An intact insect embryo for developmental neurotoxicity testing of directed axonal elongation

Abstract

Developmental neurotoxicity (DNT) of chemicals poses a serious threat to human health worldwide. Current in vivo test methods for assessing DNT require the use of large numbers of laboratory animals. Most alternative testing methods monitor readily quantifiable toxicological endpoints in cell culture, whereas the formation of a functional brain requires precisely timed navigation of axons within a complex tissue environment. We address this complexity by monitoring defects in axonal navigation of pioneer axons of intact locust embryos after exposure to chemicals. Embryos develop in serum-free culture with test chemicals, followed by immunolabeling of pioneer neurons. Defects in axon elongation of pioneer axons are quantified in concentration-response curves and compared to the general viability of the embryo, as measured by a resazurin assay.

We show that selected chemical compounds interfering with calcium signaling or cytoskeletal organization, and the ref­erence developmental neurotoxicant rotenone, can be classified as DNT positive. The pesticide rotenone inhibits pioneer neuron elongation with a lower IC50 than viability. The rho kinase inhibitor Y27632 can partially rescue outgrowth inhi­bition, supporting the classification of rotenone as a specific DNT positive compound. Since mechanisms of axonal guidance, such as growth cone navigation along molecular semaphorin gradients are conserved between locust and mammalian nervous systems, we will further explore the potential of this invertebrate preparation as an assay, including a prediction model, for testing the DNT potential of chemicals in humans.