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NOBEL PRIZE WINNER AARON CIECHANOVER
HOW PROTEIN DISPOSAL MECHANISM UNITES NEURODEGENERATIVE DISEASES

NEW YORK CITY—In cities and towns across the nation, we take our garbage collectors for granted – they stop by late at night or early in the morning when most of us are asleep, and most of us have little idea where they take their full trucks.

The same disregard for cellular garbage existed for many years among researchers who studied proteins when Aaron Ciechanover, PhD, began working on the problem as a graduate student in the late 1970s, he told a crowded auditorium here in a lecture at Columbia University in April. But the fundamental biochemistry of cellular garbage disposal is now bringing new insights to a host of neurological diseases and may lead to novel therapies.

Last October, Dr. Ciechanover and Avram Hershko, MD PhD, both from Technion University in Haifa, Israel, and Irwin Rose, PhD, of the University of California-Irvine, were awarded the Nobel Prize in Chemistry for their discovery 30 years ago of ubiquitin-mediated protein degradation – a mechanism found in every living cell, targeting damaged or used proteins, but not the active ones.

A cell achieves a delicate state of equilibrium by existing in a constant state of flux, synthesizing and destroying proteins at a hefty turnover rate of 5 percent per day. On top of simply removing cellular trash, the ubiquitin system acts as a control gauge for crucial cell processes such as cell division, DNA repair, and aspects of the immune system by rapidly clearing away specific proteins. “Destruction is a form of regulation,” Dr. Ciechanover explained.

UBIQUITIN: TWO-STAGE PROCESS

Protein destruction via ubiquitin is a two-stage process. First, ubiquitin molecules, which act as tagging units, bind to the target proteins forming a chain – a process that biochemists have dramatically termed “the kiss of death.” This poly-ubiquitin chain is then recognized by a molecular complex called the proteasome – essentially a mill that breaks down the tagged protein into short segments of amino acids that can then be recycled for further protein synthesis.

“It's like a court process,” Dr. Ciechanover explained. “Initially, you indict the person for wrong-doing, or in this case malfunctioning, and then you execute him.”

In the years since its discovery, the ubiquitin-mediated pathway has become one of the biggest fields in biology. Researchers studying proteins involved in everything from cancer to cystic fibrosis to Down syndrome simply began bumping into it, Dr. Ciechanover told Neurology Today. “People didn't walk into the ubiquitin field because they chose to work on degradation. They worked on their own proteins, but each one discovered that their favorite protein was short-lived.”

A COMMON DISEASE PATHOLOGY

Seven years ago, scientists first linked the neurodegenerative condition spinocerebellar ataxia to an inability of the ubiquitin system to clear a mutated protein called ataxin-1. Since then, associations between the ubiquitin system and disease pathology have either been shown or suspected in a long list of neurodegenerative disorders that includes Parkinson disease, Alzheimer disease, Huntington disease, amyotrophic lateral sclerosis, and prion diseases. These conditions have varying – and in many cases poorly understood – etiologies, but what unites them, is that they all show accumulation of clumps in the CNS made up of misfolded proteins, a pattern suggesting problems in the disposal of intracellular garbage.

Figure

The molecule that would later prove to be the label that marks out a protein for degradation was isolated as early as 1975. This 76-amino-acid-long polypeptide was isolated from calf sweetbread and was assumed to participate in the maturation of white blood cells. Since the molecule was subsequently found in numerous different tissues and organisms – but not in bacteria – it was given the name ubiquitin.

Although some commonality was long suspected, “it is now known what the similarity is,” says Patrik Brundin, MD, PhD, Professor of Neuroscience at Lund University in Sweden. “I would say that's a major paradigm shift.”

What's trickier still is determining ubiquitin's role in disease pathology. Is the cell's machinery for disposing of these proteins broken, or does the problem lie in the proteins themselves, with a still-unknown deficit causing them to accumulate and to gum up the degradation system?

Differentiating these two possibilities would shed much-needed light on the pathology of these diseases, says Huda Y. Zoghbi, MD, a Howard Hughes Medical Institute Investigator in Molecular Genetics at the Baylor College of Medicine in Houston. Her team bred mice with spinocerebellar ataxia with a special mutant in which ubiquitin molecules are labeled with a fluorescent marker, allowing researchers to observe when and how the misfolded proteins accumulate.

They found that the mice developed symptoms of the disease long before problems in the ubiquitin pathway were evident. “That is good news,” Dr. Zoghbi said. Since the machinery seems to be intact, at least early in the disease, “it means if we can just reduce the protein load we can help these patients.”

But others have found evidence for the opposite hypothesis. “You look at isolated studies, and they're all good,” says George N. DeMartino, PhD, Professor of Physiology at the Southwestern Medical Center at the University of Texas, who collaborated with Dr. Zoghbi on the study. “But you get these contradicting theories.”

THE PATH TO NEW THERAPIES

A clearer understanding of ubiquitin's role in neurodegenerative diseases will undoubtedly lead to new medicines, researchers say. “We need to look at the ubiquitin system not only as a problem, but as an opportunity,” said Nico P. Dantuma, PhD, Senior Scientist in Cell and Molecular Biology at the Karolinska Institute in Sweden. “Even if it's not the bad guy, we can still target it therapeutically to make it more efficient.”

Already, biochemistry has yielded one powerful new medicine called bortezomib (Velcade), which was approved by the US Food and Drug Administration in 2003 to treat multiple myeloma, a lymphoproliferative disease. Bortezomib inhibits the proteasome and probably kills cancer cells indirectly, by preventing the breakdown of proteins that suppress the cancer cells' growth.

But bortezomib highlights two major challenges in developing treatments based on the ubiquitin-mediated pathway. First, Dr. Ciechanover told Neurology Today, developing ubiquitin system-based drugs to treat neurological diseases is simply a more difficult pharmaceutical problem than developing drugs that treat cancer. Administering a small molecule like bortezomib is the biochemical equivalent of throwing a spanner in the works – it slows down the cell's normal degradation process by physically blocking enzymes from interacting with each other. But it is not clear what a drug would have to do to get the process to speed up.

Second, drugs that interfere with such a ubiquitous biochemical process are likely to have strong side effects. In fact, researchers have been surprised that bortezomib works so well, and point out that the long-term side effects are still unknown. “There's nobody around who has used [it] for more than a few years,” said Dr. DeMartino.

It may possible to rev the system up just a little bit, not enough to be toxic, says Dr. Zoghbi, but more disease-specific approaches will also needed – and that will require a better understanding of the basic biology of each one. “In Parkinson disease, we don't even know what protein is accumulating,” she said.

Meanwhile, researchers in industry and academia are screening for drugs that will act not on the ubiquitin system, but on the misfolded proteins themselves. Molecules called chaperones, which help proteins take their correct three-dimensional shape, are the most promising candidates. “In cell culture, if you over-express certain kinds of chaperones, you really can prevent aggregation,” says Dr. DeMartino. It may be possible to find a molecule that boosts their production or to synthesize molecules, which mimic the way they lend a helping hand.

REFERENCE

• Ciechanover A. The ubiquitin-proteasome pathway: on protein death and cell life. EMBO J 1998;17:7151–1760.