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The Algae’s Third Eye

Published on 18. Februar 2019, 16:57 h
Scientists at the Universities of Würzburg and Bielefeld in Germany have discovered an unusual new light sensor in green algae. The sensor triggers a reaction that is similar to one in the human eye.

Just like land plants, algae use sunlight as an energy source. Many green algae actively move in the water; they can swim to the light or move away from it. For light perception they use special sensors (photoreceptors).


In dieser reproduktiven Zelle der mehrzelligen Volvox-Alge wurde der neuartige Lichtsensor 2c-Cyclop mit Fluoreszenz (grün) markiert. Er zeigt sich in Membranen und um den Zellkern herum. Foto: Eva Laura von der Heyde
In this reproductive cell of the multicellular Volvox alga, the novel light sensor 2c-cyclop was labeled with fluorescence (green). It shows up in membranes and around the nucleus. Image: Eva Laura von der Heyde
The decades-long search for these light sensors in algae led to a first success in 2002, when two so-called channelrhodopsins have been discovered and characterized. These ion channels absorb light, then open up and transport ions. They were named after the visual pigments of humans and animals, the rhodopsins.

Now a third “eye” in algae is known: Researchers discovered a new light sensor with unexpected properties. The research groups of Professor Armin Hallmann (Bielefeld University) and Professor Georg Nagel (Julius-Maximilians-Universität Würzburg, JMU) report this finding in the journal BMC Biology.

Light reduces cGMP production
The surprise: The new photoreceptor is not activated by light but inhibited. It is a guanylyl cyclase which is an enzyme that synthesizes the important messenger cGMP. When exposed to light, cGMP production is severely reduced, leading to a reduced cGMP concentration – and that's exactly what happens in the human eye as soon as the rhodopsins there absorb light.
The newly discovered sensor is regulated by light and by the molecule ATP. Such "two component systems" are already well known in bacteria, but not in higher evolved cells. The researchers have named the new photoreceptor "Two Component Cyclase Opsin," 2c-cyclop for short. They found it in two green algae, in the unicellular alga Chlamydomonas reinhardtii as well as in the multicellular alga Volvox carteri.

Function shown in oocytes and algae
Prof. Dr. Armin Hallmann<br> Photo: Bielefeld University
Prof. Dr. Armin Hallmann
Photo: Bielefeld University
"I was pleasantly surprised that we were able to reveal the nature of these photoreceptors, which for so long escaped investigation. It is exciting to see that simple green algae developed multidomain proteins, which perform complicated intramolecular cascade reactions and which are even controlled by light”, says Armin Hallmann.
"So far, nobody could demonstrate the function of these light sensors," says Nagels co-researcher Dr. Shiqiang Gao. Only the research groups from Bielefeld and Würzburg have succeeded in doing so: They have installed the new rhodopsin in oocytes of the frog Xenopus laevis and in all cells of the spherical alga Volvox carteri. In both cases, its function could be shown and characterized.

Perspectives for Optogenetics
Zwei Moleküle des neuen Lichtsensors 2c-Cyclop, der im Dunklen den Botenstoff cGMP produziert. Grünes Licht hemmt die Signalkette. Bild: Shiqiang Gao, Armin Hallmann, Georg Nagel
Two molecules of the new light sensor 2c-cyclop, which in darkness produces the messenger cGMP. Green light inhibits the signal chain. Image: Shiqiang Gao, Armin Hallmann, Georg Nagel
The authors believe that the 2c-Cyclop light sensor offers new opportunities for optogenetics. With this methodology, the activity of living tissues and organisms can be influenced by light signals. By means of optogenetics, many basic biological processes in cells have already been elucidated. For example, it provided new insights into the mechanisms of Parkinson's disease and other neurological diseases. She also brought new insights into diseases like autism, schizophrenia, and depression or anxiety disorders.

Publication
"Two-component cyclase opsins of green algae are ATP-dependent and light-inhibited guanylyl cyclases", Yuehui Tian, Shiqiang Gao, Eva Laura von der Heyde, Armin Hallmann, and Georg Nagel, BMC Biology, 6 December 2018, 16: 144 , https://doi.org/10.1186/s12915-018-0613-5


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