The Search for Secretases

Alzheimer's disease (AD) researchers at Amgen Inc. of Thousand Oaks, Calif., appear to be the first winners in the race to find secretases, enzymes believed critical to AD onset and, potentially, to AD treatment. Considered the most promising potential drug targets for the disease, two secretases, dubbed gamma and ß, have pivotal roles in generating the neuron-suffocating plaques in the brains of AD sufferers. They work by cleaving amyloid precursor proteins (APPs) to produce amyloid ß(Aß), the peptides that accumulate to form plaques. A third enzyme, alpha-secretase, actually cuts through the middle of Aß, preventing amyloid peptide formation--thus making its inhibition an unattractive drug target. Since around 1990, researchers have had a pretty good idea of where these enzymes reside, what they look like, and how they operate, but they couldn't actually identify and isolate them. The race to do so began a couple of years later with the advent of improved cell culture models. Several labs had reported that Aß is not only generated in the brain cells of AD patients, but in all persons at some low level, and everywhere in the body. As a result, researchers were able to establish cell culture models that continually generate A ß, greatly facilitating the secretase search. But not until a paper in the October 22 issue of Science had there been proof of the cloning of a secretase.1 Aided by the considerable resources rarely afforded academic labs, Amgen investigators engaged in a labor-intensive expression cloning strategy that yielded a ß-secretase candidate dubbed BACE (ß-site APP-cleaving enzyme). "People have been worried about taking that strategy ... because it's a huge amount of work," says Alzheimer's researcher John Hardy, a professor of pharmacology at the Mayo Clinic in Jacksonville, Fla. Most secretase researchers have, without success, sought out the secretases using biochemical purification. "We decided to go for a strategy in which we go from the gene to the activity, rather than from the activity to the gene," explains senior author Martin Citron, an Amgen researcher. "That's essentially why we've succeeded." "There's no question that they've got a ß-secretase," says Hardy. "It's an extremely impressive piece of work, and their data is completely convincing." Hardy, whose group researches both gamma- and ß-secretases, adds that the search for the enzyme might not yet be over since there could be more than one ß-secretase. Gamma secretase researcher Charles Glabe, a professor of molecular biology and biochemistry at the University of California, Irvine, calls the Amgen research a "breakthrough," but cautions that there could be other pathways that circumvent BACE to produce amyloid. Also a possibility: ß-secretase (and, if found, gamma-secretase) may have critical activities outside of the production of Aß, so that decreasing their activity would have unwanted side effects. But regardless, says Glabe, finding an inhibitor of BACE, even if ineffective, will tell researchers something very important about the pathogenesis of the disease. Indeed, the stage might be set for a whole new race. Allen Roses, vice president and worldwide director of genetics at the United Kingdom-based Glaxo Wellcome PLC, suggests that other companies could purify--or may have already purified--the enzyme, and that if so, "the race begins now ... to get it into the clinic as fast as possible." Getting an inhibitor is certainly the first priority for Amgen, according to Citron. As would be the case with most drug companies, Amgen will skip the obvious time-consuming follow-up study--namely, demonstrating A ß reduction in a mouse knockout model lacking BACE--and get right into the drug hunt. As for gamma-secretase candidates, the search continues. The only real candidate right now is presenilin-1 (PS1), a protein already implicated in the biochemical pathway that leads to AD. PS1's biggest advocate is Alzheimer's researcher Dennis Selkoe, a professor of neurology and neuroscience at Harvard Medical School and Brigham and Women's Hospital. At a seminar for science writers and editors at Harvard Medical School in early October, Selkoe predicted that within 18 months some companies would be starting clinical trials of agents that can block the disease. But opinions are divided on PS1. According to an Oct. 28 review in Science,2 the fact that cleavage at the Notch receptor pathway, which has been linked to the presenilins, is sensitive to mutations, while APP processing is not, may suggest that PS1 and gamma-secretase are not one and the same. Even so, the review concludes that the presenilins along with ß- and gamma-secretases will likely be valuable molecular AD targets. The PS1 theory's biggest sticking point, says Glabe: Researchers haven't yet been able to show, as Amgen investigators did with BACE, that the purified PS1 enzyme can actually cleave the substrate in the correct position. "It may be more of a technical problem because of the location of the cleavage," Glabe comments. "I don't know if they'll ever get that." Although the discovery of BACE will have little direct impact on the search for gamma-secretase, Hardy suggests that gamma-secretase researchers could benefit by looking to the laborious but straightforward strategy employed by Citron's group. Eugene Russo can be contacted at erusso@the-scientist.com. References 1. R. Vassar et al., " ß-secretase cleavage of Alzheimer's amyloid precursor protein by transmembrane aspartic protease BACE," Science, 286:735-41, Oct. 22, 1999. 2. C. Haass, B. De Strooper, "The presenilins in Alzheimer's disease--proteolysis holds the key," Science, 286:916-9, Oct. 29, 1999.

The Search for Secretases

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