Evolution of mammalian Opn5 as a specialized UV-absorbing pigment by a single amino acid mutation” by Takahiro Yamashita et al.
Yamashita et al., 2014
Light-detecting proteins called opsins sit in the back of the retina and kickstart your sense of vision. But one of these proteins, opsin 5, lives deep inside the brain and detects light without help from the eyes. Violet light, a wavelength abundant in natural light, can pass through the skull straight to opsin 5 neurons.
These neurons (dark purple) reside in the preoptic area (light purple) of mice, a brain region that regulates body temperature and other aspects of homeostasis. They also receive information from a skin temperature-sensing brain circuit and send information to brown adipose tissue. The “good” type of fat, brown adipose tissue, burns lipids and carbs to generate heat and raise body temperature. The tissue is unique to mammals, equipping them to endure the cold and survive on sparse diets. But heat generation is an energy-sucking process, so brown adipose tissue springs into action only when it’s needed.
When opsin 5 neurons detect violet light, they inhibit brown adipose tissue activity and keep the body temperature stable. In the absence of violet light, opsin 5 neurons allow brown adipose tissue to generate heat and raise the body temperature. Detecting changes in light lets mammals calibrate their body temperature to the environment. But for modern humans, artificial light lacking the violet wavelengths may leave opsin 5 neurons under-stimulated. This disrupts the body’s ability to break down fats and carbs, which can lead to metabolic dysfunction.
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Zhang, K. X., D’Souza, S., Upton, B. A., Kernodle, S., Vemaraju, S., Nayak, G., … Lang, R. A. (2020). Violet-light suppression of thermogenesis by opsin 5 hypothalamic neurons. Nature, 585(7825), 420–425. doi: 10.1038/s41586-020-2683-0