Expression and light-dependent translocation of β-arrestin in the visual system of the terrestrial slug Limax valentianus

Abstract

Vertebrates, cephalopods and arthropods are equipped with eyes that have the highest spatiotemporal resolution among the animal phyla. In parallel, only animals in these three phyla have visual arrestin specialized for the termination of visual signaling triggered by opsin, in addition to ubiquitously expressed β-arrestin that serves in terminating general G protein-coupled receptor signaling. Indeed, visual arrestin in and rodents translocates to the opsin-rich subcellular region in response to light to reduce the overall sensitivity of photoreceptors in an illuminated environment (i.e. light adaptation). We thus hypothesized that, during evolution, visual arrestin has taken over the role of β-arrestin in those animals with eyes of high spatiotemporal resolution. If this is true, it is expected that β-arrestin plays a role similar to visual arrestin in those animals with low-resolution eyes. In the present study, we focused on the terrestrial mollusk, a species related to cephalopods but that has only β-arrestin, and generated antibodies against β-arrestin. We found that β-arrestin is highly expressed in photosensory neurons, and translocates into the microvilli of the rhabdomere within 30 min in response to short wavelength light (400 nm), to which the eye exhibits a robust response. These observations suggest that β-arrestin functions in the visual system of those animals that do not have visual arrestin. We also exploited anti-β-arrestin antibody to visualize the optic nerve projecting to the brain, and demonstrated its usefulness for tracing a visual ascending pathway.

Light avoidance by non-ocular photosensing system in the terrestrial slugLimax valentianus

“…2A), but they escaped only from short wavelength light if the irradiance was low (10 μW cm −2 ; Fig. 2B), consistent with the higher sensitivity of the slugs' eyes to short-wavelength light (Suzuki et al, 1979;Matsuo et al, 2017Matsuo et al, , 2019. We judged the avoidance behaviour based on whether more than half of the slug's body was within the dark compartment (criterion number 1).…”

“…However, a larger percentage of animals was expected to avoid short-wavelength light if monochromatic light with an equivalent photon flux density was used instead. We and other groups also demonstrated that the eye of Limax is more responsive to short-wavelength blue light, especially when the eye is light adapted (Suzuki et al, 1979;Matsuo et al, 2017Matsuo et al, , 2019. The preferential use of short-wavelength light is thought to be a widely conserved phenomenon in the animal kingdom (Gehring and Rosbash, 2003;Erren et al, 2008), and the slugs also appear to have inherited this trait.…”

“…The terrestrial gastropod slug Limax valentianus is nocturnal, and avoids light places in order not to lose water from its body. To do this, the slugs have a well-developed lens eye on the tip of their superior tentacles (STs), which can detect light with high sensitivity (Kataoka, 1975;Suzuki et al, 1979;Zieger et al, 2009;Land and Nilsson, 2012;Matsuo et al, 2017). Therefore, the slugs exhibit negative phototaxis behaviour, which is driven by comparing the intensities of incident light coming into the bilateral eyes (Crozier and Cole, 1929;Matsuo et al, 2014).…”

Co‐expression of opsins in the eye photoreceptor cells of the terrestrial slug Limax valentianus

“…Electron microscopy was performed as described previously (Matsuo, Takatori, Hamada, Koyanagi, & Matsuo, ). Briefly, an isolated ST was fixed in 2% paraformaldehyde 2% glutaraldehyde in 20 mM phosphate buffer (pH 7.3) at 4°C overnight.…”

“…Spectral sensitivity was measured by a modified method adopted from Suzuki et al (). The ERG was recorded from the surface of an isolated eye as described previously (Matsuo et al, ), except that the optic nerve was at the internal side of the recording electrode in order to record the spikes in the optic nerve fibers. The isolated eye was either kept in the dark (dark adaptation) or illuminated with 400‐nm light (11.2 μW/cm 2 , light adaptation) for 2 hr.…”