An international research team led by scientists at University of Utah Health, Stanford University, and University of Copenhagen has developed a new form of fast-acting injectable insulin based on venom from the marine snail, Conus kinoshitai. By introducing biochemical features that enable the snail’s insulin to start working quickly, the team created an a modified form of human insulin, which they hope could give patients with diabetes better, more immediate control over their blood sugar.
“There was always this idea that if one could design a very rapidly acting insulin analogue, one could get much better control of blood sugar levels in people with diabetes,” said Helena Safavi, PhD, a biologist at University of Copenhagen. As well as being a promising candidate for therapeutic development, the new molecule has also, more broadly, revealed an unexpected biochemical strategy for converting human insulin into a fast-acting compound, the investigators suggest.
Safavi is co-corresponding author of the team’s published paper in Nature Chemical Biology, which is titled “Symmetric and asymmetric receptor conformation continuum induced by a new insulin.” Co-corresponding authors on the paper are biochemist Christopher Hill, DPhil, Vice Dean of Research for University of Utah School of Medicine, and Stanford protein chemist Danny Hung-Chieh Chou, PhD.
For millions of people with diabetes insulin is essential medicine. But for some ocean-dwelling predators, insulin is used as a weapon. With a burst of insulin venom, a fish-hunting cone snail can drop the blood sugar of its prey so precipitously that the prey quickly becomes paralyzed and defenseless.
Normally, human insulin is produced and stored in the pancreas until it is needed to manage blood sugar and energy levels. To facilitate efficient storage, individual molecules of insulin come together, linking up first into pairs, or dimers, and then into groups of six. “In vertebrates, including fish and human, insulin is secreted as a hexamer that dissociates into a dimer and then a monomer to bind and activate the IR [insulin receptor],” the authors wrote.
But for people who rely on insulin injections, the molecule’s tendency to pair up is an impediment. Insulin can’t make its way from the injection site to the bloodstream until the clustered molecules dissociate. This creates a delay that can make it difficult for people with diabetes to keep their blood glucose within the optimal range, increasing the risk of complications.
“Unlike physiological release of insulin from pancreatic β-cells, subcutaneous injection results in relatively slow dissolution to the monomer, which can delay diffusion and compromise effective glucose control in individuals with diabetes,” the authors noted. Moreover, they pointed out, “Designing insulin analogs that do not form dimers and hexamers has proven challenging because the region affecting dimerization (near the C terminus of the B chain) is also critically important for IR activation.”
There are about 1,000 species of marine cone snail in existence today, and these animals use complex venoms to capture prey such as fish, worms or other snails, the authors explained. Most of the cone snail toxins target ion channels in the prey’s nervous system, resulting in rapid paralysis. But some species of cone snail also use insulin as a toxin. “Venom insulins rapidly bind and activate the prey’s IR and, consequently, induce dangerously low blood glucose levels, rendering the envenomated animal unable to escape,” the investigators stated.
The cone snails’ venomous insulins, which Safavi first discovered in a species called Conus geographus, while working as a postdoctoral researcher in the lab of University of Utah professor Baldomero Olivera, caught the research team’s attention because these molecules don’t form clusters. “The cone snail doesn’t need to have insulin for storage. It wants to have something that very quickly acts to paralyze fish,” Safavi said. “And when we looked at the insulin, we found that it doesn’t come together in six insulin molecules. It’s just a single insulin that acts in the fish prey.” As the authors further explained, “Specifically, the venom insulins have dispensed with residues near the B-chain C terminus of the hormone that, in mammalian insulins, mediate the receptor binding essential for activity and the dimerization that makes human and therapeutic insulins slow acting when injected subcutaneously.”
Helena Safavi, left, helps her colleague, José Rosado from Maputo, Mozambique, sort cone snails collected by scuba divers near the Solomon Islands in the south Pacific. The scientists set up a mobile lab on the diving ship to dissect and preserve the biological samples. [Adam Blundell]Some insulins that form fewer clusters than natural human insulin have become available to patients. Hill explained that these therapeutic insulins do form pairs, but they separate more readily than human insulin. “But the snails have been able to do even better than that,” he said. “The snails been particularly good at shifting the balance all the way over to the monomeric [singular] form.”
In 2020, a team led by Chou, then a professor at U of U Health, achieved that same shift to the monomeric form by incorporating a few key molecular features of C. geographus insulin into human insulin. “Our earlier work demonstrated that an insulin analog inspired by a venom insulin from C. geographus maintains potency in the absence of the C-terminal B-chain residues through four substitutions in the core of the insulin structure,” the authors noted.
Then Safavi discovered that C. geographus isn’t the only cone snail that makes insulin. About 150 species of cone snails feed on fish, and each species makes its own complicated cocktail of toxins to subdue its prey. By exploring a U of U collection of cone snail venoms, Safavi found several that contained insulin-like molecules. Surprisingly, one of those venomous insulins was structured quite differently from the insulin made by C. geographus, even though it, too, was fast acting and cluster free. “…unlike any other reported insulin in nature, C. kinoshitai insulin displays a four-amino acid C-terminal elongation of the A chain,” the investigators explained. “It’s just amazing, because they are using very different methods to engage the [insulin] receptor,” Chou said.
Once the team recognized Conus kinoshitai’s unique biochemical tactics, Chou used that knowledge to develop a new hybrid insulin. The new molecule maintains the ability to bind to the human insulin receptor but does not form clusters, just like the original Conus geographus-inspired insulin. Chou says that at this point, the two hybrid insulin molecules, each based on venom from one of the two cone snail species, hold similar promise as potential therapeutics.
It took detailed images captured by Alan Blakely, a graduate student in Hill’s lab, to reveal how the new hybrid insulin works. Blakeley used cryo-electromagnetic microscopy to visualize the structure of the new insulin and how it interacts with its receptor.
Normally, the human insulin receptor is activated by the same region of insulin that links the molecules to one another. To create the snail-human insulin hybrids, this segment has been removed to prevent clustering. The Hill lab’s structural analysis clarified how the new insulin manages to activate the receptor without it. “We reveal how an extended A chain can compensate for deletion of B-chain residues, which are essential for activity of human insulin but also compromise therapeutic utility by delaying dissolution from the site of subcutaneous injection,” they stated. “This finding suggests approaches to developing improved therapeutic insulins.” Understanding exactly how the two molecules interact may also help to guide further development of potential fast-acting insulins. “ … our structural and functional data also demonstrate multiple opportunities for further optimization as a fast-acting insulin with the potential to improve therapeutic options for the treatment of diabetes.”
“What’s really beautiful about this study is the way it spans a wide range of science, starting with the study of a fascinating question in animal behavior and leading to the multidisciplinary, collaborative development of a potential therapeutic,” noted Hill. “This research has opened an exciting avenue for developing better therapeutics for people with diabetes.”