Embryonic exposure to environmental levels of Roundup affects neurodevelopment in zebrafish

N-(phosphonomethyl)glycine (glyphosate) is the most sprayed herbicidal chemical in the world. Although glyphosate is considered safe in humans and higher animals due to its targeting of the shikimate pathway found only in plants and microorganisms, recent in vivo and in vitro studies show evidence of toxicity across a range of systems, including the vertebrate brain. Given that developing brains with immature blood-brain barriers can be especially sensitive to environmental contaminants, here we test the effects of embryonic exposures to environmental levels of glyphosate or its commercial formulation Roundup. Embryonic zebrafish exposed to an environmental concentration of Roundup (10 µg/L acid equivalent (a.e.)) from 10 to 48 hours post-fertilization (hpf) showed defects in morphology, respiratory capacity, and hatching at 48 hpf as well as changes in locomotion, light startle response, and thigmotaxis behaviours at 5 days post-fertilization (dpf), with some effects lasting in the juvenile. To understand neurodevelopmental changes underlying these behavioural abnormalities, we tested for changes to the timing of neurogenesis and conducted bulk RNA-sequencing. We found increased neurogenesis and uncovered dysregulation of axonogenesis pathways, which was confirmed using immunohistochemistry. Finally, to test if axonal deficits might manifest as dysregulation of neuronal circuits, we observed changes in phospho-ERK levels and Ca2+ dynamics as indicators of activity disruptions at 5 dpf following embryonic exposure to Roundup. These results suggest that Roundup at current environmental levels may not be safe for organismal exposure, and glyphosate toxicity warrants closer attention.