The sketchy picture of amyloid formation in type 2 diabetes has become a little more detailed, thanks to a structural study that applied the techniques of 2D infrared (IR) spectroscopy and isotope labeling. These techniques allowed the characterization of a critical intermediate generated during amyloid formation. The intermediate, previously postulated but largely beyond the reach of direct measurement, consists of misfolded human islet amyloid polypeptide (hIAPP), or amylin.
By characterizing this intermediate, scientists have provided a structural explanation for the lag between amyloid clumping and the ultimate formation of amyloid deposits. In addition, they may be able to explain why some species develop hIAPP plaques while others do not.
The 2D IR technique was developed in the laboratory of Martin Zanni, Ph.D., a University of Wisconsin-Madison chemistry professor. Dr. Zanni then applied the technique to the problem of amylin clumping while working with collaborators from the University of California, Irvine; University of Chicago, Argonne National Lab; and the State University of New York at Stony Brook, as described in a paper published November 11 in the Proceedings of the National Academy of Sciences (“Mechanism of IAPP amyloid fibril formation involves an intermediate with a transient β-sheet”).
"For about 30 years, we thought this problem was solved, because a lot of experiments pointed to the middle part of amylin molecules as the cause," Dr. Zanni said.
Named for its amino acid structure, the FGAIL regions of amylin proteins were believed to lock together into rigid sheets, called beta-sheets. These sheets break apart, forming the dangerous plaque. But experiments published in 2007 showed that the FGAIL section of amylin is floppy and loose, like a loop of rope. “This result made no sense compared to the 30 years of prior studies,” Dr. Zanni said.
It was known that subtle differences in the shape of amylin protected some species and endangered others. For example, while rats, cats, and dogs escape type 2 diabetes, humans do not. “Why should the small differences in the amylin protein of various mammals play such a deciding role if those differences are located in a flexible, floppy, and forgiving region of the protein?”
Dr. Zanni and his collaborators showed that the floppy FGAIL region can contribute to the formation of plaque, but first, the amylin proteins must clump together in an arrangement in which the FGAIL region is indeed a rigid beta-sheet. These investigators suspect that the intermediate clumping step is where animal species resistant to type 2 diabetes are making their move.
“Our results indicate that the proteins in rats, dogs, and other animals do not stop the plaques themselves, but instead target this upstream step,” Zanni says, “preventing the intermediate from forming and thereby the plaques as well.”
Determining the structural details of plaque formation may have profound implications. For example, determining how amylin proteins differ may provide targets for new treatments for diabetes and other plaque-involved diseases such as Alzheimer’s and Parkinson’s.
“Good drugs work by fitting into nooks and crannies,” said Dr. Zanni. “Thus, it is much easier to design a drug when the shape of the toxic protein is known, which is what our data is beginning to provide.”