By Kelsey Calhoun
Vision has been studied inside and out for more than a century, resulting in some textbooks presenting the visual system as essentially understood. But María Gomez and Enrico Nasi, adjunct scientists at the Marine Biological Laboratory (MBL), don’t agree. They have spent the last several years investigating non-visual photoreceptors, cells whose function remains elusive in eyes filled with rods and cones. They reveal an important clue to how these cells work—how calcium triggers the electrical light response— in a recent paper published in Proceedings of the National Academy of Sciences.
Studies of vision traditionally divided light-sensitive cells into two distinct classes: those of vertebrates and those of invertebrates. The two classes were so different from each other that they were thought to represent two separate lines of evolution. But a few phenomena presented problems with this view. The most dramatic is the fact that blind people, who lack functioning rods and cones—the only photoreceptive cells previously thought to exist in vertebrates—can recover from jet lag, somehow sensing the light that resets their circadian rhythms. “A new type of photosensitive cell was later discovered in the mammalian eye that is responsible for these functions,” says Nasi. “Another dogma bites the dust.”
It is these non-visual photoreceptors, sometimes called circadian photoreceptors, that Nasi and Gomez, both professors at the Universidad Nacional de Colombia, were interested in studying. “What are these sensors? The idea that they might be just like photoreceptors of invertebrates—this is beyond blasphemy,” says Nasi. If true, “this leads to rewriting the evolutionary history of vision.” But studying these cells presented a few practical challenges. In vertebrates, the cells are few and far between, and have no unique shapes or markers to make them easy to find.
So Gomez and Nasi turned to an unassuming, fish-like invertebrate called a lancelet or amphioxus. This creature holds a unique place on the evolutionary tree of life, at the branching point between vertebrates and invertebrates. It has other advantages: the photoreceptors that interest Gomez and Nasi are easy to find in the organism, and manipulate. The evidence they found in the simple amphioxus suggests that vertebrates’ non-visual photoreceptors may mimic those found in amphioxus—that the visual systems of vertebrates and invertebrates are not as different as previously thought.
Their paper tackles the final step of the pathway that lets these photoreceptors translate incoming light into signals to the organism. Most of the pathway was already known, but solid evidence for the last step was elusive: How was light converted to an electrical cell signal that could be communicated to other cells?
Gomez and Nasi investigated the flood of calcium that is released when the circadian photoreceptors were exposed to light. They showed that calcium provoked the electrical cell signal, very similar to what happens with normal light stimulation. “It reproduces the native response,” says Gomez. This flood of calcium is the link that lets these photoreceptors communicate with the rest of the organism.
“We’re rather happy to see something that fully reproduces the light response for the first time,” Nasi says. But, he adds, “We don’t want to make claims that this is going to be general to all species.” Whether this discovery proves to be common in other species or not, it’s clear the field of vision and light-sensing cells still has much to reveal.
Peinado G, Orsano T, Gomez M, and Nasi E (2015). Calcium activates the light-dependent conductance in melanopsin-expressing photoreceptors in amphioxus. PNAS, DOI: 10.1073/pnas.1420265112