Molecular and functional identification of a novel photopigment in Pecten ciliary photoreceptors

Ciliary photoreceptors in sea urchin larvae indicate pan-deuterostome cell type conservation

BACKGROUND

The evolutionary history of cell types provides insights into how morphological and functional complexity arose during animal evolution. Photoreceptor cell types are particularly broadly distributed throughout Bilateria; however, their evolutionary relationship is so far unresolved. Previous studies indicate that ciliary photoreceptors are homologous at least within chordates, and here, we present evidence that a related form of this cell type is also present in echinoderm larvae.

RESULTS

Larvae of the purple sea urchin Strongylocentrotus purpuratus have photoreceptors that are positioned bilaterally in the oral/anterior apical neurogenic ectoderm. Here, we show that these photoreceptors express the transcription factor Rx, which is commonly expressed in ciliary photoreceptors, together with an atypical opsin of the G_O family, opsin3.2, which localizes in particular to the cilia on the cell surface of photoreceptors. We show that these ciliary photoreceptors express the neuronal marker synaptotagmin and are located in proximity to pigment cells. Furthermore, we systematically identified additional transcription factors expressed in these larval photoreceptors and found that a majority are orthologous to transcription factors expressed in vertebrate ciliary photoreceptors, including Otx, Six3, Tbx2/3, and Rx. Based on the developmental expression of rx, these photoreceptors derive from the anterior apical neurogenic ectoderm. However, genes typically involved in eye development in bilateria, including pax6, six1/2, eya, and dac, are not expressed in sea urchin larval photoreceptors but are instead co-expressed in the hydropore canal.

CONCLUSIONS

Based on transcription factor expression, location, and developmental origin, we conclude that the sea urchin larval photoreceptors constitute a cell type that is likely homologous to the ciliary photoreceptors present in chordates.

The crowns have eyes: multiple opsins found in the eyes of the crown-of-thorns starfish Acanthaster planci

Background

Opsins are G protein-coupled receptors used for both visual and non-visual photoreception, and these proteins evolutionarily date back to the base of the bilaterians. In the current sequencing age, phylogenomic analysis has proven to be a powerful tool, facilitating the increase in knowledge about diversity within the opsin subclasses and, so far, at least nine types of opsins have been identified. Within echinoderms, opsins have been studied in Echinoidea and Ophiuroidea, which do not possess proper image forming eyes, but rather widely dispersed dermal photoreceptors. However, most species of Asteroidea, the starfish, possess true eyes and studying them will shed light on the diversity of opsin usage within echinoderms and help resolve the evolutionary history of opsins.

Results

Using high-throughput RNA sequencing, we have sequenced and analyzed the transcriptomes of different Acanthaster planci tissue samples: eyes, radial nerve, tube feet and a mixture of tissues from other organs. At least ten opsins were identified, and eight of them were found significantly differentially expressed in both eyes and radial nerve, with R-opsin being the most highly expressed in the eye.

Conclusion

This study provides new important insight into the involvement of opsins in visual and nonvisual photoreception. Of relevance, we found the first indication of an r-opsin photopigment expressed in a well-developed visual eye in a deuterostome animal. Additionally, we provided tissue specific A. planci transcriptomes that will aid in future Evo Devo studies.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1276-0) contains supplementary material, which is available to authorized users.

Light control of G protein signaling pathways by a novel photopigment

Channelopsins and photo-regulated ion channels make it possible to use light to control electrical activity of cells. This powerful approach has lead to a veritable explosion of applications, though it is limited to changing membrane voltage of the target cells. An enormous potential could be tapped if similar opto-genetic techniques could be extended to the control of chemical signaling pathways. Photopigments from invertebrate photoreceptors are an obvious choice—as they do not bleach upon illumination -however, their functional expression has been problematic. We exploited an unusual opsin, pScop2, recently identified in ciliary photoreceptors of scallop. Phylogenetically, it is closer to vertebrate opsins, and offers the advantage of being a bi-stable photopigment. We inserted its coding sequence and a fluorescent protein reporter into plasmid vectors and demonstrated heterologous expression in various mammalian cell lines. HEK 293 cells were selected as a heterologous system for functional analysis, because wild type cells displayed the largest currents in response to the G-protein activator, GTP-γ-S. A line of HEK cells stably transfected with pScop2 was generated; after reconstitution of the photopigment with retinal, light responses were obtained in some cells, albeit of modest amplitude. In native photoreceptors pScop2 couples to G_o; HEK cells express poorly this G-protein, but have a prominent Gq/PLC pathway linked to internal Ca mobilization. To enhance pScop2 competence to tap into this pathway, we swapped its third intracellular loop—important to confer specificity of interaction between 7TMDRs and G-proteins—with that of a G_q-linked opsin which we cloned from microvillar photoreceptors present in the same retina. The chimeric construct was evaluated by a Ca fluorescence assay, and was shown to mediate a robust mobilization of internal calcium in response to illumination. The results project pScop2 as a potentially powerful optogenetic tool to control signaling pathways.

Another way of seeing things

JGP study explores a novel photoreception pathway in a marine mollusk.

JGP study explores a novel photoreception pathway in a marine mollusk.