CHAMPAIGN, Ill. -- Researchers at the University of Illinois have isolated, sequenced and characterized an insulin gene from the marine mollusk Aplysia californica. The gene's structure -- though similar in form and function to that of human insulin -- possesses an additional peptide, the purpose of which remains unknown.
"Almost all animals, from corals to humans, make and use insulin as a hormonal control of metabolism," said Jonathan Sweedler, a professor of chemistry and a researcher in the university's Beckman Institute for Advanced Science and Technology. "Although the presence of an insulin gene in Aplysia had long been inferred, the specific gene had never been identified."
By employing a combination of advanced biochemical and mass spectrometric methods, Sweedler and colleagues at the U. of I., Columbia University and the Mount Sinai School of Medicine in New York City successfully identified the gene and fully characterized its chemical structure. The researchers reported their findings in the Sept. 15 issue of the Journal of Neuroscience.
"Using matrix-assisted laser desorption/ionization mass spectrometry of single neuron cells, we discovered a peak in the spectrum that did not correspond to any known neuropeptide," Sweedler said. "We then extracted the unknown mass using very-small-scale liquid chromatography and sequenced it in an attempt to identify potential neuroendocrine peptides, specifically insulin."
One of Sweedler's research thrusts is trying to understand the chemical cues that trigger egg-laying behavior in Aplysia. "Like studying genetics in fruit flies, snails -- especially marine mollusks -- make nice model systems for understanding behavior at its simplest," he said. "The animals are easy to study and they have readily accessible cells."
In the body, insulin causes certain physiological responses that are normally associated with food intake and metabolism. A decrease in the concentration of blood glucose is one of the usual effects of insulin injection. In their study, Sweedler and his colleagues showed that injecting insulin indeed changed the glucose level in Aplysia -- thus regulating metabolism -- as had been anticipated.
"But this is just one piece in a very large puzzle," Sweedler said. "Previous studies have shown that injecting human insulin into Aplysia causes the snail to go into egg-laying behavior. Some of our future work will focus on the role that Aplysia insulin plays in egg laying."
Future work will also attempt to resolve a mystery surrounding the Aplysia insulin structure. Normally, the insulin protein consists of two peptide chains joined together. The 'A' chain and 'B' chain are separated by a 'C' peptide, of which little is known. Surprisingly, the Aplysia insulin protein also contains an additional 'D' peptide, which hangs off the end of the 'A' chain.
"This is the first known case of a 'D' peptide," Sweedler said. "For now, its purpose eludes us."