The identification of a protein important for insulin synthesis may hold clues for understanding the pathogenesis of diabetes.
Although the protein, called TRAP-alpha, was first discovered more than 30 years ago, its biological function has been unclear. The new findings, reported Dec. 4 in Science Advances, demonstrate that TRAP-alpha is required for both early and late steps in insulin synthesis.
Variants in the TRAP-alpha gene were previously associated with susceptibility for Type 2 diabetes and pancreatic beta cell dysfunction, supporting the notion that defects in insulin synthesis contribute to the development of diabetes.
The findings were a bit of a surprise to Patrick Hu, MD, PhD, associate professor of Medicine at Vanderbilt, and his colleagues at the University of Michigan, who were using an unbiased genetic screen in the nematode C. elegans to identify new components of a signaling pathway that extends the worm's lifespan.
"These genetic screens are humbling," said Hu, who is also associate professor of Cell and Developmental Biology. "We found TRAP-alpha, and we weren't quite sure what to make of it, because it didn't seem to be involved in any part of the downstream signaling pathway we are interested in."
They knew that TRAP-alpha is part of the translocon (TRAP stands for translocon-associated protein), which moves newly synthesized proteins that will be secreted from the cell into the endoplasmic reticulum (ER) for further processing. With additional genetic analysis in C. elegans, they found that deletion of TRAP-alpha affects the worm's insulin signaling pathway.
Since insulin is a secreted protein, the investigators wondered if TRAP-alpha moves insulin into the ER.
Biochemical studies in pancreatic beta cells confirmed its role: TRAP-alpha is required for translocation of preproinsulin (the precursor to insulin) into the ER. Deletion of TRAP-alpha also impaired later steps in insulin processing and secretion.
"TRAP-alpha was not on anyone's radar in terms of being required for insulin biogenesis," Hu said. "Our work highlights the value of using a model organism like C. elegans to do an unbiased genetic screen. It led us to a molecule that seems to be important in making insulin and that could very well shed light on the pathogenesis of diabetes, a common disease that affects about 10% of the U.S. population."
Understanding mechanistic details of TRAP-alpha action could lead to new approaches to prevent and treat Type 2 diabetes.
Preproinsulin is the first "client" protein for TRAP-alpha, and the investigators are interested in identifying other proteins that TRAP-alpha shepherds into the ER.
They also want to understand why TRAP-alpha deletion affects late steps in insulin processing and secretion. They explored a role for TRAP-alpha in promoting ER homeostasis -- the balance between incoming secretory proteins and ER proteins that assist with folding and processing. Imbalance results in "ER stress," which can cause cell death. They found that deletion of TRAP-alpha triggers ER stress.
Hu and his colleagues are now conducting genetic screens to identify other components of the ER stress pathway.
"We think that the genes we identify may also be involved in diabetes pathogenesis, because beta cells are so sensitive to ER stress," he said. "ER stress has also been implicated in other diseases including cancer, neurodegenerative diseases and pulmonary fibrosis."
Hu's colleagues in the studies included Omar Itani, PhD, now at Washington University in St. Louis, and Peter Arvan, MD, PhD, and Ming Liu, MD, at the University of Michigan. The research was supported by the American Heart Association, National Institutes of Health (grants DK111174, DK048280, AG041177) and the American Cancer Society.