A team led by scientists at The Scripps Research Institute (TSRI) has created the first comprehensive roadmap of the protein interactions that enable cells in the pancreas to produce, store and secrete the hormone insulin. The finding makes possible a deeper scientific understanding of the insulin secretion process—and how it fails in insulin disorders such as type 2 diabetes.
The new study uncovered critical protein interactions in insulin-producing cells. Here, beta cells grown in culture illustrate the importance of TMEM24 in mature storage container granules (yellow) that deliver highly concentrated insulin packets to manage glucose levels after and between meals. (Image courtesy of the Balch lab, The Scripps Research Institute.)
“The development of this insulin interaction map is unprecedented, and we expect it to lead us to new therapeutic approaches for type 2 diabetes,” said William E. Balch, professor and member of the Skaggs Institute for Chemical Biology at TSRI.
Balch was the senior author of the study, which was recently reported online in the journal Cell Reports.
A Widespread Problem
Type 2 diabetes and a similar insulin-related condition known as metabolic syndrome currently affect hundreds of millions of people worldwide. One of the common features of these conditions as the disease develops is the inability of the secretion of insulin by the pancreas in response to mealtime surges of bloodstream glucose to meet demand. Insulin secretion normally signals to cells throughout the body to draw in glucose, which reduces the levels of this sugary fuel in the bloodstream—and a weakened insulin response allows blood glucose levels to stay too high for too long. Chronic elevated levels of glucose in the blood contribute to a wide variety of serious medical problems, including loss of circulation in the lower legs, degeneration of the retinas of the eyes, kidney failure, heart attacks and strokes, and even Alzheimer’s disease.
Boosting or repairing the insulin production and secretion process in the special pancreatic cells, called “beta cells,” where the hormone is made, would seem an obvious therapeutic strategy for type 2 diabetes. However, so far almost no diabetes drugs are targeted at improving the efficiency of this process, in part because so few details are known about it.
Scientists do know that within beta cells, insulin precursor proteins are synthesized, folded and processed into mature insulin as it is packed tightly into ready-to-go containers called granules, and eventually secreted into the bloodstream on demand when glucose levels rise. Researchers also have found and catalogued hundreds of individual proteins that appear to be relatively abundant within beta cells. But such studies haven’t made clear which of these beta cell proteins actually interact with insulin and therefore participate directly in the insulin production, storage and secretion process.
“Our understanding of the beta cell in generating and releasing sufficient insulin, the real culprit responsible for progression of type 2 diabetes, has been extremely poor,” said Balch.
A New View of Insulin
For the new study, Balch laboratory Research Associate Anita Pottekat used antibodies that bind to early and late forms of the hormone in the synthesis/secretion process. By pulling insulin proteins out of the soup of beta cell molecules, these antibodies also isolated whatever proteins happened to have bound to the insulin—and thus presumably had functional interactions with insulin. Pottekat then applied an extremely sensitive form of an established technique called mass spectrometry to identify these proteins.
In this way, the scientists were able not only to determine the insulin-interacting proteins within beta cells, but also to distinguish those that seem to play a role in the early, insulin-synthesis phase of the process from those that seem to function in the later stages of insulin storage and secretion. “We were able to see which proteins appear to be interacting more with the precursor ‘proinsulin’ protein versus mature insulin,” Pottekat said.
Collaborating scientist Scott Becker, a postdoctoral fellow in the laboratory of Gerard Manning, then at the Salk Institute for Biological Studies in La Jolla, handled the statistical analyses of the data. In this way they were able to assemble what is in effect the first-ever map of insulin’s “biosynthetic interaction network.”