Huntington Disease Pathology Unfolds

Courtesy: Larry Marsh and Judit Pallos IN A FLY'S EYE: The regular repeating structure of the Drosophila melanogaster compound eye is disrupted when polyglutamine (polyQ 108) is expressed, making the fly eye an excellent model for huntingtin-derived pathology. Dissecting the mechanism of neuronal demise in Huntington disease (HD) has had its share of twists and turns. It took a decade to go from marker1 to gene,2 and now, after another decade, the details are beginning to come together. Three papers published in 2001 focus on two key roles for the pathological form of huntingtin protein--interfering with transcription3,4 and plugging up proteasomes, the cellular trash compactors for misfolded proteins.5 While different, these hypotheses are not mutually exclusive, and may point to multiple effects that inevitably cause the devastating symptoms of HD. Data derived from the Science Watch/Hot Papers database and the Web of Science (ISI, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age. F. Nucifora et al., "Interference by huntingtin and atrophin-1 with CBP-mediated transcription leading to cellular toxicity," Science, 291:2423-8, March 23, 2001. (Cited in 142 papers) J. Steffan et al., "Histone deacetylase inhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila," Nature, 413:739-43, Oct. 18, 2001. (Cited in 70 papers) N. Bence et al., "Impairment of the ubiquitin-proteasome system by protein aggregation," Science, 292:1552-5, May 25, 2001. (Cited in 114 papers) In the late 1800s, Long Island physician George Huntington lent his name to this disorder, which is characterized by uncontrollable, dancelike movements and personality changes.6 After a long illness, individuals with HD die from complications such as choking or infection. The genetic marker discovered in 1983 had the uninspiring name G8 and the gene discovered 10 years later had the equally ineloquent IT15, for "interesting transcript 15." HD was one of the first expanded triplet repeat disorders identified, caused by extra glutamines at huntingtin's amino end. Normal huntingtin has 34 glutamine repeats and is likely antiapoptotic. Mutant proteins with more than 40 repeats cause HD. Wild-type huntingtin is cleaved and stays in the cytoplasm. Elongated, it invades the nucleus, where its effects are dual. "Mutant huntingtin enters the nucleus through pores, losing its antiapoptotic function and generating toxic products. So HD is both a gain of function through the generation of polyglutamine fragments, and a loss of antiapoptotic function," explains Michael Hayden, a professor of medical genetics at the University of British Columbia. Abnormally extended huntingtin may disable transcription factors as well as clog proteasomes. "It is becoming increasingly challenging to make sense of it all, to see all the effects of polyglutamine and to identify those important to the disease mechanism, as opposed to the so-called epiphenomena that just happen," says Kenneth Fischbeck, director of the neurogenetics division at the National Institute of Neurological Disorders and Stroke. HD may reflect an unfortunate pairing: polyglutamine is more likely to aggregate than other amino acids, and neurons are more likely to feel the effects than other cells.7,8 "Neurons appear to be exquisitely sensitive to expanded stretches of polyglutamines. This may be because they no longer divide, they have different sets of proteins expressed, and as they age, their ability to deal with cellular insults decreases," explains Larry Marsh, professor of developmental and cell biology at the University of California, Irvine.4 Adds co-author and colleague Joan Steffan, associate professor of psychiatry and human behavior, "Neurons have a different set of transcription factors expressed than other cells, and these transcription factors are more sensitive to interference by polyglutamine-containing proteins." TRANSCRIPTIONAL DYSREGULATION Two of the Hot Papers3,4 address the interaction between huntingtin and a transcription factor, CREB binding protein (CBP), which includes an 18-glutamine stretch. Abnormal huntingtin binds CBP, hampering its ability to turn on transcription of other genes. But the teams identified different places where the proteins meet. Courtesy: Larry Marsh and Judit Pallos SHEDDING LIGHT: Huntingtin inclusions result in an accumulation of green fluorescent protein. Johns Hopkins' Christopher Ross, Ted Dawson, and Frederick Nucifora looked at cells cultured from transgenic mice and people who had died of HD.3 They found that mutant huntingtin binds only CBP if the 18 glutamines are present, suggesting that the polyglutamine tracts attract. They also showed that excess CBP can dilute mutant huntingtin's effect. Crippling a transcription factor can have targeted and broad effects, and this is why the paper is "hot," says Nucifora, a graduate student at the time. The paper describes an HD pathogenesis mechanism, and has implications for other neurodegenerative diseases with protein aggregation and inclusion bodies, such as Parkinson disease and Alzheimer disease. "This suggests that aggregates of the disease protein are not directly toxic, but can provide a downstream marker of other pathogenic events, such as redistribution of critical proteins," he adds. In contrast, the second Hot Paper4 used cell-free assays, mammalian cells in culture, and Drosophila to show that the interaction point on CBP is the active site for its acetyltransferase activity, not its poly- glutamines. CBP turns on transcription by adding acetyl groups to histones, an action countered by deacetylases. A drug that inhibits histone deacetylase overcame the effects of mutant huntingtin, implicating