Some amyloid-β accumulates in mitochondria, where it has been blamed for all manner of malfunctions, from respiration problems to wonky organelle morphology. How could one small peptide be responsible for so much mayhem? Researchers think they may have uncovered a mechanism. Aβ throws a wrench into the assembly line that produces mature mitochondrial proteins, according to a paper in the August 28 Cell Metabolism. Researchers led by Chris Meisinger at the University of Freiburg, Germany, reported that Aβ retards this coming-of-age process, triggering the destabilization of several key mitochondrial proteins.
“There is substantial evidence for mitochondrial deficits in Alzheimer’s disease; however, the reasons have been obscure,” commented Flint Beal of Weill Cornell Medical College in New York. “This paper presents a potential mechanism.”
The vast majority of proteins in the mitochondria are cytosolic emigrants. Most flash an N-terminal passport sequence to hitch a ride on the organelle’s import machinery (see Vögtle et al., 2009). Once inside, the proteins surrender this presequence to mitochondrial processing peptidase (MPP), which releases mature, stable proteins that tackle various jobs. A metalloprotease called PreP then degrades the remnant presequence peptides.
Protein Interrupted. Normally, immature mitochondrial proteins carrying N-terminal presequences are trimmed by MPP, then Cym1 disposes of the clippings (left panel). In AD (right), Aβ seems to derail this process by blocking Cym1, leading to a presequence build-up that blocks maturation of preproteins. [Image courtesy of Mossmann et al., Cell Metabolism, 2014.]
Meisinger and colleagues had previously noticed that immature preproteins accumulated in yeast cells lacking PreP activity. Interestingly, researchers have reported that PreP also degrades Aβ in mitochondria, and that the activity of the protease is weak in AD patients (see Jul 2006 news story and Alikhani et al., 2011). Meisinger and colleagues wondered whether degradation of presequence peptides was necessary for the protein maturation machinery to work, and if so, whether interference from Aβ could foul up the process.
To address this question, co-first authors Dirk Mossmann and F.-Nora Vögtle started by generating yeast mutants that lacked the PreP homolog, Cym1. Mass spectroscopic analysis revealed an increase in immature preproteins in the mutants’ mitochondria. The functions of those proteins ran the gamut from ATP synthesis to maintenance of mitochondrial DNA, and oxidative stress response. In a cell-free system using mitochondrial extracts spiked with a known presequence peptide from the mitochondrial protein cytochrome oxidase 4 (Cox4), the researchers determined that the peptides failed to be degraded in extracts from Cym1-deficient yeast. These peptides, in turn, inhibited MPP’s ability to cleave preproteins. Interestingly, the yeast lacking Cym1 displayed many of the mitochondrial dysfunctions observed in people with AD, including boosted levels of reactive oxygen species (ROS), reduced oxygen consumption, and decreased membrane potential.
Because Aβ accumulates in AD mitochondria and is a known substrate for PreP (Cym1), the researchers tested whether an abundance of Aβ could thwart protein maturation in yeast mitochondria. In a cell-free system, the researchers found that Aβ was indeed degraded by Cym1. However, the peptide blocked the efficient degradation of the Cox4 preseqence peptide, indicating that Aβ may derail the turnover of presequence peptides, which in turn slows preprotein maturation. To confirm this in whole cells, they generated a yeast strain that produces Aβ peptides driven by the galactose promoter, and found that after three days on galactose, several immature proteins had accumulated. After five days, signs of mitochondrial dysfunction started to appear, including a 30 percent bump in ROS production and a dip in both membrane potential and oxygen consumption to 80 percent of wild-type levels.
The researchers next searched for hints of the same mechanism in human tissue. Using a combination of western blotting and mass spectrometry, they compared mitochondrial protein maturation in postmortem brain samples from four AD patients to that of four controls. Mossmann and Vögtle identified an abundance of several immature preproteins in AD patients, including the mitochondrial ribosomal subunit MRPL23, respiratory chain protein NDUFA9, several enzymes in the citric acid cycle, and MPP itself.
The human data suggests that this mechanism could be relevant in for AD, commented Xiongwei Zhu of Case Western Reserve University in Cleveland. However, he added that the impairment appeared to be limited to just several mitochondrial proteins, and wondered whether this apparent selectivity would be sufficient to cause the broad mitochondrial defects observed in AD.
Meisinger suggested that the maturation impairment probably was broader than reported in this initial paper. Because the researchers had access to limited amounts of each postmortem sample, identifying preproteins was difficult, especially since such preproteins are known to be rapidly degraded. He believes that many more proteins may have been affected. If the level of each mitochondrial preprotein increased by just a few percent, it could impair total mitochondrial proteostasis, he wrote in an email to Alzforum.
Russell Swerdlow of the University of Kansas in Kansas City praised the study. Swerdlow is a proponent of a mitochondrial hypothesis of Alzheimer’s disease (Swerdlow et al., 2014). He added that the findings stimulate more questions. “The authors do not discuss data that indicate mitochondrial function in AD subjects is also altered outside the brain, for example in platelets and fibroblasts,” Swerdlow said. “Does Aβ also account for that biochemical phenomenon?”
Meisinger told Alzforum that answering this question is a major focus of his lab. “Our main interest at the moment is to test if we can find immature mitochondrial proteins in blood cells from AD patients,” he said. If so, he predicted that such preproteins could eventually become useful diagnostics.—Jessica Shugart
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- Mossmann D, Meisinger C, Vögtle FN. Processing of mitochondrial presequences. Biochim Biophys Acta. 2012 Sep-Oct;1819(9-10):1098-106. Epub 2011 Dec 7 PubMed.
- Morris JK, Honea RA, Vidoni ED, Swerdlow RH, Burns JM. Is Alzheimer's disease a systemic disease?. Biochim Biophys Acta. 2014 Sep;1842(9):1340-9. Epub 2014 Apr 18 PubMed.
- Mossmann D, Vögtle FN, Taskin AA, Teixeira PF, Ring J, Burkhart JM, Burger N, Pinho CM, Tadic J, Loreth D, Graff C, Metzger F, Sickmann A, Kretz O, Wiedemann N, Zahedi RP, Madeo F, Glaser E, Meisinger C. Amyloid-β Peptide Induces Mitochondrial Dysfunction by Inhibition of Preprotein Maturation. Cell Metab. 2014 Aug 27; PubMed.