|
||||||||
|
NEUROLOGY Enzyme Linked to Pathology of Parkinson's Disease Appears Two-facedProtein's Reckless Side May Lead to Deadly Pileups in the Cell An enzyme implicated in Parkinson's disease performs two opposing actions related to protein degradation, according to a study in the Oct. 18 Cell. The laboratory of Peter Lansbury found that the enzyme, UCH-L1, exhibits an activity in vitro that counteracts its previously known function. They believe that UCH-L1's newly uncovered flip side may be the real pathological culprit, allowing proteins to clog the normal path of degradation. The finding adds a new wrinkle to the story of Parkinson's pathology and insight into how failure to dispose of proteins can wreak havoc on a cell.
From left to right, Peter Lansbury, Yichin Liu, Hilal Lashuel, and Zhihua Liu uncovered a dual role for UCH-L1 that explains why a genetic variant of the enzyme may be protective against Parkinson's disease. (Photo by Graham Ramsay) Much research has focused on how the aggregated proteins found in neurodegenerative diseases like Parkinson's are produced, overproduced, folded, and eventually clumped together. But a defect in protein degradation could also lead to a pathogenic traffic jam in the cell. "The idea is that the concentration of these proteins needs to be kept below some threshold in order to avoid aggregation," said Lansbury, HMS associate professor of neurology at Brigham and Women's Hospital. Roles in RecyclingWhile heredity seems to play a minor role in Parkinson's, the three mutated genes that have been identified in familial cases support a role for protein degradation in the disease. Those genes encode alpha-synuclein, parkin, and UCH-L1; alpha-synuclein is the protein that accumulates in the dopamine-carrying neurons of Parkinson's patients, and the latter two proteins are thought to have a role in degrading the first.Lansbury's team, led by research fellow Yichin Liu, decided to pursue UCH-L1 because occasional experiments with the enzyme did not seem to go as planned. Normally, strings of ubiquitin molecules are attached to proteins to signal which ones will get canned--chewed up into amino acids by the proteasome and recycled for later use. UCH-L1's function in humans is not known, but in vitro studies suggested it helps cut the ubiquitin strings from peptide products of the proteasome so they can be broken down into amino acids. But when the team added alpha-synuclein with ubiquitin strings attached to purified UCH-L1 in vitro, the reaction actually added further ubiquitins onto the protein rather than chopping them off. Meanwhile, conflicting genetic evidence supported a dual personality for the protein. The original mutation that implicated UCH-L1 in Parkinson's was a point mutation shared by the siblings of one family. It was assumed that the mutation interfered with the enzyme's ability to cut ubiquitin, thereby allowing alpha-synuclein to pile up. Further searching, however, did not uncover more cases of this mutation--instead, a common polymorphism was discovered elsewhere in the gene. Studies of populations of Parkinson's patients vs. controls showed that this polymorphism was associated with decreased likelihood of acquiring Parkinson's. Many scientists assumed that the polymorphic variant, called S18Y, was simply more efficient at cleaving ubiquitin, but Liu and her colleagues found this was not the case: both the variant and the wild type neuronal protein can remove ubiquitin in vitro equally well. Lansbury's team believes it has found an answer to this puzzle: UCH-L1 has the ability to both unravel ubiquitin strings and knit them together. It is this knitting action that S18Y lacks, an action that is potentially pathogenic. Disease LinksBy studying the behavior of the enzyme in nerve cells and in cell-free systems, the team found that the S18Y version of UCH-L1 had a reduced ability to link ubiquitin molecules together. To make an in vitro model of how the polymorphic variant might influence Parkinson's susceptibility, the team compared mixtures of UCH-L1 variants that would mimic possible human genotypes: all S18Y, half S18Y and half wild type, and all wild type. The ubiquitin-cleaving activity was constant, but the ability to create strings of ubiquitin increased with the proportion of wild type, tracking the increase in Parkinson's risk. "We believe that the polymorphism may be silent throughout life," but would appear under certain conditions like old age, Lansbury said.At the same time, both the wild type form of the protein and the form with the disease-causing point mutation tend to stick together in pairs, but S18Y did not. The team believes that molecular pairing is the mechanism that allows UCH-L1 to form ubiquitin strings, essentially functioning as a switch from one activity to the other. What triggers this dimerization is still a mystery. Lansbury suspects that factors currently thought to cause protein aggregation in neurodegenerative disease might also have an effect here. Ole Isacson, HMS professor of neurology at McLean Hospital, said that the study fits in with a growing recognition that the disturbances in the proteasome pathway contribute to protein aggregation in Parkinson's. "I think it fits with our data," he said, adding that even in cases of Parkinson's in which heredity does not come into play "we find that the proteasome is very compromised." The behavior of the protein in vitro does not yet prove its in vivo role. "What we'd like to do is essentially knock out one of the activities," Lansbury said. The group is taking advantage of the drug discovery laboratory at the Harvard Center for Neurodegeneration and Repair and is beginning screens for small molecules that could disrupt one of the enzyme's functions while leaving the other intact. The case of this duplicitous enzyme illustrates the advantage of this more refined approach: a more traditional method--knocking out the gene in an animal model--would not allow study of both functions separately. --Courtney Humphries |