the acetylation active site. Both teams suspect that mutant huntingtin can grab CBP at either site, perhaps depending on circumstances. Ross, a professor of psychiatry and neuroscience, suggests that binding the acetylase site disrupts a normal function, whereas binding the 18 polyglutamines causes abnormal function. Leslie Thompson, a professor of psychiatry and human behavior at UC-Irvine and coauthor,4 agrees that "expanded polyglutamine repeats may attract like Velcro, gumming up the works in a way that does not have anything to do with the normal role of the polyglutamines in transcription factors." But her team also attributes the groups' differing results on technique. "In Nucifora's data, other proteins may act as bridges in the cellular interactions between CBP and huntingtin that lead to colocalization or coimmunoprecipitation." The in vitro interactions, she says, may involve direct interaction between two soluble proteins. BLOCKING THE PROTEASOME Mutant huntingtin also disrupts the ubiquitin-proteasome system, the cell's plumbing. Explains Ron Kopito, a professor of biological sciences at Stanford University, and coauthor5 with Neil Bence and Roopal Sampat, "In protein folding, a partly structured intermediate undergoes a structural transition to a stable state. There are multiple trajectories and many intermediates on a pathway to a folded native molecule." The ubiquitin-proteasome system seeks and destroys misfolded proteins that have been polyubiquitinated. "The protein then unfolds at a proteasome particle, and is fed into it," Kopito explains. But, even the most sophisticated garbage disposals can get clogged. The protein-aceous spheroids of amyotrophic lateral sclerosis, the tangles of Alzheimer disease, and the Lewy bodies of Parkinson disease, can block it. Mutant huntingtin might do the same. To measure proteasomal function, they linked green fluorescent protein (GFP) to a destabilizing peptide. If the proteasome works, then the signal vanishes in a half-hour as the protein is processed. But if the proteasome is clogged, like a backed-up sewer, the fluorescence persists. The researchers attached the destabilized reporter to two versions of the first exon of huntingtin, one with the below-threshold 25 glutamines, the other with 103. The shorter segment cleared the proteasomes with ease, quickly quenching the green flash. But the longer molecules became stuck, producing what Bence calls "a frustrated proteasome. As the proteasomes clog, other misshapen proteins accumulate. IMPLICATIONS AND APPLICATIONS Researchers want to reconcile the roles of mutant huntingtin as transcription dysregulator and proteasome destroyer. "Probably both processes are occurring simultaneously, and either would kill the cell. The combination may accelerate cell death," says Dawson, a professor of neurology and neuroscience. Adds Ross, "I think the proteasome effect is separate, and could be a mechanism shared by all protein misfolding diseases." The success of histone deacetylase inhibitors in saving the eyesight and lives of flies led to a similar demonstration in mice.9 The drug SAHA (suberoylanilide hydroxamic acid) dramatically improved their motor skills. Gillian Bates, professor of neurogenetics, medical, and molecular genetics at King's College London, notes, "We carried out the mouse trial as a direct result of the fly work. This is the first time that a compound/drug has been used in cell culture, yeast, and fly systems and then gone on to work in mice. It partly validates the pipeline of models for developing therapeutics." Ricki Lewis (rickilewis@nasw.org) is a freelance writer in Scotia, NY. References 1. J. F. Gusella et al., "A polymorphic DNA marker genetically linked to Huntington's disease," Nature, 306:234-8, 1983. 2. The Huntington's Disease Collaborative Research Group, "A novel gene containing a trinucleotide repeat that is unstable in Huntington's disease chromosomes," Cell, 72:971-83, 1993. 3. F. Nucifora et al., "Interference by huntingtin and atrophin-1 with CBP-mediated transcription leading to cellular toxicity," Science, 291:2423-8, March 23, 2001. (Cited in 142 papers) 4. J. Steffan et al., "Histone deacetylase inhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila," Nature, 413:739-43, Oct. 18, 2001. (Cited in 70 papers) 5. N. Bence et al., "Impairment of the ubiquitin- proteasome system by protein aggregation," Science, 292:1552-5, 2001. (Cited in 143 papers) 6. G. W. Huntington, "On chorea," Medical Surgical Reporter, 26:317-21, 1872. 7. M.F. Perutz et al., "Glutamine repeats as polar zippers: their possible role in inherited neurodegenerative diseases," Proc Natl Acad Sci, 91:5355-8, 1994. 8. A. Kazantsev et al, "Insoluble detergent-resistant aggregates form between pathological and nonpathological lengths of polyglutamine in mammalian cells," Proc Natl Acad Sci, 96:11404-9, 1999. 9. E. Hockly et al., "Suberoylanilide hydroxamic acid, a histone deacetylase inhibitor, ameliorates motor deficits in a mouse model of Huntington's disease," Proc Natl Acad Sci, 100:2041-6, Feb. 18, 2003. function sendData() { document.frm.pathName.value = location.pathname; result = false if (document.frm.score[0].checked) result = true; if (document.frm.score[1].checked) result = true; if (document.frm.score[2].checked) result = true; if (document.frm.score[3].checked) result = true; if (document.frm.score[4].checked) result = true; if (!result) alert("Please select") return result; } .myradio{background-color : #94bee5} Please indicate on a 1 - 5 scale how strongly you would recommend this article to your colleagues? Not recommended 1 2 3 4 5 Highly recommended Please register your vote

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