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A new study shines light on why some people suffer from sleep and mood disorders and how light therapy should be performed.
By Katie Neitz
If the dark days of winter make you feel blue, low-level signaling by melanopsin may be to blame.
Melanopsin, a light-sensitive protein, plays a role in the body’s circadian rhythm, as it detects changing levels of light and notifies the brain when it’s time to be active. In dim conditions, melanopsin doesn’t trigger a response, but in bright light, like sunshine, it makes you feel alert.
This is relatively new knowledge: Melanopsin was only identified in 1998, and its discovery (in an African-clawed frog) gave new insight into biological clocks and who’s at risk for certain mood and sleep disorders.
Jim Dearworth, associate professor of biology and neuroscience, has been working to further the scientific community’s understanding of this protein. And that led to this new finding: There’s a high probability that melanopsin is present in red-eared slider turtles.
This discovery was the result of an interdisciplinary research effort including several Lafayette contributors: Sze Cheng ’17, Christopher Anderson, professor of chemical and biomolecular engineering, Joseph Sherma, professor emeritus of chemistry, and Eric Ho, professor of biology and computer science. The results of their research, six years in the making, will be published in the December issue of Journal of Herpetology.
“We identified that the mRNA—the genetic signal that would support the presence of the protein—is in the iris of the turtle,” Dearworth says. “It has been studied in other animals, but very little has been done in reptiles. The turtle has a very slow pupil response to light, compared to other animals, like mice and monkeys and humans. That can give us more insight and a different perspective on how melanopsin might fit in with our understanding of circadian rhythm and seasonal affective disorder.”
Dearworth has been studying red-eared slider turtles for 24 years, since he was in graduate school at University of Delaware. Back then, there was a misconception about how turtles’ eyes functioned.
“For a long time, it was thought that the turtle’s pupil was not reactive to light, that it didn’t change when exposed to bright light,” he says. “But we clearly showed that it did react—it was just a super-slow reaction. The turtle’s pupil does in minutes what we do in split seconds. That was a breakthrough.”
Then, in the late 1990s came the discovery of melanopsin, which Dearworth says really transformed how researchers thought about vision. For a hundred years prior to that finding, it was believed that rod and cone cells were solely responsible for the eye’s reaction to light.
Knowing that melanopsin is at work too—and understanding that it drives a slow response to light—enables doctors to better understand why some people suffer from sleep and mood disorders, like seasonal affective disorder, and how light therapy should be performed.
Armed with this knowledge, Dearworth says that one day doctors might be able to identify people who are at risk for developing seasonal affective disorder by evaluating how their pupils react to light.
The paper, “Melanopsin mRNA in the Iris of Red-Eared Slider Turtles,” goes further than most studies in advancing understanding of the photopigment. For example, there are two different forms of melanopsin, but very few researchers have differentiated them in their research. Dearworth and his colleagues studied both types (known as Opn4m and Opn4x) and discovered that the retina of the turtle has more of Opn4m than Opn4x, and it’s vice-versa in the iris.
Another significant aspect of the study is its genetic component: Ho’s computational biology expertise took the research a step closer to determining the genetic sequencing of melanopsin in the red-eared slider turtle.
“The gene Jim is interested in has not been sequenced yet, so we used the sequencing from a close relative, the painted turtle, and compared it to the mRNA in the red-eared slider turtle, which enabled us to predict the full sequence for melanopsin in the red-eared slider turtle,” Ho says. “A lot of biology involves computational work, and I’m glad I’m able to help my colleagues.”
Ho’s contribution—the gene sequencing—is critical for the next phase of Dearworth’s research.
“We can now carry out immunohistochemistry, which will enable us to identify if the melanopsin is actually in the tissues of the turtle,” Dearworth says. “We are fairly certain based on the genetic analysis we did for this study, but going to this next level will enable us to know absolutely for sure.”
Work on that phase will begin in the spring. Bright days ahead for Dearworth, indeed.
This research was supported by the Academic Research Committee, EXCEL Scholars program, Howard Hughes Medical Institute, biology department’s Science Horizons program, and Roger Newton ’72.