Antenna cells of ascidian larvae are gravity-sensing neurons
Ascidians are the closest living relatives of vertebrates, and they share a basic body plan including the dorsal central nervous system. The central nervous system of ascidian tadpole larvae controls their swimming behavior. In particular, for the first few hours after hatching, larvae swim upward. Gravity is sensed by an organ called the otolith, which resides in a brain vesicle. Although this sensory information is thought to be transmitted to two antenna cells abutting the otolith, direct evidence that the antenna cells function as gravity-sensory neurons has been lacking. In the present study, we first identified a regulatory element of the Vglut gene that drives expression in antenna cells and otolith-associated ciliated cells. Using this upstream regulatory element, we specifically overexpressed the apoptosis-inducing gene, Bax, in antenna cells and otolith-associated ciliated cells, and succeeded in their targeted ablation. These Bax-overexpressing larvae lost their preference for upward swimming. Because antenna cells have axons and otolith-associated ciliated cells lack neurites, our observation indicates that antenna cells are the sensory neurons for gravity. In addition, our experimental method for targeted disruption of cells using an enhancer element and Bax provides a valuable tool to examine neural circuit functions at cellular resolution.