15 September 2004. Mitochondria, those bags of energy once revered for keeping us all respiring, have come in for some bad press since they were discovered to initiate programmed cell death in addition to spewing out toxic reactive oxygen species (ROS). But the heat may be turned off somewhat as evidence grows that another organelle, the endoplasmic reticulum (ER), may be responsible for a similar brand of cellular destruction. In the September 10 Molecular Cell, Antony Cooper and colleagues from the University of Missouri, Kansas City, report that in yeast the unfolded protein response (UPR), initiated in the ER in reaction to the presence of misfolded proteins, also contributes to cell death by generating ROS.
Misfolded proteins are, of course, a hallmark of many neurodegenerative disorders, including Alzheimer disease. The UPR, which leads to expression of a variety of proteins that aid in the folding and trafficking of proteins through the organelle, also stimulates the ER-associated degradation system, or ERAD, which eliminates the miscreants. This combined response is normally adequate; however, the ERAD can become overwhelmed, as may be the case in Huntington disease (see the report of Martin Duennwald’s presentation at FASEB Summer Research Conferences on “Protein Misfolding, Amyloid and Conformational Disease). Does this mean that failure of the ERAD has wider consequences?
To answer this, first author Cole Haynes and colleagues eliminated one of the major ERAD proteins in yeast, ERV29. Cells deficient in this gene began to die soon after overexpressing a model misfolding protein, a mutant carboxypeptidase Y, even though the UPR was stimulated about fivefold. Significantly, the dying cells had many characteristics of cells undergoing apoptosis, including DNA fragmentation and the appearance of the programmed cell death marker phosphatidylserine on the outside of the plasma membrane. (This lipid is normally found inside the cell membrane.)
To test the role of the UPR in this cell death, Haynes removed genes for either Ire1p or Hac1p, signaling proteins that mediate the UPR response. Repeating the same experiments, the authors found that misfolded carboxypeptidase now failed to initiate fragmentation of DNA, indicating that full transduction of the UPR signal was necessary for DNA damage to occur.
So how might the UPR lead to fragmentation of DNA and cell death? Recently it was shown that the ER can generate reactive oxygen species as a side product of oxidative protein folding (see Harding et al., 2003). When the authors examined ERV29-negative cells overexpressing the carboxypeptidase, they found ROS accumulations, as judged by ROS-sensitive fluorescent dyes. Significantly, in cells void of Ire1p or Hac1p, and which cannot fully activate the UPR, no ROS accumulations were found.
“Our results support a model in which the ultimate phenotype of sustained, severe ER stress is cell death due to prolonged UPR activation and the subsequent accumulation of ROS,” state the authors.
In the ER, where protein thiols must often be oxidized to disulfide bonds to ensure proper protein folding, reducing equivalents can be ultimately transferred to molecular oxygen through the oxidation/reduction proteins Pd1 and Ero1. The authors suggest that when protein folding goes awry, repeated formation and reduction of the wrong disulfide bonds can result in a futile redox cycle that consumes glutathione (GSH), a major cellular reductant, and generates toxic levels of ROS. In support of this, they found that removing all the cysteine amino acids from carboxypeptidase stimulated growth in cells overexpressing this mutant.
“Persistent ER stress, GSH depletion, ROS accumulation, and cell death are also associated with a number of age-related neurological diseases including Alzheimer’s, Huntington’s, Parkinson’s and the prion-based diseases. Our data suggest a mechanism by which persistent ER stress and prolonged UPR activation can account for some of these phenotypes,” write the authors.—Tom Fagan.
Haynes CM, Titus EA, Cooper AA. Degradation of misfolded proteins prevents ER-derived oxidative stress and cell death. Mol. Cell. 2004. September 10; 15:767-776. Abstract