Optogenetic activation of primary sensory neurons induces metamorphosis in Ciona

Metamorphosis is a key event in development that is conserved in many marine organisms. Ciona intestinalis type A induces metamorphosis through the settlement of papillae onto the substrate. The papilla consists of collocytes (CCs), primary sensory neurons (PSNs), and axial columnar cells (ACCs), but it remains unclear whether PSNs alone can induce metamorphosis. Manipulating single neurons is crucial for elucidating the neural network system that drives metamorphosis. In this study, we developed an optogenetic system in which ChrimsonR, a red-shifted mutant of channelrhodopsin, was expressed exclusively in PSNs, enabling metamorphosis to be induced by light stimulation. A Ciona-optimized self-cleaving peptide, T2A, was used to co-express the Ca²⁺ indicator GCaMP6s, allowing us to monitor neural activity during light stimulation. Activation of PSNs alone induced a series of metamorphic events, including epidermal backward movement, mesenchymal cell extravasation, and tail regression. Furthermore, we confirmed that metamorphosis proceeded to the juvenile stage. Metamorphosis was induced even with intermittent light stimulation, and the total stimulation time required for its initiation was approximately 6 min. The optogenetic system developed in this study may significantly contribute to elucidating the link between neuronal function and metamorphosis at the single-cell level.