Eugene Bell Center

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.

Enrico Nasi and Maria del Pilar Gomez

Enrico Nasi and Maria del Pilar Gomez

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.

Amphioxus can grow as long as 2.5 inches, and it is very difficult to tell their head from their tail. Other than that, they are a very useful animal model. Photo by Hans Hillewaert.

Amphioxus can grow as long as 2.5 inches, and it can be difficult to tell the head from the tail. Photo by Hans Hillewaert

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

Akash Srivastava, a student in the Brown-MBL Graduate Program in Biological and Environmental Sciences, successfully defended his Ph.D. dissertation entitled “Transdifferentiation of Liver to Pancreas” on August 18 at Brown University. Srivastava, a student in the Molecular Biology, Cell Biology and Biochemistry Department (MCB) at Brown, conducted his research at the MBL’s Eugene Bell Center for Regenerative Biology and Tissue Engineering under the advisement of MBL Associate Scientist Marko Horb. His doctoral committee members also included Rich Freiman, Kristi Wharton, and Eric Morrow from the MCB Department.

Srivastava’s doctoral research helped to elucidate the molecular mechanisms involved in transdifferentiation of liver to pancreas in the frog Xenopus laevis. He identified a previously unknown role of a highly conserved beta-catenin inhibitor protein (Chibby) in transdifferentiation of liver to pancreas and in normal pancreas development. His research also provided a better understanding of the function of Wnt/beta catenin signaling in pancreas development. Funding for his research came from the Horb lab at with grant support from the National Institutes of Health.

After completing his final manuscripts, Srivastava plans to work as a validation consultant in the pharmaceutical industry.

Akash Srivastava defends his thesis at Brown University for his Ph.D. in the Brown-MBL Graduate Program.

Akash Srivastava defends his thesis at Brown University for his Ph.D. in the Brown-MBL Graduate Program.

By Laurel Hamers

One of the brain’s amazing abilities is self-repair: Although injury or illness may disrupt neural circuits, many connections will reform over time.

Artur Llobet, an MBL Research Awardee from the University of Barcelona, is spending his second consecutive summer in the Whitman Center for Visiting Research investigating olfactory neuron repair in Xenopus laevis, the African clawed frog.

Calcium labeling of olfactory sensory neurons' presynaptic terminals in a Xenopus laevis tadpole. Photo credit: Artur Llobet

Labelling of presynaptic terminals of olfactory sensory neurons in a Xenopus laevis tadpole using calcium green dextran. Image is pseudocolored so that yellow represents higher and blue lower calcium concentration.
Photo and caption: Artur Llobet

“One of the advantages of working with frogs is that they have fantastic regenerative capabilities,” says Llobet. Tadpoles are able to repair damaged neural circuits in a few days, making them ideal test subjects.

Llobet is working with a transgenic line of Xenopus tadpoles that express green fluorescent protein (GFP) in their neurons, allowing him to easily see the neural connections. Last year, he studied the timeframe of Xenopus neural repair by measuring how long snipped olfactory nerves took to regrow. Now, he is trying to understand in greater detail the mechanisms behind the repair process.

Neurons pass electrochemical messages between each other at junctions called synapses; when a neuron fires, the voltage change propagates along the nerve fiber (axon) and calcium increases at the presynaptic terminal, which releases neurotransmitters. By labeling the tadpoles’ synaptic terminals with calcium indicators, Llobet can visualize the functionality of the re-grown connections and determine when during the repair process the new synapses start signaling.

“In a GFP animal, we can see that the nerve has re-grown, but we don’t know if that nerve is actually working or not,” says Llobet. “So we look at the synapses and see whether the calcium concentration increases when we stimulate olfactory sensory neurons.” This calcium accumulation indicates that the new nerve is not just present, but also functional.

By examining neural repair in frogs, scientists hope to gain insight into this process in more complex systems such as the human brain.

Llobet’s research is taking place through the National Xenopus Resource (NXR) at the MBL, a center that maintains breeding stocks of frogs and provides training on advanced imaging and experimental technologies. According to Llobet, the specialized resources offered by the NXR make this research project possible. He is one of six MBL Research Awardees in 2014 to be using the animals and research services of the NXR, which is one of 28 National Institutes of Health-funded Animal Resource Centers nationwide and a cornerstone facility of the MBL’s Bell Center for Regenerative Biology and Tissue Engineering.

Carlo Bocconcelli, a senior at Falmouth Academy and student researcher working in the lab of MBL scientist Joel Smith under the guidance of postdoctoral associate Sarah Tulin, won second place in the Intel International Science Fair Competition, held last week in Los Angeles. Carlo’s project involved developing a method for detecting regulatory regions in the sea urchin genome which produced promising results the lab intends to submit for publication this summer. Carlo will be joining Harvard’s freshman class this coming fall. Congratulations, Carlo!