Mutant digestive enzymes aggregate in nearby insulin-producing beta cells, triggering an inherited condition that may shed light on other diseases of the pancreas.

In the pancreas, insulin-producing beta cells are clustered with other hormone-producing endocrine cells and surrounded by pancreatic exocrine cells that secrete digestive enzymes. Joslin Diabetes Center researchers now have shown how one form of the rare inherited disease known as mature onset diabetes of the young (MODY) is driven by mutated digestive enzymes generated in pancreatic exocrine cells that are then taken up by neighboring insulin-secreting beta cells.

This finding may help in understanding other diseases of the pancreas, including type 1 or type 2 diabetes, in which abnormal molecular crosstalk between these two groups of cells might play a damaging role, said Joslin senior investigator Rohit N. Kulkarni, MD, PhD, Co-Section Head of Joslin’ Islet and Regenerative Biology Section and a Professor of Medicine at Harvard Medical School.

Most versions of MODY are caused by a single mutation in genes expressing proteins in the beta cells. But in one form of MODY called MODY8, a mutated gene in nearby exocrine cells is known to kick off this damaging process, said Kulkarni, corresponding author on a Nature Metabolism paper presenting the work. Scientists in his lab discovered that in MODY8, digestive enzymes generated by this mutated gene aggregate in the beta cells and impair their health and insulin-releasing function.

"While the endocrine and exocrine pancreas form two distinct parts with disparate functions, their close anatomical relationship shapes their fate," said Sevim Kahraman, PhD, a postdoctoral researcher in the Kulkarni lab and lead author of the paper. "The pathological condition developing in one part impairs the other."

“Although MODY8 is a very rare disease, it may shed light on general mechanisms involved in the development of diabetes," said Anders Molven, PhD, a contributing author and Professor at the University of Bergen in Norway. "Our findings demonstrate how a disease process that starts in the exocrine pancreas eventually may affect the insulin-producing beta-cells. We think that such negative exocrine-endocrine crosstalk could be particularly relevant for understanding some cases of type 1 diabetes.”

Kulkarni explained that the mutated CEL (carboxyl ester lipase) gene in MODY8 also is considered a risk gene for type 1 diabetes. That raises the question of whether some cases of type 1 diabetes also feature these aggregated mutant proteins in beta cells, he said.

The study began by modifying a human exocrine (acinar) cell line to express the mutant CEL protein. When beta cells were bathed in solution from either mutated or normal exocrine cells, the beta cells took up both the mutated and normal proteins, bringing in higher numbers of the mutated proteins. The normal proteins were degraded by regular processes in the beta cells and disappeared over several hours, but the mutant proteins did not, instead forming protein aggregates.

So how did these aggregates affect the function and health of the beta cells? In a series of experiments, Kahraman and her colleagues proved that the cells didn't secrete insulin as well on-demand, proliferated more slowly and were more vulnerable to death.

She confirmed these findings from cell lines with experiments in cells from human donors. Next, she transplanted human exocrine cells (again expressing either the mutated or the normal digestive enzyme) along with human beta cells into a mouse model designed to accept human cells. "Even in that scenario, she could show that the mutated protein is again taken up more by the beta cell in comparison to the normal protein, and it forms insoluble aggregates," Kulkarni said.

Additionally, examining pancreases from people with MODY8 who died from other causes, the investigators saw that the beta cells contained the mutated protein. "In healthy donors, we did not find even the normal protein in the beta cell," he said.

"This MODY8 story originally started with the clinical observation of patients with diabetes also having digestive problems, which led to the finding of a common genetic denominator," said Helge Raeder, MD, a co-author and Professor at the University of Bergen. "In the current study, we close the circle by mechanistically linking these clinical findings. Contrary to our expectations, a digestive enzyme normally destined for the gut was instead misled to enter the pancreatic islet in the diseased state, ultimately compromising insulin secretion."

Today, people with MODY8 are treated with insulin or oral diabetes drugs. Kulkarni and his colleagues will look for ways to design more tailored and personalized therapeutics. "For example, can we dissolve these protein aggregates, or limit their aggregation in the beta-cell?" he said. "We can take cues from what has been learned in other diseases like Alzheimer's disease and Parkinson's disease that have a similar aggregation mechanism in the cells."

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Joslin's Ercument Dirice, Giorgio Basile, Danielle Diegisser, Manoj Gupta and Jiang Hu contributed to the work. Other co-authors include Jahedul Alam and Bente Johansson of the University of Bergen, Ling Huang and Senthil Muthuswamy of Beth Israel Deaconess Medical Center, and Chew-Li Soh and Danwei Huangfu of Memorial Sloan Kettering Cancer Center. Lead funding came from the National Institutes of Health.