By David N. Leff
BOSTON — What the doctor knows as "essential hypertension" he interprets for his patient as "high blood pressure; cause unknown."
"Hypercholesterolemia" translates as "high cholesterol," forerunner of atherosclerosis — hardening of the arteries.
"Acquired immunodeficiency syndrome" needs no translator; it spells AIDS, as in HIV.
And then there's Alzheimer's disease (AD).
What does AD have in common with the above widely disparate diseases? In a word, proteases — enzymes that clip, or cleave, proteins.
People diagnosed with hypertension look forward to a lifetime of popping pills, such as Captopril, that prevent angiotensin-converting enzyme (ACE) from doing its job of constricting blood vessels.
Individuals with cholesterol levels way out of bounds are prescribed Mevacor (lovastatin) to rein in a more jaw-breaking enzyme: 3-hydroxy-3 methylglutaryl coenzyme A reductase. Its day job is to promote the synthesis of cholesterol.
And the current triple-drug anti-AIDS regimen features one or another protease inhibitor to clip vital fragments off the HIV virus.
Pills For Alzheimer's Under Development
AD patients could certainly use a protease inhibitor against the enzyme most responsible for their affliction.
For lack of a more precise moniker, it's called gamma secretase. This weird enzyme stands accused of unleashing the main perpetrator of AD, amyloid beta-peptide, from its large beta-amyloid precursor protein (APP).
"Amyloid beta-peptide has assumed a center-stage position in AD research," said pioneer AD researcher Dennis Selkoe on Tuesday. He was speaking to attendees at the annual Biotechnology and Trade Exposition of the Massachusetts Biotechnology Council here. A professor of neurology at Harvard Medical School, Selkoe is co-director of the Center for Neurologic Diseases at Harvard-affiliated Brigham and Women's Hospital.
"In 1992," he recalled, "we discovered that amyloid beta can be secreted into the media of cultured cells. This happens in vivo too," he told his audience. "You and I have nanomolar levels of amyloid beta in our spinal fluid, and picomole levels in plasma. This production occurs in all of us throughout life," he added, "in neurons, muscle cells, fibroblasts — throughout the body"
Once secreted beyond the neuronal cell membrane in which gamma secretase cuts it loose from APP, the small but deadly amyloid beta-peptide molecule begins to build up in the intraneuronal space of AD patients' brains. The proteinaceous fibrils that accumulate form globular clumps called senile neuritic (amyloid) plaques — the defining hallmark of the disease.
A protease inhibitor to block gamma secretase from liberating amyloid beta-peptide from APP "would be a very attractive drug discovery target," Selkoe observed, adding, "a number of pharmaceutical companies have publicly announced that they are working on finding candidate compounds."
He cited Bristol-Myers Squibb Co., of New York, "which said they have a very active program;" and Eli Lilly & Co., of Indianapolis, Ind., which is working in collaboration with Athena Neurosciences, of South San Francisco, Calif.
Selkoe pointed out that "as one of Athena's founding scientists in 1986, I'm aware of their work in that regard."
But he noted that none of these companies, or others unidentified, "to my knowledge, have shown the structure of their gamma-secretase-inhibiting compounds. They have stated that they have small-molecule inhibitors that lower amyloid beta-peptide. Some have administered them orally to transgenic mice."
None of these candidate AD medicaments is the product of rational drug design, but rather of irrational design.
"Using 96-well robotic screening," Selkoe told the symposium, "several companies have put more than 100,000 compounds through a whole-cell assay, and came up with candidate gamma secretase inhibitors. They are orally bioavailable and cross the blood-brain barrier. In some cases, they have reportedly lowered amyloid beta-peptide production in transgenic mice."
Selkoe added: "They've been highly modified by medicinal chemistry to make them more potent and hopefully non-toxic. But none of these compounds has yet seen any human subjects."
The Harvard neurologist's own "armchair prediction is that we're probably one to three years away from clinical trials with the gamma secretase inhibitors that are now ready to hand."
Enzyme's Gene Still Being Sought
He made the point that "gamma secretase is a very unusual enzyme. The curious thing about it is that it cleaves the amyloid beta-peptide from a transmembrane orientation, and that clearly is a very bizarre proteolytic processing event."
He continued: "It is a protease that seems to reside within the phospholipid membranes of the cell, both the internal membranes and the cell-surface membrane. Generally, proteases are soluble proteins that are found in the cytosolic compartment of the cell, the cytoplasmic compartment.
"Not so for gamma secretase," Selkoe went on. "Unfortunately, no one knows the enzyme. It stays in the membrane, and we believe that right inside the membrane it clips APP, the precursor protein."
Besides being bizarre, gamma secretase is secretive.
"No one to my knowledge," Selkoe said, "has identified its gene, or solved the structure of the enzyme itself. 'Gamma secretase' is only a nickname we apply to it.
"We could do much better drug discovery," he observed, "if we had the enzyme in hand; [we] could crystalize it with an inhibitor in its active site, etcetera. But that's not been done yet."
Gamma secretase's oddball membrane cleavage behavior, Selkoe noted, "is one of only two such proteases known. The other instance of an intramembrane proteolysis that occurs while the membrane is intact is the cleavage of a protein that regulates cholesterol uptake and synthesis."
Selkoe ended his overflight of where AD research stands today by announcing his latest discovery, still unpublished.
"We recently found," he said, "that a protease that clips the insulin molecule in half, to stop it from doing what it normally does, is also able to do this clipping of the amyloid peptide from APP.
"It turns out," he added, "that the insulin-degrading enzyme — IDE — can also degrade glucagon, atrial natriuretic factor and a number of other small peptides. Now we add to the list the amyloid beta-peptide; it can also cut that up. Whether it does it in you and me during life, we don't know," Selkoe said. "We've not done definitive experiments to prove that it's working right now in our brains. But we've shown it in animals and cultured cells."
He doesn't see any obvious therapeutic role for IDE, but observed, "in the future, when we know more about whether this really happens in the brains of living people, we could try to regulate or stimulate IDE to break down more amyloid peptide, so it wouldn't build up into plaques.
"It would be like taking something to clear cholesterol," Selkoe concluded, "so you wouldn't build it up and get atherosclerosis." *