Tool to Measure ER Stress in Cells Promises Diabetes Insight

Feroz Papa

In the current online edition of the journal Cell, UCSF endocrinologist and scientist Feroz Papa, MD, PhD, reports on a way to accurately measure a cellular phenomenon that may play an important role in the development of diabetes. The work focuses within the cell on the protein-folding factory called the endoplasmic reticulum, or ER. Papa believes that during the gradual development of type 2 diabetes, the stress of processing large amounts of insulin within the ER compartments of beta cells can stress the ER. Using new techniques, Papa is sorting out the molecular details of ER stress and cell death. He believes that the concept of stress in the ER needs to be better defined. To address this, his lab has developed new molecular gauges to measure ER stress. It turns out that cells have a protective mechanism. It’s called the unfolded protein response (UPR). The UPR kicks into gear when the ER is overtaxed. However, it is unclear whether the UPR actually causes cells to revert back to their healthy, pre-stressed states. The ER lies deep within the cell. There has been no way to directly observe its inner workings in living cells. Therefore, it was not possible to determine whether the UPR had effectively done its job. “The endoplasmic reticulum of a cell is equipped to deal with emergencies, like the emergency room of a hospital,” says Papa. “Coincidentally, both even have the same abbreviation — ER. In the case of the cell’s ER, challenges to protein folding are addressed by molecules called chaperones. These chaperones act as triage nurses and doctors. They isolate sick, unfolded proteins and attempt to revive them. “But because it is difficult to peer into the ER of a cell with existing tools to ask how it is faring under stress, researchers have simply said that the ER must be stressed whenever the corrective measures are activated. This is a bit like trying to figure out what is going inside the ER at San Francisco General Hospital by just listening to the siren wails of ambulances as they arrive. “Sure, there are lots of ambulances around, but are the doctors and nurses inside able to effectively cope with the emergencies they are facing? Without ways to look inside the ER, we really wouldn’t know what’s going on inside.” To monitor ER stress, Papa and colleagues dispatch green fluorescent proteins (GFPs) into the ER compartments of living cells. There, the GFPs report on a vital aspect of protein folding — protein oxidation. Papa and colleagues have discovered that protein oxidation becomes compromised during states of ER stress. The GFPs, by emitting light in different spectra, can report back on the state of protein oxidation and folding within the ER in real time. Researchers can thereby determine whether the ER is coping or whether it is failing under the burden of having to fold too much protein. The researchers then use red fluorescent proteins to track the unfolded protein response, also in real time. By simultaneously measuring fluorescence from both red and green proteins within the same cells, Papa’s lab team can answer the question of whether this corrective response actually is effective in reducing ER stress within individual cells. Papa thinks such gauges of ER physiology may eventually be useful to relate the level of ER stress to the progression of beta cell death in type 2 diabetes. The ER of a pancreatic beta cell under continuous stress sends out death signals that result in the suicide of the stressed cell, Papa explains. “It takes many years before enough beta cells die so that an individual becomes diabetic,” Papa says. “But why do the beta cells of some obese individuals cope, while the beta cells of others die?” Papa believes that when a tipping point of ER stress is exceeded, the UPR kicks into suicide mode in some cells. Indeed, the ER gauges accurately revealed previously undetected stress differences in individual cells within large populations of cells. The tipping point tolerated by the UPR may vary in different people, according to Papa. The new gauges for ER stress may help point out such differences between individuals, and could identify new targets against which to develop antidiabetic drugs, Papa says. The research report will be featured as the cover article in the Nov. 28, 2008, print edition of Cell. In 2007, Papa received a National Institutes of Health Director’s New Innovator Award to advance this research. He was one of just 29 recipients out of more than 2,100 applicants.