity. Typically, liquid chromatography—mass spectrometry is used to measure the changes in lipid abundance in whole-body homogenates of daphnids, each only ca. 3 mm in length which limits any biochemical interpretation of site-specific toxicity. Here, we applied mass spectrometry imaging of Daphnia magna to combine untargeted lipidomics with spatial resolution to map the molecular perturbations to defined anatomical regions. A desorption electrospray ionization—mass spectrometry (DESI-MS) method was optimized and applied to tissue sections of daphnids exposed to bisphenol-A (BPA) compared to unexposed controls, generating an untargeted mass spectrum at each pixel (35 µm2/pixel) within each section. First, unique lipid profiles from distinct tissue types were identified in whole-body daphnids using principal component analysis, specifically distinguishing appendages, eggs, eye, and gut. Second, changes in the lipidome were mapped over four stages of normal egg development and then the effect of BPA exposure on the egg lipidome was characterized. The primary perturbations to the lipidome were annotated as triacylglycerides and phosphatidylcholine, and the distributions of the individual lipid species within these classes were visualized in whole-body D. magna sections as ion images. Using an optimized DESI-MS workflow, the first ion images of D. magna tissue sections were generated, mapping both their baseline and BPA-perturbed lipidomes.
Highlights
Daphnia magna is a key sentinel species for environmental toxicology owing to its position in freshwater food webs as one of the most voracious primary consumers of phytoplankton
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Summary
Daphnia magna is a key sentinel species for environmental toxicology owing to its position in freshwater food webs as one of the most voracious primary consumers of phytoplankton. Changes to Daphnia fitness can induce significant perturbations to higher trophic levels. This species is amenable to laboratory toxicity testing due to their rapid life cycle, asexual reproduction, and large numbers of offspring that are produced per adult. For these reasons, D. magna is utilized in the international OECD. Adverse outcome pathways (AOPs) provide a knowledgebase for developing new chemical test methods that will focus on the detection of molecular perturbations that are predictive of adverse effects, i.e., toxicity endpoints [5]. The core of an AOP that is relevant to environmental toxicology comprises of early response molecular biomarkers that are indicative of adverse outcomes in larger biological structures up to an entire ecosystem
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