Light sensing machinery transforms sunlight into body chemistry
Whether it comes from the sun, the stars, or our phones, we are often bathed in light. We use both natural and man-made light to help us get through modern life, and we’ve even figured out how to harvest its energy through the use of solar panels. But the ability to capture the energy in sunlight isn’t new to humans. In fact, it’s coded in our DNA. The human body is capable of building specialized proteins that transform light energy into chemical energy. One of these proteins, known as melanopsin, uses light energy to set our biological clock and may be involved in embryonic development.
Light carries energy with it—you can sometimes feel that energy in the form of heat. Most life forms have evolved to capture and use this energy for various purposes. Plants, for example, have developed the ability to use energy from light to build complex sugars. Though humans can’t do this, we have evolved other impressive skills that involve light. We’re able to produce proteins known as opsin and retinal which work together to detect light. To function, retinal and opsin are chemically bound to one another. When light waves come into contact with the retinal, it passes energy into the protein which causes it to move slightly. This movement disrupts the chemical bond between retinal and opsin, causing the opsin protein to move a little as well. These small movements can send a chemical chain reaction through a cell and cause neurons to signal back to the brain. This is the basic concept behind vision1,2.
Across the animal kingdom, there are many different versions of the opsin proteins. Within humans alone there are multiple forms, each of which serves a unique function1. We’re able to detect different colors in light thanks to small differences in the DNA coding for the various opsin proteins. These differences can either strengthen or weaken the chemical bond between opsin and retinal. When the bond between retinal and opsin is strong, it requires more energetic light—like blue light—to stimulate the chain reaction. When there’s a weak bond, less energetic light—like red light—is sufficient2. In humans, light sensitive proteins can be found in specialized cells of the eye. These cells are known as rods and cones. Light enters our eye and comes into contact with light sensitive proteins in the rod or cone cells. This contact triggers a relay of signals from the eye to the brain and results in an image. Thanks to these cells, and more specifically the opsin and retinal proteins, our brains are able to form colorful images2.
Not all light sensitive proteins are used to help our brains form images, however. Almost twenty years ago, scientists discovered another type of opsin which appeared to be involved in non-image forming processes3. This opsin is coded by the gene OPN4 and is called melanopsin1-3. Through a series of well-designed studies, researchers showed that melanopsin was involved in multiple processes that integrate light with our physiology4. Setting the biological clock is a good example of this. Many processes (like sleep) are regulated by an internal molecular clock referred to as the circadian rhythm. By training our biological clock according to light, we’re able to have innate desires to sleep during the night, and seek out food during the day. Cells in our eyes that produce melanopsin reach all the way back to a region of the brain that is known as our internal clock’s master regulator (or the suprachiasmatic nucleus). By detecting light for these cells, melanopsin is able to help regulate a person’s circadian rhythm3-5.
Other processes may also be influenced by the OPN4 gene. Studies in mice have shown that some light is able to penetrate into their body and can affect embryonic development6. Researchers found that melanopsin proteins sensed this light and helped direct the formation of blood vessels and neurons in the developing mouse eye. This evidence suggests that human eye development may be under similar influence from melanopsins.
It is fascinating to see how light is manipulated by our bodies. Consider this: Light from the sun travels 93 million miles through space. It blows past planets, enters the atmosphere, and collides with your eyes. That cosmic energy is not wasted on us—with the help of genes like OPN4, we put it to work!
3Graham, Dustin M. “Melanopsin-Expressing, Intrinsically Photosensitive Retinal Ganglion Cells (IpRGCs).” Webvision: The Organization of the Retina and Visual System [Internet]., U.S. National Library of Medicine, 2 Nov. 2016, www.ncbi.nlm.nih.gov/books/NBK27326/.
4Hatori, Megumi, and Satchidananda Panda. “The Emerging Roles of Melanopsin in Behavioral Adaptation to Light.” Trends in molecular medicine 16.10 (2010): 435–446. PMC. Web. 27 Feb. 2018.
5Besharse, Joseph C., and Douglas G. McMahon. “The Retina and Other Light Sensitive Ocular Clocks.” Journal of biological rhythms 31.3 (2016): 223–243. PMC. Web. 27 Feb. 2018.
6Rao, Sujata et al. “A Direct and Melanopsin-Dependent Fetal Light Response Regulates Mouse Eye Development.” Nature 494.7436 (2013): 243–246. PMC. Web. 27 Feb. 2018.