UV-sensing protein in brain of marine annelid zooplankton

Most animals use external light signals for vision and "non-visual" photoreceptive functions, such as regulation of circadian behaviors. In some cases, photoreceptor cells outside eyes are involved in non-visual photoreception. Previous studies have shown that larvae of the annelid Platynereis dumerilii (marine ragworm), which are studied as a zooplankton model, possess photoreceptor cells in the brain, and the cells regulate circadian swimming behaviors. Interestingly, the brain photoreceptor cells in Platynereis express an opsin that is closely related to visual pigments in our visual photoreceptor (rod and cone) cells. Zooplankton show a synchronized circadian movement known as diel vertical migration (DVM), moving upward in water at night and downward in daytime. DVM is probably the largest daily movement of biomass, comparable to human commuting. Since a major cause of DVM is to avoid damaging UV (ultra-violet) irradiation, light-dependent DVM regulation via the brain photoreceptor cells was suggested.

This study showed that the Platynereis opsin can receive and transmit UV signals. Unlike vertebrate visual opsins, the opsin can directly bind exogenous all-trans-retinal. This suggests that the opsin enables the brain photoreceptor cells to detect UV signals, even without the supply of 11-cis-retinal, which is specifically produced in eyes. Mutagenesis analyses identified that a single amino acid residue is responsible for not only UV sensing but also direct binding of exogenous all-trans-retinal. Thus, the single residue is essential for the opsin to achieve the characteristics suitable for UV reception in the brain. Taken together, the opsin possesses ideal properties enabling the brain photoreceptor cells in Platynereis to sense ambient UV signals.

As summarized above, this study revealed molecular basis of the opsin to function as a UV-sensor in the brain of the zooplankton model. Since detection of ambient UV signals should be necessary for DVM, the molecular properties of the opsin are helpful to understand the physiology, ecology and evolution of zooplankton species.