G protein-coupled receptor (GPCR) signaling is at the heart of many biological processes—these receptors sense stimuli such as smell, taste, neurotransmitters, and hormones in the brain. Now, a new optogenetics-based system is furthering this understanding.
The system, developed by a team at Osaka Metropolitan University in Japan, is based on two G protein-coupled photoreceptor (light-sensitive) proteins called opsins (see video). They have a bistable nature and possess unique properties that have developed throughout their evolution. Being bistable means the receptors switch between stable states when active or inactive and without breaking down.
“We’ve made a system with high spatiotemporal control of various kinds of GPCR-signaling, which underlie so many biological processes, using light stimuli,” says Mitsumasa Koyanagi, a biology professor in Osaka’s Graduate School of Science who led the research with fellow biology professor Akihisa Terakita. “Using our tools, high spatiotemporal control—which lets us control specific areas of tissue with precise timing—can be achieved with much higher sensitivity than conventional methods.”
He adds that it can also be achieved in a color-dependent manner, which allows their system to use multiple signals with different wavelengths of light.
“The existing systems for optical control of GPCR signaling use bleaching opsins, like our visual opsins we use to see, which have weaknesses when used in tissues outside the eye,” Koyanagi says. “Because they photo-bleach and require 11-cis retinal—the light-sensitive component of rod and cone photoreceptors—they do not function sustainably. Our new system uses bistable opsins to overcome these issues.”
The team has been working specifically with Caenorhabditis elegans (C. elegans), a transparent nematode more commonly known as a roundworm. For decades, roundworms have been used as a model organism in biological research, given their multicellular structure and nervous system.
In their study, published in the Proceedings of the National Academy of Sciences, opsins isolated from MosOpn3, a protein-coding gene in mosquitoes, were introduced into a roundworm’s sensory cells—these types of cells detect information, including light, sound, and temperature, through receptors on their surface, which travels through nerves to the brain. Upon detecting such information, the roundworm flees.
“MosOpn3 is involved in photoreception in extra-ocular tissues, which means it has evolved to be suitable for photoreception outside the eye, and we applied these photoreceptive properties to optical controls in other tissues,” Koyanagi says, noting the sensory cells.
The researchers exposed a roundworm, in an isolated setting, to white light. This triggered the “avoidance behavior” and with significantly higher sensitivity than is possible with channelrhodopsin-2, an optogenetic protein traditionally used in the study of neurons and cells. When the researchers turned the light off, the roundworms were completely still.
UV-sensitive opsins found in the pineal organ, a photosensory organ that contains photoreceptor cells similar to those in the retina, of lamprey eels (referred to as LamPP) were then introduced into the roundworm’s motor neurons.
“LamPP distinguishes color in the pineal organ by generating color opponency,” Koyanagi says. “This means LamPP has evolved to be suitable for color-dependent photoreception, which we applied to create color-dependent controls for different tissues.”
In the study, the roundworm stopped moving when exposed to UV light, but when exposed to green light they began moving again.
This “highly sensitive, robust, and versatile tools for optical control of GPCR signaling could accelerate basic life science and medical/pharmaceutical research,” Koyanagi says, “It could also accelerate the development of photo-stimulation devices, which could be used to noninvasively stimulate these biological processes.”
The team’s next steps will include expanding the tools’ application into other animal models, including mice.
