Unconventional Roles of Opsins

Abstract

Rhodopsin is the classical light sensor. While rhodopsin is important for image formation in the eye, the requirements for opsins in non-image formation and in extra-ocular light sensation were revealed later. Most recent is the demonstration that an opsin in the fruit fly, Drosophila melanogaster, is expressed in pacemaker neurons in the brain and functions in circadian photoentrainment. After more than a century of analysis, the dogma has been that opsins are exclusive light sensors. Remarkably, through studies in Drosophila, light-independent roles for opsins in multiple senses are emerging. These include roles in temperature sensation and hearing. While these findings are uncovered in the fruit fly, there are hints that opsins have light-independent roles in a wide array of animals, including mammals. Thus, despite the decades of focus on opsins as light detectors, they represent an important new class of polymodal sensory receptors.

Proprioceptive Opsin Functions in Drosophila Larval Locomotion

“…Work on Drosophila has shown that opsins, the protein moieties of visual photopigments, not only detect light (Leung and Montell, 2017). Opsins were implicated in larval temperature preference behaviors (Shen et al, 2011;Sokabe et al, 2016) and in hearing in the adult flies (Senthilan et al, 2012).…”

“…Accordingly, opsins were surmised to have light-independent ancestral functions (Shen et al, 2011;Senthilan et al, 2012), yet what these functions are remains a mystery. In thermosensation, opsins, though usually thermo-stable, were proposed to act as thermosensors (Shen et al, 2011;Leung and Montell, 2017), and how opsins contribute sound transduction is unclear (Albert and Gö pfert, 2015;Leung and Montell, 2017). Locomotion is a hallmark of animal life, and the need for movement control presumably was a driving factor in early nervous system evolution (Jé kely et al, 2015).…”

Seeing Nemo: molecular evolution of ultraviolet visual opsins and spectral tuning of photoreceptors in anemonefishes (Amphiprioninae)

“…Increased SWS1 opsin expression has been found in the damselfish, P. nagasakiensis during winter months and may be a visual tuning response for taking advantage of the higher UV . Seasonal changes in opsin gene expression levels have been reported to alter colour perception in widespread taxa (see review by Shimmura et al 2018 Leung & Montell, 2017) and most recently in taste (Leung et al 2020).…”

Glass confers rhabdomeric photoreceptor identity inDrosophila, but not across all metazoans

“…Therefore we expect that, in the near future, increasing both the number and the quality such single cell transcriptomes for these and other species will be useful to address several questions about the evolution of specific cell fates. For example, some opsins may have other functions apart from light-sensing [73], and it would be relevant to know if glass regulates the expression of any such opsin outside the Platynereis eye (for example), at any stage.…”